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Gas-Phase Kinetics of the Hydroxyl Radical Reaction with Allene: Absolute Rate Measurements at Low Temperature, Product Determinations, and Calculations
Abstract
The gas phase reaction of the hydroxyl radical with allene has been studied theoretically and experimentally in a continuous supersonic flow reactor over the range 50 ≤ T/K ≤ 224. This reaction has been found to exhibit a negative temperature dependence over the entire temperature range investigated, varying between (0.75 and 5.0) × 10(-11) cm(3) molecule(-1) s(-1). Product formation from the reaction of OH and OD radicals with allene (C(3)H(4)) has been investigated in a fast flow reactor through time-of-flight mass spectrometry, at pressures between 0.8 and 2.4 Torr. The branching ratios for adduct formation (C(3)H(4)OH) in this pressure range are found to be equal to 34 ± 16% and 48 ± 16% for the OH and OD + allene reactions, respectively, the only other channel being the formation of CH(3) or CH(2)D + H(2)CCO (ketene). Moreover, the rate constant for the OD + C(3)H(4) reaction is also found to be 1.4 times faster than the rate constant for the OH + C(3)H(4) reaction at 1.5 Torr and at 298 K. The experimental results and implications for atmospheric chemistry have been rationalized by quantum chemical and RRKM calculations.
... Additional reactions include the photolysis of pernitric acid (Atkinson et al., 2004), reaction of methyl peroxy radical with OH (Assaf et al., 2016;Caravan et al., 2018), reaction of hydroxymethyl hydrogen peroxide (HMHP) with OH (Allen et al., 2018), and oxidation of propadiene (C 3 H 4 ). For the latter, we use the OH reaction rate coefficient of Atkinson and Arey (2003) rather than the 1.8 times slower rate coefficient of Daranlot et al. (2012), owing to the similar observed decay rates of propadiene and ethene. Subsequent propadiene chemistry follows the mechanisms of Daranlot et al. (2012) and Xu et al. (2019). ...
... For the latter, we use the OH reaction rate coefficient of Atkinson and Arey (2003) rather than the 1.8 times slower rate coefficient of Daranlot et al. (2012), owing to the similar observed decay rates of propadiene and ethene. Subsequent propadiene chemistry follows the mechanisms of Daranlot et al. (2012) and Xu et al. (2019). We also update rate coefficients for reaction of peroxyacetic acid with OH (Berasategui et al., 2020) and peroxyacyl radicals with HO 2 (Jenkin et al., 2019), which are slower/faster than MCM default values by factors of 123/1.33, ...
Large wildfires influence regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key connections within gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOCs), oxidants, and reactive nitrogen over 12 h of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl
nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOCs and nitrogen oxides (NOx).
Observations and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating process interactions. Simulations examine the effects of (1) previously unmeasured reactive VOCs identified in recent laboratory studies
and (2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOCs leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO
amplifies radical cycling and serves as a downwind NOx source, although impacts depend on how HONO is introduced. The addition of initial HONO (representing primary emissions) or particulate nitrate photolysis amplifies ozone production, while heterogeneous conversion of NO2 suppresses ozone formation. Analysis of radical initiation rates suggests that
oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 h of aging. Ozone production chemistry transitions from VOC sensitive to NOx sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.
... Additional reactions include photolysis of pernitric acid (Atkinson et al., 2004), reaction of methyl peroxy radical with OH (Assaf et al., 2016;Caravan et al., 2018), reaction of hydroxymethyl hydrogen peroxide (HMHP) with OH (Allen et al., 2018), and oxidation of propadiene (C3H4). For the latter, we use the OH reaction rate coefficient of Atkinson and Arey (2003) rather than the 1.8-times slower rate coefficient of Daranlot et al. (2012), based on the similar observed decay 195 rates of propadiene and ethene. Subsequent propadiene chemistry follows the mechanisms of Daranlot et al. (2012) and Xu et al. (2019). ...
... For the latter, we use the OH reaction rate coefficient of Atkinson and Arey (2003) rather than the 1.8-times slower rate coefficient of Daranlot et al. (2012), based on the similar observed decay 195 rates of propadiene and ethene. Subsequent propadiene chemistry follows the mechanisms of Daranlot et al. (2012) and Xu et al. (2019). We also update rate coefficients for reaction of peroxyacetic acid with OH (Berasategui et al., 2020) and peroxyacyl radicals with HO2 (Jenkin et al., 2019), which are slower/faster than MCM default values by factors of 123/1.33, ...
Large wildfires markedly alter regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key facets of gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOC), oxidants, and reactive nitrogen over 12 hours of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOC and nitrogen oxides (NOx). Measurements and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating key processes. Simulations examine the effects of 1) previously-unmeasured reactive VOC identified in recent laboratory studies, and 2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOC leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NOx source, although two different HONO production mechanisms (particulate nitrate photolysis and heterogeneous NO2 conversion) exhibit markedly different effects on ozone, NOx, and PNs. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 hours of aging. Ozone production chemistry transitions from VOC-sensitive to NOx-sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.
... × 10 −12 cm 3 / molecule/s for allene + OH 17−20 and in the range 0.95−6.21 × 10 −12 cm 3 /molecule/s for propyne + OH. 17,21−24 At temperatures lower than ambient, Daranlot et al. 25 carried out experimental and theoretical work on the gas phase reaction of the OH radicals with allene over 50−228 K. Their study was conducted at low gas pressure (∼1 Torr) in a continuous supersonic flow reactor. ...
... A wave-meter (Bristol 610) was used to monitor the visible laser wavelength. To minimize the noise caused by laser intensity fluctuations, common-mode-rejection scheme was applied by splitting the UV beam with a beam 18 299−424 100 5.59 × 10 −12 exp(305 ± 300 (cal/mol))/RT) Ohta 19 297 760 (1.03 ± 0.11) × 10 −11 Atkinson and Aschmann 20 295 735 (9.65 ± 0.96) × 10 −12 Liu et al. 25 305−1173 760 (6.7 ± 0.9) × 10 −12 exp(100 ± 50)/T) Daranlot et al. 26 50−298 ...
Allene (H2C=C=CH2; a-C3H4) and propyne (CH3C≡CH; p-C3H4) are of great importance in various chemical environments. In combustion processes, the reactions of hydroxyl radicals with a-C3H4 and p-C3H4 have been shown to be critical in the overall fuel oxidation system. In this work, rate coefficients of OH radicals with allene (R1: OH + H2C=C=CH2 → products) and propyne (R2: OH + CH3C≡CH → products) were measured behind reflected shock waves over the temperature range of 843 – 1352 K and pressures near 1.5 atm. Hydroxyl radicals were generated by rapid thermal decomposition of tert-butyl-hydroperoxide ((CH3)3−CO−OH), and monitored by narrow linewidth laser absorption of the well-characterized R1(5) electronic transition of the OH A–X (0,0) electronic system near 306.7 nm. Allene is found to react faster with OH radicals than propyne over the temperature range of this study. Measured rate coefficients can be expressed in Arrhenius form as follows: k1(T) = (2.62 ± 0.4)×10–11exp((–768 ±13) ⁄T) T = 843 – 1352 K k2(T) = (2.45 ± 0.5)×10–11exp((–1630 ±24) ⁄ T) T = 846 – 1335 K
... This product formation occurs in two steps (see, Sec The high reactivity of aldehydes with species like OH radicals is well-known, even at low temperatures, and well-studied on simple models (primarily for reactions OH + CH 2 O and acetaldehyde [96][97][98][164][165][166][167]. The preferred reaction here is the abstraction of aldehydic H-atom leading to negative activation energy (NTC phenomenon), as opposed to the OH + olefins reactions [117][118][119] , whereas the addition-barrier to C=O bond is considerable (5-15 kcal/mol). ...
... The parameter k pp is specific to propadiene, with the total rate coefficient being k pp + 2k v . Daranlot et al. (2012) have inferred that the reaction occurs 80 % via addition to the internal carbon atom, based on a combination of experimental results and theoretical calculations. This branching ratio was therefore used to constrain the relative values of k pp and k v in the present work, i.e. k pp = 8k v . ...
Reaction with the hydroxyl (OH) radical is the dominant removal
process for volatile organic compounds (VOCs) in the atmosphere. Rate
coefficients for reactions of OH with VOCs are therefore essential parameters
for chemical mechanisms used in chemistry transport models, and are required
more generally for impact assessments involving the estimation of atmospheric
lifetimes or oxidation rates for VOCs. Updated and extended
structure–activity relationship (SAR) methods are presented for the reactions
of OH with aliphatic organic compounds, with the reactions of aromatic
organic compounds considered in a companion paper. The methods are optimized
using a preferred set of data including reactions of OH with 489 aliphatic
hydrocarbons and oxygenated organic compounds. In each case, the rate
coefficient is defined in terms of a summation of partial rate coefficients
for H abstraction or OH addition at each relevant site in the given organic
compound, so that the attack distribution is defined. The information can
therefore guide the representation of the OH reactions in the next generation
of explicit detailed chemical mechanisms. Rules governing the representation
of the subsequent reactions of the product radicals under tropospheric
conditions are also summarized, specifically their reactions with
O2 and competing processes.
