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The gas-phase chemistry of carbon chains in dark cloud chemical models
Abstract and Figures
We review the reactions between carbon chain molecules and radicals, namely Cn, CnH, CnH2, C2n+1O, CnN, HC2n+1N, with C, N and O atoms. Rate constants and branching ratios for these processes have been re-evaluated using experimental
and theoretical literature data. In total 8 new species have been introduced, 41 new reactions have been proposed and 122
rate coefficients from kida.uva.2011 have been modified. We test the effect of the new rate constants and branching ratios
on the predictions of gas–grain chemical models for dark cloud conditions using two different C/O elemental ratios. We show
that the new rate constants produce large differences in the predicted abundances of carbon chains since the formation of
long chains is less effective. The general agreement between the model predictions and observed abundances in the dark cloud
TMC-1 (CP) is improved by the new network and we find that C/O ratios of 0.7 and 0.95 both produce a similar agreement for
different times. The general agreement for L134N (N) is not significantly changed. The current work specifically highlights
the importance of O + CnH and N + CnH reactions. As there are very few experimental or theoretical data for the rate constants of these reactions, we highlight
the need for experimental studies of the O + CnH and N + CnH reactions, particularly at low temperature.
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... Destruction routes are dominated by reactions with ions such as C + , H + , + H 3 , and HCO + . Loison et al. (2014) published a critical and general review of the reactions involving carbon chains, including cyanopolyynes up to HC 9 N. They only list reactions (1), where they roughly evaluate the rate and branching ratios based on the capture theory and exothermicity of the products, respectively. ...
... Since the rate constants of reactions (3) are considered the same for HC 7 N and HC 9 N, and the destruction also occurs at the same rate, the ratios HC 5 N:HC 7 N:HC 9 N would reflect the parent species ratios, namely HC 6 :HC 8 :HC 10 . In other words, the HC n N/HC n+2 N ratios are inherited from the HC n /HC n+2 one, as no reaction directly links HC n N to HC n+2 N in the Loison et al. (2014) scheme. Anyway, Loison et al. (2014) modeled the TMC-1 case and predicted the HC 5 N:HC 7 N: HC 9 N ratios to be 1:0.14-0.3:0.10-0.13 at around 1 × 10 5 yr, where the model predictions agree better with observations (of several carbon chains). ...
... In other words, the HC n N/HC n+2 N ratios are inherited from the HC n /HC n+2 one, as no reaction directly links HC n N to HC n+2 N in the Loison et al. (2014) scheme. Anyway, Loison et al. (2014) modeled the TMC-1 case and predicted the HC 5 N:HC 7 N: HC 9 N ratios to be 1:0.14-0.3:0.10-0.13 at around 1 × 10 5 yr, where the model predictions agree better with observations (of several carbon chains). The two low and high values of each ratio are obtained assuming an elemental C/O abundance equal to 0.7 and 0.95, respectively. ...
We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100 m Robert C. Byrd Green Bank Telescope, we observe three emission lines of HC 3 N, nine lines of HC 5 N, five lines of HC 7 N, and nine lines of HC 9 N. HC 9 N is detected for the first time toward the source. The high spectral resolution (∼0.05 km s ⁻¹ ) reveals double-peak spectral line profiles with the redshifted peak a factor 3–5 brighter. Resolved maps of the core in other molecular tracers indicate that the southern region is redshifted. Therefore, the bulk of the cyanopolyyne emission is likely associated with the southern region of the core, where free carbon atoms are available to form long chains, thanks to the more efficient illumination of the interstellar field radiation. We perform a simultaneous modeling of the HC 5 N, HC 7 N, and HC 9 N lines to investigate the origin of the emission. To enable this analysis, we performed new calculation of the collisional coefficients. The simultaneous fitting indicates a gas kinetic temperature of 5–12 K, a source size of 80″, and a gas density larger than 100 cm ⁻³ . The HC 5 N:HC 7 N:HC 9 N abundance ratios measured in L1544 are about 1:6:4. We compare our observations with those toward the well-studied starless core TMC-1 and with the available measurements in different star-forming regions. The comparison suggests that a complex carbon chain chemistry is active in other sources and is related to the presence of free gaseous carbon. Finally, we discuss the possible formation and destruction routes in light of the new observations.
... Destruction routes are dominated by reactions with ions such as e.g., C + , H + , H + 3 and HCO + . Loison et al. (2014) published a critical and general review of the reactions involving carbon chains, including cyanopolyynes up to HC 9 N. They only list reactions (1), where they roughly evaluate the rate and branching ratios based on the capture theory and exothermic- HC5N, HC7N and HC9N. ...
... Since the rate constants of reactions (3) are considered the same for HC 7 N and HC 9 N, and the destruction also occurs at the same rate, the ratios HC 5 N:HC 7 N:HC 9 N would reflect the parent species ratios, namely HC 6 :HC 8 :HC 10 . In other words, the HC n N/HC n+2 N ratios is inherited from the HC n /HC n+2 one, as no reaction directly link HC n N to HC n+2 N in the Loison et al. (2014) scheme. Anyway, Loison et al. (2014) modeled the TMC-1 case and predicted the HC 5 N:HC 7 N:HC 9 N ratios to be 1:0.14-0.3:0.10-0.13 at around 1 × 10 5 yr, where the model predictions agree better with observations (of several carbon chains). ...
... In other words, the HC n N/HC n+2 N ratios is inherited from the HC n /HC n+2 one, as no reaction directly link HC n N to HC n+2 N in the Loison et al. (2014) scheme. Anyway, Loison et al. (2014) modeled the TMC-1 case and predicted the HC 5 N:HC 7 N:HC 9 N ratios to be 1:0.14-0.3:0.10-0.13 at around 1 × 10 5 yr, where the model predictions agree better with observations (of several carbon chains). The two low and high value of each ratio are obtained assuming an elemental C/O abundance equal to 0.7 and 0.95, respectively. ...
We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100m Robert C. Byrd Green Bank Telescope (GBT) we observe 3 emission lines of HC$_3$N, 9 lines of HC$_5$N, 5 lines of HC$_7$N, and 9 lines of HC$_9$N. HC$_9$N is detected for the first time towards the source. The high spectral resolution ($\sim$ 0.05 km s$^{-1}$) reveals double-peak spectral line profiles with the redshifted peak a factor 3-5 brighter. Resolved maps of the core in other molecular tracers indicates that the southern region is redshifted. Therefore, the bulk of the cyanopolyyne emission is likely associated with the southern region of the core, where free carbon atoms are available to form long chains, thanks to the more efficient illumination of the interstellar field radiation. We perform a simultaneous modelling of the HC$_5$N, HC$_7$N, and HC$_9$N lines, to investigate the origin of the emission. To enable this analysis, we performed new calculation of the collisional coefficients. The simultaneous fitting indicates a gas kinetic temperature of 5--12 K, a source size of 80$\arcsec$, and a gas density larger than 100 cm$^{-3}$. The HC$_5$N:HC$_7$N:HC$_9$N abundance ratios measured in L1544 are about 1:6:4. We compare our observations with those towards the the well-studied starless core TMC-1 and with the available measurements in different star-forming regions. The comparison suggests that a complex carbon chain chemistry is active in other sources and it is related to the presence of free gaseous carbon. Finally, we discuss the possible formation and destruction routes in the light of the new observations.
... Unfortunately, a vast majority (80%) of the rates and branching ratios of the reported gas-phase reactions have not been measured or computed and are often based on approximate estimates (e.g., using the capture theory; Su & Chesnavich 1982;Herbst 2006;Woon & Herbst 2009;Loison et al. 2013), educated guesses on similarity principles, or simple chemical intuition. In addition, even when some experimental measurements are available, the estimated rate constants may have substantial uncertainties, as they are often based on experiments at room temperature (e.g., Anicich 2003). ...
The gas-phase reaction networks are the backbone of astrochemical models. However, due to their complexity and nonlinear impact on the astrochemical modeling, they can be the first source of error in the simulations if incorrect reactions are present. Over time, following the increasing number of species detected, astrochemists have added new reactions, based on laboratory experiments and quantum mechanics (QM) computations, as well as reactions inferred by chemical intuition and the similarity principle. However, sometimes no verification of their feasibility in the interstellar conditions, namely their exothermicity, was performed. In this work, we present a new gas-phase reaction network, GRETOBAPE , based on the KIDA2014 network and updated with several reactions, cleaned from endothermic reactions not explicitly recognized as such. To this end, we characterized all the species in the GRETOBAPE network with accurate QM calculations. We found that ∼5% of the reactions in the original network are endothermic, although most of them are reported as barrierless. The reaction network of Si-bearing species is the most impacted by the endothermicity cleaning process. We also produced a cleaned reduced network, GRETOBAPE-red , to be used to simulate astrochemical situations where only C-, O-, N-, and S-bearing species with less than six atoms are needed. Finally, the new GRETOBAPE network, its reduced version, and the database with all the molecular properties are made publicly available. The species property database can be used in the future to test the feasibility of possibly new reactions.