... The parameter k pp is specific to propadiene, with the total rate coefficient being k pp + 2k v . Daranlot et al. (2012) have inferred that the reaction occurs 80 % via addition to the internal 10 carbon atom, based on a combination of experimental results and theoretical calculations. This branching ratio was therefore used to constrain the relative values of k pp and k v in the present work, i.e. k pp = 8k v . ...
Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry-transport models, and are required more generally for impact assessments involving estimation of atmospheric lifetimes or oxidation rates for VOCs. Updated and extended structure-activity relationship (SAR) methods are presented for the reactions of OH with aliphatic organic compounds, with the reactions of aromatic organic compounds considered in a companion paper. The methods are optimized using a preferred set of data including reactions of OH with 489 aliphatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The information can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the subsequent reactions of the product radicals under tropospheric conditions are also summarized, specifically their reactions with O2 and competing processes.
We report the detection of methyl ketene towards TMC-1 with the QUIJOTE line survey. Nineteen rotational transitions with rotational quantum numbers ranging from J = 3 up to J = 5 and K a ≤ 2 were identified in the frequency range 32.0–50.4 GHz, 11 of which arise above the 3 σ level. We derived a column density for CH 3 CHCO of N = 1.5 × 10 ¹¹ cm ⁻² and a rotational temperature of 9 K. Hence, the abundance ratio between ketene and methyl ketene, CH 2 CO/CH 3 CHCO, is 93. This species is the second C 3 H 4 O isomer detected. The other, trans-propenal (CH 2 CHCHO), corresponds to the most stable isomer and has a column density of N = (2.2 ± 0.3)×10 ¹¹ cm ⁻² , which results in an abundance ratio CH 2 CHCHO/CH 3 CHCO of 1.5. The next non-detected isomer with the lowest energy is cis-propenal, which is therefore a good candidate for future discovery. We have carried out an in-depth study of the possible gas-phase chemical reactions involving methyl ketene to explain the abundance detected, achieving good agreement between chemical models and observations.
The reaction of the OH radical with cyclopentadiene (C5H6) was investigated at room temperature using multiplexed photoionization mass spectrometry. OH radicals in their ground electronic state were generated in the gas phase by 248 nm photolysis of H2O2 or 351 nm photolysis of HONO. Analysis of photoion spectra and temporal profiles reveal that at room temperature and over the 4-8 Torr pressure range, the resonance-stabilized 5-hydroxycyclopent-2-en-1-yl (C5H6OH) is the main observed reaction product. Abstraction products (C5H5) were not detected. The C5H6OH potential energy surface calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G** level of theory suggests that the resonance-stabilized radical product is formed through barrierless addition of the OH radical onto cyclopentadiene's π system to form a van der Waals complex. This weakly bound adduct isomerizes through a submerged energy barrier to the resonance-stabilized addition adduct. Master Equation calculations, including two OH-addition entrance pathways, predict that 5-hydroxycyclopent-2-en-1-yl remains the sole addition product up to 500 K. The detection of an OH-containing resonance-stabilized radical at room temperature further highlights their importance in carbon- and oxygen-rich environments such as combustion, planetary atmospheres, and the interstellar medium.
The C3H5O and C3H5O2 potential energy surfaces accessed by the oxidation reactions of allyl radical with atomic and molecular oxygen have been mapped out at the CCSD(T)-F12b/cc-pVTZ-F12//ωB97XD/6-311G(d,p) level of electronic structure theory to unravel the reaction mechanism. Temperature- and pressure-dependent reaction rate constants have been evaluated using variable reaction coordinate transition state and RRKM-Master Equation theoretical kinetics methods. The C3H5 + O reaction is found to be fast in the 300-2500 K temperature interval, with the total rate constant of 1.8-1.0 × 10⁻¹⁰ cm³ molecules s⁻¹ independent of pressure in the considered 30 Torr – 100 atm range. Acrolein + H formed by a simple O addition/H elimination mechanism via the CH2CHCH2O association complex are predicted to be the major reaction products with the branching ratio decreasing from 61% at 300 K to 44% at 2500 K. Other substantial bimolecular products include C2H3 + formaldehyde (32%-26%) formed through the C-C bond β-scission in CH2CHCH2O and two minor products C2H4 + formyl radical HCO and allene + OH, where the latter, formed via direct H abstraction by O from the central carbon atom of allyl, becomes significant only at high temperatures above ∼1500 K. The C3H5 + O2 reaction studied in the 200-2500 K temperature range shows a peculiar kinetic behavior characteristic for a bimolecular reaction with a low entrance barrier, shallow association well, and high ensuing isomerization and decomposition barriers to bimolecular products. This behavior is described in terms of three distinct temperature regimes: the low-temperature one with slightly negative dependence of the rate constant, the intermediate one where the rate constant sharply drops, and the high-temperature regime with an Arrhenius-like rate constant. The rate constant is relatively high (10⁻¹³-10⁻¹² cm³ molecule⁻¹ s⁻¹) in the low-temperature regime when the reaction produces the peroxy association complex CH2CHCH2OO, but slow at higher temperatures when it leads to bimolecular products. Under combustion relevant conditions, the C3H5 + O2 rate constant is by 5 to 3 orders of magnitude lower than that for C3H5 + O, but since O2 concentrations in flames could be as much as 3 orders of magnitude larger than O concentrations, C3H5 + O2 cannot be ruled out as a significant allyl radical sink. Above 1500 K, the C3H5 + O2 reaction can form vinoxy radical + formaldehyde (40%-15%) via the five- or four-membered ring closure and opening mechanism starting from the initial peroxy intermediate, acrolein + OH (41%-49%) via the H shift from the attacked carbon to the terminal oxygen followed by the immediate O-O bond cleavage, and allene + OH (18-25%) via the H migration from the central CH group to the terminal O atom accompanied or followed by the C-O bond rupture. Modified Arrhenius expressions fitting the computed rate constants for both reactions have been generated and proposed for kinetic models.
We report the detection of the oxygen-bearing complex organic molecules propenal (C 2 H 3 CHO), vinyl alcohol (C 2 H 3 OH), methyl formate (HCOOCH 3 ), and dimethyl ether (CH 3 OCH 3 ) toward the cyanopolyyne peak of the starless core TMC-1. These molecules were detected through several emission lines in a deep Q -band line survey of TMC-1 carried out with the Yebes 40m telescope. These observations reveal that the cyanopolyyne peak of TMC-1, which is a prototype of a cold dark cloud rich in carbon chains, also contains O-bearing complex organic molecules such as HCOOCH 3 and CH 3 OCH 3 , which have previously been seen in a handful of cold interstellar clouds. In addition, this is the first secure detection of C 2 H 3 OH in space and the first time that C 2 H 3 CHO and C 2 H 3 OH have been detected in a cold environment, adding new pieces to the puzzle of complex organic molecules in cold sources. We derive column densities of (2.2 ± 0.3) × 10 ¹¹ cm ⁻² , (2.5 ± 0.5) × 10 ¹² cm ⁻² , (1.1 ± 0.2) × 10 ¹² cm ⁻² , and (2.5 ± 0.7) × 10 ¹² cm ⁻² for C 2 H 3 CHO, C 2 H 3 OH, HCOOCH 3 , and CH 3 OCH 3 , respectively. Interestingly, C 2 H 3 OH has an abundance similar to that of its well-known isomer acetaldehyde (CH 3 CHO), with C 2 H 3 OH/CH 3 CHO ∼ 1 at the cyanopolyyne peak. We discuss potential formation routes to these molecules and recognize that further experimental, theoretical, and astronomical studies are needed to elucidate the true formation mechanism of these O-bearing complex organic molecules in cold interstellar sources.
More than 50 years have passed since Haszeldine reported the first addition of a trifluoromethyl radical to an allene; in the intervening years, both the chemistry of allenes and the reactivity of single-electron species have become topics of intense interest. In this Review, we provide an overview of the fundamentals of radical additions to allenes and highlight the emergence of theoretical and experimental evidence that reveals unique reactivity patterns for radical additions to allenes as compared with other unsaturated compounds. Factors capable of exerting control over the chemo-, regio-, and stereoselectivities of the attack of carbon- and heteroatom-based radicals at each of the three potential reactive sites in an allene substrate are described. These include reaction conditions, the nature of the attacking radical, the substitution pattern of the allene, and the length of the linker between the radical center and the proximal allene carbon in the substrate. Cycloaddition reactions between allenes and partners containing π-bonds, which are likely to proceed through radical pathways, are presented to highlight their ability to rapidly access complex polycyclic scaffolds. Finally, the synthetic utility of the products arising from these chemistries is described, including their applications to the construction of complex molecules.
The field of astrochemistry concerns the formation and abundance of molecules in the interstellar medium, star-forming regions, exoplanets and solar system bodies. These astrophysical objects contain the chemical material from which new planets and solar systems are formed. Around 200 molecules have thus far been observed in the interstellar medium; almost half containing six or more atoms and considered ``complex'' by astronomical standards. All of these complex molecules consist of at least one carbon atom and thus the term complex organic molecules (COMs) has been coined by the astrochemical community. In order to understand the formation and destruction of these COMs under the extreme conditions of star-forming regions, three kinds of activity are involved: (1) the astronomical identification of complex molecules present in the ISM; (2) the construction of astrochemical models that attempt to explain the formation routes of the observed molecules; and (3) laboratory measurements and theoretical calculations of critical kinetic parameters that are included in the models. In the following review, we present recent laboratory efforts to produce quantitative kinetic data for gas-phase reactions at low temperatures. We discuss the use of the CRESU technique, a French acronym standing for \textit{Cin\'{e}tique de R\'{e}action en Ecoulement Supersonique Uniforme}, which means reaction kinetics in uniform supersonic flow, to measure reactions of astrochemical importance. In particular, we highlight recent and future advances in the measurement of product-specific reaction kinetics at low temperatures.
The interstellar medium attracts our great attention, as the place where stars and planets are born and from where, probably, the molecular precursors of life have come to Earth. To understand the chemical pathways to the formation of stars, planets, and biological molecules, astronomical observations, astrochemical modelling, and laboratory astrochemistry should go hand in hand. In this paper we review the laboratory experiments devoted to investigations of the reaction dynamics of species of astrochemical interest at the temperatures of the interstellar medium and performed by using one of the most popular techniques in the field, CRESU. We discuss new technical developments and scientific ideas for CRESU, which, if realized, will bring us one more step closer to an understanding of the astrochemical history and future of our Universe.
The interstellar medium attracts our great attention, as the place where stars and planets are born and from where, probably, the molecular precursors of life have come to Earth. To understand the chemical pathways to the formation of stars, planets, and biological molecules, astronomical observations, astrochemical modelling, and laboratory astrochemistry should go hand in hand. In this paper we review the laboratory experiments devoted to investigations of the reaction dynamics of species of astrochemical interest at the temperatures of the interstellar medium and performed by using one of the most popular techniques in the field, CRESU. We discuss new technical developments and scientific ideas for CRESU, which, if realized, will bring us one more step closer to an understanding of the astrochemical history and future of our Universe.
Self-recombination and cross-reactions of large resonant stabilized hydrocarbon radicals such as fulvenallenyl (C7H5) are predicted to form polycyclic aromatic hydrocarbons in combustion and the interstellar medium. Although fulvenallenyl is likely to be present in these environments, large uncertainties remain about its formation mechanisms. We have investigated the formation of fulvenallenyl by reacting the OH radical with fulvenallene (C7H6) over the 298 K to 450 K temperature range and at a pressure of 5 Torr (667 Pa). The reaction rate coefficient is found to be 8.8(±1.7)×10(-12) cm(3) s(-1) at room temperature with a negative temperature dependence that can be fit from 298 K to 450 K to k(T)=8.8(±1.7)×10(-12) (T/298K)(-6.6(±1.1))exp(-8.72(±3.03)kJ mol-1/R (1/T-1/298K)) cm(3) s(-1). The comparison of the experimental data with calculated abstraction rate coefficients suggests that over the experimental temperature range, association of the OH radical to fulvenallene plays a significant role likely leading to a low fulvenallenyl branching fraction.
We mapped out the stationary points and the corresponding conformational space on the C3H5O potential energy surface relevant for the OH + allene and OH + propyne reactions systematically and automatically using the KinBot software at the UCCSD(T)-F12b/cc-pVQZ-F12//M06-2X/6-311++G(d,p) level of theory. We used RRKM-based 1-D master equations to calculate pressure- and temperature-dependent, channel-specific phenomenological rate coefficients for the bimolecular reactions propyne + OH and allene + OH, and for the unimolecular decomposition of the CH3CCHOH, CH3C(OH)CH, CH2CCH2OH, CH2C(OH)CH2 primary adducts, and also for the related acetonyl, propionyl, 2-methylvinoxy, and 3-oxo-1-propyl radicals. The major channel of the bimolecular reactions at high temperatures is the formation propargyl + H2O, which makes the title reactions important players in soot formation at high temperatures. However, below ∼1000 K the chemistry is more complex, involving the competition of stabilization, isomerization and dissociation processes. We found that the OH addition to the central carbon of allene has a particularly interesting and complex pressure dependence, caused by the low-lying exit channel to form ketene + CH3 bimolecular products. We compared our results to a wide range of experimental data and assessed possible uncertainties arising from certain aspects of the theoretical framework.
In this work, the first and rate determining steps of the mechanism of the OH addition to 2-methyl-2-propen-1-ol (MPO221) and methylpropene (M2) have been studied at the DFT level, employing the BH and HLYP functional and the cc-pVDZ and aug-cc-pVDZ basis sets. The thermochemical properties of equilibrium (enthalpy, entropy and Gibbs free energies) have been determined within the conventional statistical thermodynamics relations and the rate coefficients have been determined on the basis of the variational transition state theory. The adoption of the microcanonical variational transition state theory was proved to be crucial for the description of the kinetics of OH addition to these unsaturated compounds. The rate coefficients obtained for the OH reactions with MPO221 and M2 at 298.15 K deviate, respectively, 27% and 13% from the experimental rate coefficient available in the literature. A non-Arrhenius profile is observed for the rate coefficients. Moreover, the values of the rate coefficients for the MPO221 + OH reaction are greater than those for the M2 + OH reaction, suggesting that the substitution of the hydrogen atom in an alkene by the -OH functional group increases the reactivity with respect to the hydroxyl radical.
The present review is focused on the application of computational/theoretical methods to the wide and rich chemistry of allenes. Special emphasis is made on the interplay and synergy between experimental and computational methodologies, rather than on recent developments in methods and algorithms. Therefore, this review covers the state-of-the-art applications of computational chemistry to understand and rationalize the bonding situation and vast reactivity of allenes. Thus, the contents of this review span from the most fundamental studies on the equilibrium structure and chirality of allenes to recent advances in the study of complex reaction mechanisms involving allene derivatives in organic and organometallic chemistry.
Cumulenes are a highly varied class of compounds, including such species as ketenes, allenes, ketenimines, and isocyanates, as well as analogues where carbon is replaced by silicon or germanium, oxygen is replaced by sulfur or selenium, and nitrogen by phosphorus or arsenic. The unique structural characteristics of cumulenes lead to their distinctive patterns of reactivity, and the investigations of their structures and properties have enhanced their utility. Silylcyclobutenones prepared by new methodology from dimethoxycyclobutenedione are readily converted to the corresponding stabilized 2-silylvinylketenes, and these give efficient [4 + 2] cycloadditions with tetracyanoethylene (TCNE) and also with imines. There is increasing interest in cumulenes, including the preparation and identification of new members of this family, with new ways of catalyzing their reactivity, and the development of stereoselective processes.
Consecutive reactions of adducts, resulting from OH radicals and aromatics, with the tropospheric scavenger molecules O2, NO and NO2 have been studied for benzene, naphthalene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol and aniline by observing decays of OH at temperatures where the thermal back-decomposition to OH is faster than 3 s−1, typically between 300 and 340 K. The experimental technique was resonance fluorescence with flash photolysis of water as source of OH. Biexponential decays were observed in the presence of either O2 or NO, and triexponential decays were obtained in the presence of NO2. The kinetic analysis was performed by fitting the relevant rate constants of the reaction mechanism to whole sets of decays obtained at various concentrations of aromatic and scavenger. In the case of hexamethylbenzene, the biexponential decays suggest the existence of the ipso-adduct, and the slightly higher necessary temperatures show that it is even more stable. In addition, smog chamber experiments at O2 concentrations from atmospheric composition down to well below 100 ppm have been carried out for benzene, toluene and p-xylene. The drop of the effective rate constant of removal by OH occurs at reasonable O2 levels, given the FP/RF results. Comparison of the adduct reactivities shows for all aromatics of this study that the reaction with O2 predominates over that with NO2 under all tropospheric conditions, and that a reaction with NO may only occur after the reaction with O2.
This compilation updates and expands two previous evaluations of kinetic data on elementary, homogeneous, gas phase reactions of neutral species involved in combustion systems [J. Phys. Chem. Ref Data 21, 411 (1992); 23, 847 (1994)]. The work has been carried out under the auspices of the IUPAC Commission on Chemical Kinetics and the UK Engineering and Physical Sciences Research Council. Individual data sheets are presented for most reactions but the kinetic data for reactions of C2, C, ethyl, i-propyl, t-butyl, and allyl radicals are summarized in tables. Each data sheet sets out relevant thermodynamic data, experimental kinetic data, references, recommended rate parameters with their error limits and a brief discussion of the reasons for their selection. Where appropriate the data are displayed on an Arrhenius diagram or by fall-off curves. Tables summarizing the recommended rate data and the thermodynamic data for the reactant and product species are given, and their sources referenced. As in the previous evaluations the reactions considered relate largely to the combustion in air of organic compounds containing up to three carbon atoms and simple aromatic compounds. Thus the data base has been expanded, largely by dealing with a substantial number of extra reactions within these general areas.
These experiments study the preparation of and product channels resulting from OCH2CHCH2, a key radical intermediate in the O+allyl bimolecular reaction. The data include velocity map imaging and molecular beam scattering results to probe the photolytic generation of the radical intermediate and the subsequent pathways by which the radicals access the energetically allowed product channels of the bimolecular reaction. The photodissociation of epichlorohydrin at 193.3 nm produces chlorine atoms and c-OCH2CHCH2 radicals; these undergo a facile ring opening to the OCH2CHCH2 radical intermediate. State-selective resonance-enhanced multiphoton ionization (REMPI) detection resolves the velocity distributions of ground and spin-orbit excited state chlorine independently, allowing for a more accurate determination of the internal energy distribution of the nascent radicals. We obtain good agreement detecting the velocity distributions of the Cl atoms with REMPI, vacuum ultraviolet (VUV) photoionization at 13.8 eV, and electron bombardment ionization; all show a bimodal distribution of recoil kinetic energies. The dominant high recoil kinetic energy feature peaks near 33 kcal/mol. To elucidate the product channels resulting from the OCH2CHCH2 radical intermediate, the crossed laser-molecular beam experiment uses VUV photoionization and detects the velocity distribution of the possible products. The data identify the three dominant product channels as C3H4O (acrolein)+H, C2H4+HCO (formyl radical), and H2CO (formaldehyde)+C2H3. A small signal from C2H2O (ketene) product is also detected. The measured velocity distributions and relative signal intensities at m/e = 27, 28, and 29 at two photoionization energies show that the most exothermic product channel, C2H5+CO, does not contribute significantly to the product branching. The higher internal energy onset of the acrolein+H product channel is consistent with the relative barriers en route to each of these product channels calculated at the CCSD(T)/aug-cc-pVQZ level of theory, although a clean determination of the barrier energy to H+acrolein is precluded by the substantial partitioning into rotational energy during the photolytic production of the nascent radicals. We compare the measured branching fraction to the H+acrolein product channel with a statistical prediction based on the calculated transition states.
Photoabsorption and photoionization cross sections of organic molecules are systematically compared for elucidating relation between superexcitation and ionization. The cross sections examined are of alkenes, alkanes, alcohols, and ethers in the energy range of about 2 eV above the first ionization potential. Although the photoabsorption cross sections are much different from one another, the photoionization cross sections are similar in each molecular group. This result indicates that ions are formed only through direct photoionization and most of superexcited molecules dissociate to neutral fragments. Ionization efficiency curves are calculated under the assumption of no autoionization, and they well reproduce the observed ionization curves, which mainly depend on energy difference between the first and second ionization potentials.
We report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, for flame-sampling photoionization mass spectrometry (PIMS). Synchrotron radiation offers important advantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superior signal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges of current VUV laser sources. Near-threshold photoionization efficiency measurements were used to determine the absolute concentrations of the allene and propyne isomers of C3H4 in low-pressure laminar ethylene–oxygen and benzene–oxygen flames. Similar measurements of the isomeric composition of C2H4O species in a fuel-rich ethylene–oxygen flame revealed the presence of substantial concentrations of ethenol (vinyl alcohol) and acetaldehyde. Ethenol has not been previously detected in hydrocarbon flames. Absolute photoionization cross sections were measured for ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curves are presented for several additional reaction intermediates prominent in hydrocarbon flames. © 2003 American Institute of Physics.
The reaction dynamics of ground-state atomic oxygen O(3 P) with allyl radicals (C 3 H 5) has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants O(3 P) and C 3 H 5 were produced by the photodissociation of NO 2 and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of O(3 P) C 3 H 5 →C 3 H 4 OH was observed and the nascent internal state distributions of the product OH (X 2 :0,1) showed substantial bimodal internal excitations of the low-and high-N components without -doublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBS-QB3 level of ab initio theory and Rice–Ramsperger– Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of O(3 P) with C 3 H 5 is the formation of acrolein (CH 2 CHCHO)H, which is consistent with the previous bulk kinetic experiments performed by Gutman et al. J. Phys. Chem. 94, 3652 1990. The counterpart C 3 H 4 of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reaction enthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atom-radical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low N-components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high N-components implies that some fraction of reactants is sampled to proceed through the indirect short-lived addition-complex forming process.
Ab initio calculations of the reaction of ground-state atomic oxygen O(3 P) with an allyl radical (C 3 H 5) have been carried out using the density functional method and the complete basis set model. On the calculated lowest doublet potential energy surface, the barrierless association of O(3 P) to C 3 H 5 forms three energy-rich addition intermediates, which are predicted to undergo subsequent isomerization and decomposition steps leading to various products: C 3 H 4 OH, CH 2 OC 2 H 3 , C 2 H 4 CHO, C 2 H 2 OCH 3 , C 2 H 5 CO, C 3 H 4 OH, and C 2 H 4 OCH. The respective reaction mechanisms through the three addition intermediates are presented, and it has been found that the barrier height, reaction enthalpy, and the number of intermediates involved along the reaction coordinate are of extreme importance in understanding such reactive scattering processes. With the aid of Rice–Ramsperger–Kassel–Marcus calculations, the major reaction pathway is predicted to be the formation of acrolein (C 3 H 4 O)H, which is consistent with the previous gas-phase bulk kinetic experiment performed by Gutman et al. J. Phys. Chem. 94, 3652 1990. For the minor C 3 H 4 OH channel, which has been newly found in the recent crossed beam investigations, a second barrierless, direct H-atom abstraction from the central carbon of C 3 H 5 is calculated to compete with the addition process due to the little C–H bond dissociation energy and the formation of a stable allene product. The dynamic and kinetic characteristics of the reaction mechanism are discussed on the basis of the comparison of prior statistical calculations to the nascent internal distributions of the observed OH product. © 2003 American Institute of Physics.
We use a combination of crossed laser-molecular beam experiments and velocity map imaging experiments to investigate the primary photofission channels of chloroacetone at 193 nm; we also probe the dissociation dynamics of the nascent CH(3)C(O)CH(2) radicals formed from C-Cl bond fission. In addition to the C-Cl bond fission primary photodissociation channel, the data evidence another photodissociation channel of the precursor, C-C bond fission to produce CH(3)CO and CH(2)Cl. The CH(3)C(O)CH(2) radical formed from C-Cl bond fission is one of the intermediates in the OH + allene reaction en route to CH(3) + ketene. The 193 nm photodissociation laser allows us to produce these CH(3)C(O)CH(2) radicals with enough internal energy to span the dissociation barrier leading to the CH(3) + ketene asymptote. Therefore, some of the vibrationally excited CH(3)C(O)CH(2) radicals undergo subsequent dissociation to CH(3) + ketene products; we are able to measure the velocities of these products using both the imaging and scattering apparatuses. The results rule out the presence of a significant contribution from a C-C bond photofission channel that produces CH(3) and COCH(2)Cl fragments. The CH(3)C(O)CH(2) radicals are formed with a considerable amount of energy partitioned into rotation; we use an impulsive model to explicitly characterize the internal energy distribution. The data are better fit by using the C-Cl bond fission transition state on the S(1) surface of chloroacetone as the geometry at which the impulsive force acts, not the Franck-Condon geometry. Our data suggest that, even under atmospheric conditions, the reaction of OH with allene could produce a small branching to CH(3) + ketene products, rather than solely producing inelastically stabilized adducts. This additional channel offers a different pathway for the OH-initiated oxidation of such unsaturated volatile organic compounds, those containing a C=C=C moiety, than is currently included in atmospheric models.
This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H(2)COCH)CH(2)Cl. The three dominant photoproduct channels analyzed are c-(H(2)COCH)CH(2)+Cl, c-(H(2)COCH)+CH(2)Cl, and C(3)H(4)O+HCl. In the second channel, the c-(H(2)COCH) photofission product is a higher energy intermediate on C(2)H(3)O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H(2)CCO. The final primary photodissociation pathway HCl+C(3)H(4)O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H(2)COC)=CH(2); the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C(3)H(5)O radical produced from C-Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O((3)P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C(2)H(4) products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H(2)CO+C(2)H(3) product channel of the O((3)P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H(3)CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C(3)H(5)O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C(2)H(4), and H(2)CO+C(2)H(3) product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels. Using the C(2)H(4) bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C(2)H(4) and HCO yields a value of 11.6+1/-3 Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies.
The temperature dependence of the reactions of the dicarbon molecule in its ground singlet (X1Sigma(g)+) and first excited triplet (a 3Pi(u)) states with acetylene, methylacetylene, allene and propene has been studied using a recently constructed continuous supersonic flow reactor. Four Laval nozzles have been designed to access specified temperatures over the range of 77 < or = T < or = 220 K and measurements have been performed at 296 K under subsonic flow conditions. C2 was produced in its two lowest electronic states via the in situ multiphoton dissociation of C2Br4 at 266 nm. The time dependent losses of C2 in these two states in the presence of an excess of co-reagent species were simultaneously followed by laser-induced fluorescence in the Mulliken and Swan bands for the detection of singlet and triplet state C2, respectively. The rate coefficients were measured to be very fast, with values larger than 10(-10) cm3 molecule(-1) s(-1) and up to 5 x 10(-10) cm3 molecule(-1) s(-1). The reactions of 1C2 are seen to be essentially temperature independent from 77 < or = T < or = 296 K whereas the rate coefficients for the 3C2 reactions are seen to increase until they are equivalent to the 1C2 values at 77 K.
No abstract is available for this article.
We use a combination of crossed laser-molecular beam scattering experiments and velocity map imaging experiments to investigate the dissociative ionization of the CH3C(O)CH2 radical to C2H5+. We form the radical from C–Cl bond fission in the photodissociation of chloroacetone at 193 nm. Upon 10.5 eV VUV photoionization, the radical is not detected at a parent mass-to-charge ratio of 57, but instead is only detected at the fragment m/z = 29 (C2H5+). While the appearance of multiple daughter ions is expected, and indeed observed, using 200 eV electron bombardment ionization, one normally expects “soft” VUV photoionization to give signal at parent ion. We present electronic structure calculations that offer an explanation of our experimental results. The results presented herein also confirm the presence of a minor dissociation channel for the highly vibrationally excited CH3C(O)CH2 radicals – one that forms C2H5 + CO following isomerization to CH3CH2CO.
Rate coefficients k1 and k2 have been determined for the association reaction N+2+2 N2↠k1N+4+N2 and O+2+2 O2↠k2O+4+O2 in the temperature range 20–160 K, using a supersonic jet apparatus (CRESU), which is described in detail. For the highest temperatures where other measurements exist, the measured values of the rate coefficients are in excellent agreement with previous measurements. The temperature dependence of k1 can be represented by a power law of the form k1=6.0×10−29 (300/T)1.85 from 20 K to higher temperatures. For k2, only lower limits for 20 and 45 K are reported but the present data do not confirm the existence of a maximum in k2 as suggested by other authors. These results are examined in the light of recent theoretical arguments.
Using an infrared-ultraviolet double resonance method, we have measured rate coefficients at room temperature for the transfer of OH radicals from rotational levels between ji=1.5 (N=1) and ji=8.5 (N=8) in the X 2Π, Ω=3/2; v=1 vibronic state in collisions with He, Ar, N2 and HNO3. OH radicals were generated by 266 nm pulsed laser photolysis of HNO3 and promoted to selected ji using a pulsed infrared laser tuned to an appropriate line in the (1,0) infrared fundamental band of OH. The evolution of population in selected levels was observed using time-delayed laser-induced fluorescence in the (1,1) band of the A 2Σ+–X 2Π system. The results of two kinds of measurement are reported. For ji=3.5 and 6.5, a single Λ-doublet component of the selected rotational level was excited and the evolution of the populations in both Λ-doublet components was observed. The decay of the sum of the two individual populations then yields rate coefficients for the transfer of population from ji, free of complications arising from the transfer of population between the two Λ-doublet levels. For a wider range of levels, including ji=6.5, we have carried out simpler measurements in which rate coefficients for transfer from the initially excited level ji are inferred by monitoring only the change in population in the Λ-doublet level that is directly populated by the pump laser. Measurements of both kinds have been carried out for ji=6.5 and the rate coefficients derived from the two sets of measurements are in good agreement. The measured rate coefficients for rotational relaxation (kRET) show a significant dependence on both the collision partner, with kRET(He)≈kRET(Ar)<kRET(N2)<kRET(HNO3), and on the rotational level with the values of kRET generally decreasing as ji is increased beyond ji=3.5.
The kinetics and pressure dependence of the reactions of NO2 with CH3 and CH3O have been investigated in the gas phase at 298 K, at pressures from 1 to 10 Torr. A low-pressure discharge-flow laser-induced fluorescence (LIF) technique was used. In a consecutive process, CH3 reacted with NO2 to form CH3O, CH3+ NO2→ CH3O + NO (1), which further reacted with NO2 to form products, CH3O + NO2→ products (2). Reaction (1) displayed no discernible pressure dependence over the pressure range 1–7 Torr, and k1 was calculated to be (2.3 ± 0.3)× 10–11 cm3 molecule–1 s–1. Reaction (2) displayed a strong pressure dependence and an RRKM analysis yielded the following limiting low- and high-pressure rate constants in He, k0= 5.9 × 10–29 cm6 molecule–2 s–1 and k∞= 2.1 × 10–11 cm3 molecule–1 s–1. It is unrealistic to quote errors for this type of analysis. Parametrisation in the standard Troe form with Fc= 0.6 yielded k0=(5.3 ± 0.2)× 10–29 cm6 molecule–2 s–1 and k∞=(1.4 ± 0.1)× 10–11 cm3 molecule–1 s–1. Atmospheric implications and possible reaction mechanisms are discussed.
The kinetics of the reactions of CH3O and CD3O with NO have been studied using a discharge flow reactor. CH3O and CD3O were detected using laser-excited fluorescence (LEF) near 300 nm. Total rate coefficients for the reaction of CH3O with NO were measured as a function of temperature (220–473 K) and pressure (0.75–5.0 Torr) of He or Ar. Total rate coefficients for the CD3O + NO reaction were measured at ca. 294 K over the pressure range 0.75–5.0 Torr He. Using molecular-beam mass spectrometry, the CH3ONO yield of the CH3O + NO reaction was measured at 297 K (0.5 and 1.0 Torr) and 223 K (1.0 Torr), showing that disproportionation to H2CO + HNO is the major channel at low pressures. These results were combined to obtain the following expressions for the disproportionation and low-pressure recombination rate coefficients.
The rates of reaction of hydroxyl radicals with ethylene, propane, propylene, methylacetylene, and allene have been measured at room temperature in a discharge-flow system using electron spin resonance detection. The stoichiometries (n=Δ[OH]/Δ[R]) were obtained by mass spectrometric analysis of the reacted gas under similar, although not completely identical conditions. The primary rate constants for the C3-hydrocarbons obtained by combining the two are given as: OH + propane k6=(5.0 ± 1.0)× 1011 cm3 mol–1 s–1+ propylene k7=(3.0 ± 1.0)× 1012 cm3 mol–1 s–1+ methylacetylene k8=(5.7 ± 1.0)× 1011 cm3 mol–1 s–1+ allene k9=(2.7 ± 1.5)× 1012 cm3 mol–1 s–1. The values of n as well as the nature of the products provide some information on the mechanisms involved.
The rate constant for the CH+H2O reaction has been measured in a fast-flow reactor to be (0.9±0.4)×10−11cm3molecule−1s−1 at 330K. The product branching ratio of H atoms probed by resonance fluorescence is determined to be 100%. Quantum chemistry and statistical calculations show that the rate limiting process is the isomerisation of the HCOH2 adduct to H2COH and allow us to propose k=(2.8±1.4)×10−11(T/300)−1.22±0.07exp(−(12±12)/T)cm3molecule−1s−1 in the range 100–700K for the rate constant of CH+H2O→H+H2CO and H2+H+CO.
Absolute rate constants for the gas-phase reaction of the OH radical with 1,3-butadiene and allene in an argon atmosphere were measured at 1 atm over the temperature range 305-1173 K. It was not possible to determine the rate of the H abstraction reaction from either allene or 1,3-butadiene; an upper limit for these rates is about twice that which one would predict from the OH + ethylene reaction. At temperatures below 600 K, k(OH + 1,3-butadiene) = (1.4 +/- 0.1) x 10â»Â¹Â¹ exp((440 +/- 40)/T) cm³/(molecule.s) and k(OH + allene) = (6.7 +/- 0.9) x 10â»Â¹Â² exp((100 +/- 50)/T) cm³/(molecule.s). Above 600 K the rate constant of the allene reaction decreases weakly while the rate constant for the 1,3-butadiene reaction decreases markedly.
Absolute partial cross sections from threshold to 1000 eV are reported for electron-impact ionization of Hâ, Nâ, and Oâ. Data are presented for the production of Hâ{sup +} and H{sup +} from Hâ; the production of Nâ{sup +}, N{sup +}+Nâ{sup 2+}, and N{sup 2+} from Nâ; and the production of Oâ{sup +}, O{sup +}+Oâ{sup 2+}, and O{sup 2+} from Oâ. The product ions are mass analyzed with a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides clear evidence that all energetic fragment ions are collected. The overall uncertainty in the absolute cross-section values for singly charged parent ions is ±3.5% and is marginally higher for fragment ions. Previous cross-section measurements are compared to the present results. {copyright} {ital 1996 The American Physical Society.}
An approximation is suggested for the calculation of the angular momentum-conserved rotational sum of states for polyatomic molecules under a central potential, based on an interpolation between high-J and low-J forms (J = total angular momentum). A similar procedure is suggested for an approximation to the angular momentum-conserved polyatomic vibrational-rotational sum of states, which obviates a computationally intensive convolution integral. The procedure is checked against rigorous results with satisfactory results. It can be applied to various fragment symmetries in both the phase space limit and at arbitrary interfragment distance, which is of some interest in variational routines.
The photoionization (PI) cross sections of allyl and 2-propenyl radicals to form C3H5+ were measured using tunable vacuum ultraviolet (VUV) synchrotron radiation coupled with photofragment translational spectroscopy. At 10 eV, the cross sections were found to be 6.2±1.2 and 5.1±1.0 Mb, respectively. Using these values, the PI efficiency curves for each radical were placed on an absolute scale from 7.75 to 10.75 eV.
The analysis and measurements reported here emphasize the need to take into account species enrichments due to Mach‐number focusing when sampling gas mixtures using a molecular‐beam mass‐spectrometer sampling system. Depending upon source conditions and system geometry, the enrichment of a given species may vary from unity to a value which is of the order of magnitude of the ratio of the given‐species mass to the major‐species mass. Procedures for handling effects of Mach‐number focusing on species enrichments are presented. These procedures are more general than previous procedures in that they accommodate, as needed, (a) flow divergence upstream from the skimmer and (b) transition to free‐molecule flow (for one or more of the several species) upstream from the skimmer. The measurements support a procedure for handling effects of pressure diffusion near the sampling source which was suggested earlier. An empirical curve for handling modest effects of skimmer interference is included.
Absolute rate constants for the reaction of OH radicals with allene, 1,3‐butadiene, and 3‐methyl‐l‐butene have been determined over the temperature range 299–424 °K, using a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained were k (allene) =5.59×10−12 e(305±300)/RT cm3 molecule−1 sec−1, k (1,3‐butadiene) =1.45×10−11 e(930±300)/RT cm3 molecule−1 sec−1, k (3‐methyl‐1‐butene) =5.23×10−12 e(1060±300)/RT cm3 molecule−1 sec −1, with rate constants at room temperature of (9.30±0.93) ×10−12, (6.85±0.69) ×10−11, and (3.10±0.31) ×10−11 cm3 molecule−1 sec−1 for allene, 1,3‐butadiene, and 3‐methyl‐l‐butene, respectively.
Photofragment translational spectroscopy experiments employing tunable vacuum ultraviolet photoionization yielded absolute photoionization cross sections for vinyl and propargyl radicals at 10 eV of 11.1±2.2 and 8.3±1.6 Mb, respectively. From these values, the photoionization efficiency curves from 7.8–10.8 eV for these radicals were placed on an absolute scale. © 2003 American Institute of Physics.
Ionization potentials determined by examining the long wavelength limit of the ionization continuum for H2S, CS2, C6H6, toluene, p-xylene, C2H4, butadiene, and CH3I agreed with spectroscopic values obtained by Price and co-workers. Values are reported also for NH3, NO, acetone, methyl-ethyl-ketone, CH3OH, and C2H5OH.
Improved energy measurements showed that photoionization cross sections of NO in the region 1070—1340A obtained earlier were about 40 percent too low. At 1215.6A the photoionization and the total cross sections are 2.0×10—18 and 2.4×10—18 cm2, respectively, the difference being ascribed to an absorption band. The photoionization cross section of NH3 was found to rise rather gradually with decreasing wavelength in the region 1220—1060A and to reach a value of 9×10—18 cm2 at 1060A.
We employed the fast-neutral-beam technique in a measurement of absolute partial cross sections for the electron-impact ionization and dissociative ionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than 2.0×10-16 cm2 at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of 1.85×10-16 cm2 at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the D2O molecule and found good agreement between our cross sections and the most recent measurements of Straub et al. [J. Chem. Phys. 108, 109 (1998)] as well as with recent calculations.
The temperature dependence of the rate coefficient for the atmospherically important radical−molecule reaction OH + HBr has been investigated between 76 and 242 K using a pulsed uniform supersonic flow reactor. The current work indicates that the rate coefficient shows significant inverse temperature dependence only below 150 K. These results verify that within the terrestrial atmosphere, the OH + HBr reaction manifests a temperature-independent bimolecular rate coefficient k = (1.2 ± 0.2) × 10-11 cm3 s-1.
A pyrolysis source coupled to a supersonic expansion has been used to produce the CH3 radical from two precursors, iodomethane CH3I and nitromethane CH3NO2. The relative ionization yield of CH3 has been recorded at the SOLEIL Synchrotron Radiation source in the range 9.0−11.6 eV, and its ionization threshold has been modeled by taking into account the vibrational and rotational temperature of the radical in the molecular beam. The relative photoionization yield has been normalized to an absolute cross section scale at a fixed wavelength (118.2 nm, σiCH3 = 6.7−1.8+2.4 Mb, 95% confidence interval) in an independent laboratory experiment using the same pyrolysis source, a vacuum ultraviolet (VUV) laser, and a carefully calibrated detection chain. The resulting absolute cross section curve is in good agreement with the recently published measurements by Taatjes et al.,(1) although with an improved signal-to-noise ratio. The absolute photoionization cross section of CH3I at 118.2 nm has also been measured to be σiCH3I = (48.2 ± 7.9) Mb, in good agreement with previous electron impact measurements. Finally, the photoionization yield of the iodine atom in its ground state 2P3/2 has been recorded using the synchrotron source and calibrated for the first time on an absolute cross section scale from our fixed 118.2 nm laser measurement, σiI2P3/2 = 74−23+33 Mb (95% confidence interval). The ionization curve of atomic iodine is in good agreement, although with slight variations, with the earlier relative ionization yield measured by Berkowitz et al.(2) and is also compared to an earlier calculation of the iodine cross section by Robicheaux and Greene.(3) It is demonstrated that, in the range of pyrolysis temperature used in this work, all the ionization cross sections are temperature-independent. Systematic care has been taken to include all uncertainty sources contributing to the final confidence intervals for the reported results.
The absolute photoionization cross-section of the methyl radical has been measured using two completely independent methods. The CH3 photoionization cross-section was determined relative to that of acetone and methyl vinyl ketone at photon energies of 10.2 and 11.0 eV by using a pulsed laser-photolysis/time-resolved synchrotron photoionization mass spectrometry method. The time-resolved depletion of the acetone or methyl vinyl ketone precursor and the production of methyl radicals following 193 nm photolysis are monitored simultaneously by using time-resolved synchrotron photoionization mass spectrometry. Comparison of the initial methyl signal with the decrease in precursor signal, in combination with previously measured absolute photoionization cross-sections of the precursors, yields the absolute photoionization cross-section of the methyl radical; σCH3(10.2 eV) = (5.7 ± 0.9) × 10−18 cm2 and σCH3(11.0 eV) = (6.0 ± 2.0) × 10−18 cm2. The photoionization cross-section for vinyl radical determined by photolysis of methyl vinyl ketone is in good agreement with previous measurements. The methyl radical photoionization cross-section was also independently measured relative to that of the iodine atom by comparison of ionization signals from CH3 and I fragments following 266 nm photolysis of methyl iodide in a molecular-beam ion-imaging apparatus. These measurements gave a cross-section of (5.4 ± 2.0) × 10−18 cm2 at 10.460 eV, (5.5 ± 2.0) × 10−18 cm2 at 10.466 eV, and (4.9 ± 2.0) × 10−18 cm2 at 10.471 eV. The measurements allow relative photoionization efficiency spectra of methyl radical to be placed on an absolute scale and will facilitate quantitative measurements of methyl concentrations by photoionization mass spectrometry.
The absolute photoionization cross section of C2H5 has been measured at 10.54 eV using vacuum ultraviolet (VUV) laser photoionization. The C2H5 radical was produced in situ using the rapid C2H6 + F → C2H5 + HF reaction. Its absolute photoionization cross section has been determined in two different ways: first using the C2H5 + NO2 → C2H5O + NO reaction in a fast flow reactor, and the known absolute photoionization cross section of NO. In a second experiment, it has been measured relative to the known absolute photoionization cross section of CH3 as a reference by using the CH4 + F → CH3 + HF and C2H6 + F → C2H5 + HF reactions successively. Both methods gave similar results, the second one being more precise and yielding the value: σC2H5ion = (5.6 ± 1.4) Mb at 10.54 eV. This value is used to calibrate on an absolute scale the photoionization curve of C2H5 produced in a pyrolytic source from the C2H5NO2 precursor, and ionized by the VUV beam of the DESIRS beamline at SOLEIL synchrotron facility. In this latter experiment, a recently developed ion imaging technique is used to discriminate the direct photoionization process from dissociative ionization contributions to the C2H5+ signal. The imaging technique applied on the photoelectron signal also allows a slow photoelectron spectrum with a 40 meV resolution to be extracted, indicating that photoionization around the adiabatic ionization threshold involves a complex vibrational overlap between the neutral and cationic ground states, as was previously observed in the literature. Comparison with earlier photoionization studies, in particular with the photoionization yield recorded by Ruscic et al.(1) is also discussed.
Using three formulations of the master equation (ME), we have investigated theoretically the dissociation of methane in the low-pressure limit. The three forms of the ME are as follows: (1) A one-dimensional model in which E, the total energy, is the independent variable (the E model). (2) The two-dimensional strong-collision-in-J model of Smith and Gilbert (Int. J. Chem. Kinet. 1988, 20, 307−329) in which ε, the energy in the active degrees of freedom, and J, the total angular momentum quantum number, are the independent variables (the ε,J model). (3) A two-dimensional variant of the ε,J model in which E and J are the independent variables (the E,J model). The third form of the ME is the most physically realistic, and for this model we investigate the dependence of values of the energy transfer moments (ΔEd, −ΔE, and ΔE21/2) deduced from experiment on assumed forms of the energy transfer function, P(E,E‘), and on temperature. All three moments increase as the temperature rises; −ΔE increases from 20−25 cm-1 at 300 K to 110−120 cm-1 at 4000 K. The variation in the energy transfer moments with the form of P(E,E‘) depends on the particular moment and the temperature, but generally the variation is not greater than 25%. For the same input to the models, the E and E,J models give similar values of the rate coefficient at high temperature, implying that the rotational degrees of freedom behave increasingly as if they are active as temperature is increased. For T > 3000 K, the dissociation perturbs the equilibrium energy distribution of the molecule so much that the detailed-balance condition begins to fail; i.e., k0(T)/kr(T) ≠ Keq(T), where k0(T) and kr(T) are the dissociation and recombination rate coefficients and Keq(T) is the equilibrium constant.
The title reactions were investigated using pulsed laser photolysis combined with pulsed laser induced fluorescence detection of CH3O to determine the rate coefficients for CH3 + NO2 → products (3) and CH3O + NO2 → products (5) as a function of temperature and pressure, and to estimate the yield of CH3 (and thus the yield of CH3C(O)OH) from the reaction of OH with CH3C(O)CH3 (2) at two different temperatures. Reaction 3 has both bimolecular and termolecular components: a simplified falloff parametrization with Fcent = 0.6 gives = (3.2 ± 1.3) × 10-28(T/297)-0.3 cm6 s-1 and = (4.3 ± 0.4) × 10-11(T/297)-1.2 cm3 s-1 with CH3NO2 the likely product. The rate constant for the bimolecular reaction pathway to form CH3O + NO (3a) was found to be 1.9 × 10-11 cm-3 s-1. The low- and high-pressure limiting rate coefficients for reaction between CH3O and NO2 to form CH3ONO2 (5b) were derived as = (5.3 ± 0.3) × 10-29(T/297)-4.4 cm6 s-1 and = (1.9 ± 0.05) × 10-11(T/297)-1.9 cm3 s-1, respectively. Although the final result is associated with some experimental uncertainty, we find that CH3 is formed in the reaction between OH and CH3C(O)CH3 at ≈50% yield at room temperature and 30% at 233 K.
The reaction of CH{sub 2}OH with O{sub 2}, NO, and NO{sub 2} has been studied using pulse radiolysis to generate the radicals and ultraviolet absorption to observe the kinetics. Rate constant values of (0.88 {plus minus} 0.02) {times} 10{sup {minus}11}, (2.5 {plus minus} 0.02) {times} 10{sup {minus}11}, and (2.3 {plus minus} 0.4) {times} 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} have been measured at room temperature and 1 atm pressure for the O{sub 2}, NO, and NO{sub 2} reactions, respectively. Absorptions due to long-lived or stable products were observed in the same spectral region. A simple analysis of these observations suggests that formation of an adduct may dominate in the reaction of CH{sub 2}OH with NO and NO{sub 2} but that this process accounts for only a minor route in the O{sub 2} reaction.
Rate constants for the reactions of 19 gaseous diolefins with OH radicals were determined by relative rate determination methods. Hydroxyl radicals were generated by photolysis of H2O2 at 2537 Å in a mixture of two olefins in the millitorr pressure range at 24 ± 2°C and 1-atm pressure of O2/N2. Analysis of the rate constants indicated that overall rate constants of diolefins were almost the sum of the contributions from each carbon-carbon double-bond group to the the overall reaction with OH radicals and that the contributions were predictable from the rate constants for the reactions of the corresponding monoolefins with OH, based on the procedure derived in this work.
A new photochemical model of Saturn's atmosphere, which includes hydrocarbon (up to C4 compounds) and oxygen compounds, is presented. This model derives from Neptune's model established by Dobrijévic et al. (2000. Planet. Space Sci., submitted). In this one-dimensional model, we consider vertical transport driven by molecular and eddy diffusion. Downward flux of atomic hydrogen from the upper atmosphere and the ionosphere, and external fluxes of oxygenated material are included. Concerning hydrocarbons, calculated abundances agree with observations for methane and acetylene, whereas the ethane and methyl radical disagree. Methyl (CH3) abundance is widely overestimated, suggesting an underevaluation of methyl recombination rates. Moreover, further investigations of the chemical scheme and dynamics are needed. Water and carbon dioxide, whose column abundances in Saturn's stratosphere have been inferred by ISO observations (Feuchtgruber et al., 1997. Nature 389, 159-162), originate in an external flux of oxygenated material. We find an agreement with ISO values (ηH2O = 6.8 × 1014 molecules cm-2 and ηCO2 = 7.8 × 1014 molecules cm-2) for an external flux of material with a cometary-like composition (ΦH2O = 106 molecules cm-2 s-1, ΦCO = 2 × 105 molecules cm-2 s-1 and ΦCO2 = 6 × 104 molecules cm-2 s-1). These external fluxes cannot reproduce the abundance of CO inferred by Noll and Larson (1991. Icarus 89, 168 189), suggesting that CO has an internal origin. More generally, we find that a total influx of oxygen atoms between 106 and 107 atoms cm-2 s-1 is necessary to reproduce oxygen compound abundances. The precise determination of this flux depends on the nature of the entering materials.
The absolute yields of the primary products of the reactions of CH 2 OH with F and Cl atoms and OH radicals were measured in the gas phase at room temperature using a discharge flow apparatus connected to a Far Infrared Laser Magnetic Resonance (FIR‐LMR) spectrometer. Quantitative determinations of the reaction pathways were carried out by employing reference reactions with known product yields. The measurements yielded the branching ratios for the following channels
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Thus, the main reaction channels were found to be addition of the F, Cl, or OH, respectively, to the CH 2 moiety followed by elimination of CO.
Using a relative rate technique, rate constants for the gas phase reactions of the OH radical with n-butane, n-hexane, and a series of alkenes and dialkenes, relative to that for propene, have been determined in one atmosphere of air at 295 ± 1 K. The rate constant ratios obtained were (propene = 1.00): ethene, 0.323 ± 0.014; 1-butene, 1.19 ± 0.06; 1-pentene, 1.19 ± 0.05; 1-hexene, 1.40 ± 0.04; 1-heptene, 1.51 ± 0.06; 3-methyl-1-butene, 1.21 ± 0.04; isobutene, 1.95 ± 0.09; cis-2-butene, 2.13 ± 0.05; trans-2-butene, 2.43 ± 0.05; 2-methyl-2-butene, 3.30 ± 0.13; 2,3-dimethyl-2-butene, 4.17 ± 0.18; propadiene, 0.367 ± 0.036; 1,3-butadiene, 2.53 ± 0.08; 2-methyl-1,3-butadiene, 3.81 ± 0.15; n-butane, 0.101 ± 0.012; and n-hexane, 0.198 ± 0.017. From a least-squares fit of these relative rate data to the most reliable literature absolute flash photolysis rate constants, these relative rate constants can be placed on an absolute basis using a rate constant for the reaction of OH radicals with propene of 2.63 × 10−11 cm3 molecule−1 s−1. The resulting rate constant data, together with previous relative rate data from these and other laboratories, lead to a self-consistent data set for the reactions of OH radicals with a large number of organics at room temperature.
Reactions of the hydroxyl radical with propene and 1-butene are studied experimentally in the gas phase in a continuous supersonic flow reactor over the range 50≤T/K≤224. OH radicals are produced by pulsed laser photolysis of H(2)O(2) at 266 nm in the supersonic flow and followed by laser-induced fluorescence in the (1, 0) A(2)Σ(+)←X(2)Π(3/2) band at about 282 nm. These reactions are found to exhibit negative temperature dependences over the entire temperature range investigated, varying between (3.1-19.2) and (4.2-28.6)×10(-11) cm(3) molecule(-1) s(-1) for the reactions of OH with propene and 1-butene, respectively. Quantum chemical calculations of the potential energy surfaces are used as the basis for energy- and rotationally resolved Rice-Ramsperger-Kassel-Marcus calculations to determine the rate constants over a range of temperatures and pressures. The negative temperature dependences of the rate constants are explained by competition between complex redissociation and passage to the adducts by using a model with two transition states. The results are compared and contrasted with earlier studies and discussed in terms of their potential relevance to the atmosphere of Saturn.
To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molec-ular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecu-lar abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmo-sphere. From the ISO observations, we determine that the column densities of CH 3 , CH 3 C 2 H, and C 4 H 2 above 10 mbar are 4 +2 −1.5 × 10 13 cm −2 , (1.1 ± 0.3) × 10 15 cm −2 , and (1.2 ± 0.3) × 10 14 cm −2 , re-spectively. The observed ISO emission features also indicate C 2 H 2 mixing ratios of 1.2 +0.9 −0.6 × 10 −6 at 0.3 mbar and (2.7 ± 0.8) × 10 −7 at 1.4 mbar, and a C 2 H 6 mixing ratio of (9 ± 2.5) × 10 −6 at 0.5 mbar. Upper limits are provided for C 2 H 4 , CH 2 CCH 2 , C 3 H 8 , and C 6 H 2 . The sensitivity of the model results to variations in the eddy diffu-sion coefficient profile, the solar flux, the CH 4 photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are dis-cussed in detail. We find that C 4 H 2 and CH 3 C 2 H are particularly good tracers of important chemical processes and physical condi-tions in Saturn's upper atmosphere, and C 2 H 6 is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than ∼3 × 10 4 cm 2 s −1 at pres-sures greater than 1 mbar in order to reproduce the C 2 H 6 abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH 3 radicals if the low-temperature chemistry of CH 3 were better under-stood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere—diacetylene, butane, and water condense between ∼1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational se-quences and for analyzing data from the upcoming Cassini mission.
The rate of the reaction OH+CH2CO→products has been investigated over the temperature range 193–423 K, and over the pressure range 1.76–3.35 Torr at 298 K using a discharge-flow resonance-fluorescence system. The rate exhibited a negative temperature dependence corresponding to an “Arrhenius” activation energy of −4.2 kJ mol−1. The rate constant at 298 K was (3.3±0.6)×10−11 cm3 molecule−1 s−1. No evidence for a pressure dependence was found. A mechanism consistent with these observations is presented. Comparisons are made between the rate of reaction of ketene with OH and with other radical species. A possible implication for interstellar chemistry is discussed.
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Ensembles of classical trajectories are used to study collisional energy transfer in highly vibrationally excited CH(4) for eight bath gases. Several simplifying assumptions for the CH(4) + M interaction potential energy surface are tested against full dimensional direct dynamics trajectory calculations for M = He, Ne, and H(2). The calculated energy transfer averages are confirmed to be sensitive to the shape of the repulsive wall of the intermolecular potential, with an exponential repulsive wall required for quantitative predictions. For the diatomic baths, the usual "separable pairwise" approximation for the interaction potential is unable to describe the orientation dependence of the interaction potential accurately, and the ambiguity in the resulting parametrizations contributes an additional uncertainty to the predicted energy transfer averages of 20-40%. On the other hand, the energy transfer averages are shown to be insensitive to the level of theory used to describe the intramolecular CH(4) potential, with a computationally efficient semiempirical tight binding potential for hydrocarbons performing equally well as an MP2 potential. The relative collisional energy transfer efficiencies of the eight bath gases are discussed and shown to be a function of temperature. The ensemble-averaged energy transferred in deactivating collisions <ΔE(d)> for each bath is used to parametrize a single-exponential-down model for collisional energy transfer in master equation calculations. The predicted decomposition rate coefficients for CH(4) agree well with available experimental rate coefficients for M = He, Ar, Kr, and CH(4). The effect of vibrational anharmonicity on the predicted rate coefficients is considered briefly.
I use coupled-cluster theory and a modest basis set, aug-cc-pVDZ, to calculate structures and harmonic vibrational frequencies of local minima and transition states on the C(3)H(5)O potential energy surface. Accurate energies are computed using explicitly correlated coupled-cluster methods and a large basis set, cc-pVQZ-F12, to approach the one-particle basis set limit. My computations characterize eight additional stable radical structures on the global potential energy surface for this system. Additionally, this study encompasses many more isomerization and dissociation pathways, both between previously known intermediates and ones first characterized here. Analysis of the transition states and statistical transition-state theory results shows that energetically small barriers connect many of the alkenol and epoxide intermediates to the straight-chain alkoxy isomers, leading to significant branching to these alkoxy radical intermediates. Facile isomerization to these alkoxy intermediates is significant because the barrier heights leading to H + acrolein and HCO + C(2)H(4) product channels are energetically accessible even at low vibrational energies. The low dissociation barrier heights and loose transition states of these pathways result in unimolecular dissociation as opposed to isomerization to a different C(3)H(5)O intermediate.
Products of the reaction of OH radicals with propene, trans-2-butene, and 1-butene have been investigated in a fast flow reactor, coupled with time-of-flight mass spectrometry, at pressures between 0.8 and 3.0 Torr. The product determination includes H atom abstraction channels as well as site-specific OH addition. The OH radicals are produced by the H + NO(2) → OH + NO reaction or by the F + H(2)O → OH + HF reaction, the H or F atoms being produced in a microwave discharge. The gas mixture is ionized using single photon ionization (SPI at 10.54 eV), and products are detected using time-of-flight mass spectrometry (TOF-MS). The H atom abstraction channels are measured to be <2% for OH + propene, 8 ± 3% for OH + 1-butene, and 3 ± 1% for OH + trans-2-butene. Analysis of ion fragmentation patterns leads to 72 ± 16% OH addition to the propene terminal C atom and 71 ± 16% OH addition to the 1-butene terminal C atom. The errors bars represent 95% confidence intervals and include estimated uncertainties in photoionization cross sections.
Photodissociation of acetaldehyde (CH3CHO) at 266 nm produced CH3 and HCO radicals, and single-photon vacuum ultraviolet ionization was used to record velocity map ion images of both CH3+ and HCO+. Comparison of the translational energy distributions from both species indicates that secondary fragmentation of HCO is negligible for 266 nm photodissociation. Thus, the relative photoion signals for CH3+ and HCO+ in the mass spectrometer, combined with the recently measured absolute photoionization cross section of CH3, allowed the determination of the absolute photoionization cross section of HCO (σ(HCO) = 4.8 ± 1.5(2.0), 5.9 ± 1.6(2.2), and 3.7 ± 1.2(1.6) Mb at 10.257, 10.304, and 10.379 eV, respectively). The observed values are quite small but consistent with the similarly small value at threshold for the isoelectronic species NO. This behavior is discussed in terms of the character of the HOMO in both molecules.
The absolute photoionization cross section of the methyl radical was determined relative to that of NO at photon energies of 10.54 eV using the CH(3) + NO(2) --> CH(3)O + NO reaction. Kinetics of this reaction was studied in a fast flow reactor coupled with VUV laser photoionization. Simulation of the kinetics of the decrease of the methyl signal and the corresponding increase of the NO signal (in combination with the NO absolute photoionization cross section determined by Watanabe (Watanabe, K. J. Chem. Phys. 1954, 22, 1564; Watanabe, K.; Matsunaga, F. M.; Sakai, H. Appl. Opt. 1967, 6, 391)), yields the absolute photoionization cross section of the methyl radical: sigma(CH(3))(10.54 eV) = 5.1 (1.2) x 10(-18) cm(2) (95% confidence interval). This result is in good agreement with the recently published measurements by Taatjes et al. (Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Meloni, G.; Fan, H.; Pratt, S. T. J. Phys. Chem. A 2008, 112, 9336) and by Gans et al. (Gans, B.; Mendes, L. A. V.; Boyé-Péronne, S.; Douin, S.; Garcia, G.; Soldi-Lose, H.; Cunha de Miranda, B. K.; Alcaraz, C.; Carrasco, N.; Pernot, P.; Gauyacq, D. J. Phys. Chem. A 2009, 114, 3237).
Photoionization yield and absorption coefficient of nitric oxide were measured at many wavelengths in the region 580-1350 A. H emission and helium continuum sources were used with a 1-m monochromator. Absolute intensity measurements were based on a calibrated thermocouple.
The kinetics of the reactions of atomic chlorine with ethane and propane have been studied in a continuous supersonic flow reactor over the range 48 K < or = T < or = 167 K. Chlorine atoms were produced by microwave discharge upstream of the Laval nozzle and were probed in the vacuum ultraviolet wavelength range around 138 nm by resonance fluorescence. The reaction of Cl with ethane has been found to exhibit a positive temperature dependence, with a rate coefficient decreasing from (4.3 +/- 0.6) x 10(-11) cm(3) molecule(-1) s(-1) at 167 K to (2.9 +/- 0.3) x 10(-11) cm(3) molecule(-1) s(-1) at 48 K and deviates from true Arrhenius behavior below 120 K. In contrast, the rate coefficient for the reaction of Cl with propane has been found to have a constant value of (1.4 +/- 0.2) x 10(-10) cm(3) molecule(-1) s(-1) over the same temperature range. The expressed uncertainties are the combined statistical (a single standard deviation from the mean) and systematic (estimated at 10%) uncertainties.