... Unfortunately, the vast majority (≥ 80%) of the rates and branching ratios of the reported gas-phase reactions has not been measured or computed and is often based on approximate estimates (e.g. using the Capture Theory: Su & Chesnavich 1982;Herbst 2006;Woon & Herbst 2009;Loison et al. 2013), educated guesses on similarity principles or simple chemical intuition. In addition, even when some experimental measurements are available, the estimated rate constants may have substantial uncertainties as they are often based on experiments at room temperature (e.g. ...
The gas-phase reaction networks are the backbone of astrochemical models. However, due to their complexity and non-linear impact on the astrochemical modeling, they can be the first source of error in the simulations if incorrect reactions are present. Over time, following the increasing number of species detected, astrochemists have added new reactions, based on laboratory experiments and quantum mechanics (QM) computations as well as reactions inferred by chemical intuition and similarity principle. However, sometimes no verification of their feasibility in the interstellar conditions, namely their exothermicity, was performed. In this work, we present a new gas-phase reaction network, GRETOBAPE, based on the KIDA2014 network and updated with several reactions, cleaned from endothermic reactions not explicitly recognized as such. To this end, we characterized all the species in the GRETOBAPE network with accurate QM calculations. We found that 5% of the reactions in the original network are endothermic although most of them are reported as barrierless. The reaction network of Si-bearing species is the most impacted by the endothermicity cleaning process. We also produced a cleaned reduced network, GRETOBAPE-red, to be used to simulate astrochemical situations where only C-, O-, N- and S- bearing species with less than 6 atoms are needed. Finally, the new GRETOBAPE network, its reduced version, as well as the database with all the molecular properties are made publicly available. The species properties database can be used in the future to test the feasibility of possibly new reactions.
... The rate coefficients of these reactions were estimated byLoison et al. (2014) through extrapolation of the calculations byChastaing et al. (2001) andChastaing et al. (2000) on C + alkenes, alkynes, dienes, and diynes. The other major formation routes involve atomic reactions with related anions: ...
Using data from the Green Bank Telescope (GBT) Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey, we report the first astronomical detection of the C 10 H ⁻ anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C 10 H ⁻ . From the velocity stacked data and the matched filter response, C 10 H ⁻ is detected at >9 σ confidence level at a column density of 4.04 − 2.23 + 10.67 × 10 11 cm ⁻² . A dedicated search for the C 10 H radical was also conducted toward TMC-1. In this case, the stacked molecular emission of C 10 H was detected at a ∼3.2 σ confidence interval at a column density of 2.02 − 0.82 + 2.68 × 10 11 cm ⁻² . However, as the determined confidence level is currently <5 σ , we consider the identification of C 10 H as tentative. The full GOTHAM data set was also used to better characterize the physical parameters including column density, excitation temperature, line width, and source size for the C 4 H, C 6 H, and C 8 H radicals and their respective anions, and the measured column densities were compared to the predictions from a gas/grain chemical formation model and from a machine learning analysis. Given the measured values, the C 10 H ⁻ /C 10 H column density ratio is ∼ 2.0 − 1.6 + 5.9 —the highest value measured between an anion and neutral species to date. Such a high ratio is at odds with current theories for interstellar anion chemistry. For the radical species, both models can reproduce the measured abundances found from the survey; however, the machine learning analysis matches the detected anion abundances much better than the gas/grain chemical model, suggesting that the current understanding of the formation chemistry of molecular anions is still highly uncertain.
The presence of carbon-chain molecules in the interstellar medium (ISM) has been known since the early 1970s and >130\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$>130$\end{document} such species have been identified to date, making up ∼43\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim 43$\end{document}% of the total of detected ISM molecules. They are prevalent not only in star-forming regions in our Galaxy but also in other galaxies. These molecules provide important information on physical conditions, gas dynamics, and evolutionary stages of star-forming regions. Larger species of polycyclic aromatic hydrocarbons (PAHs) and fullerenes (C60 and C70), which may be related to the formation of the carbon-chain molecules, have been detected in circumstellar envelopes around carbon-rich Asymptotic Giant Branch (AGB) stars and planetary nebulae, while PAHs are also known to be a widespread component of the ISM in most galaxies. Recently, two line survey projects toward Taurus Molecular Cloud-1 with large single-dish telescopes have detected many new carbon-chain species, including molecules containing benzene rings. These new findings raise fresh questions about carbon-bearing species in the Universe. This article reviews various aspects of carbon-chain molecules, including observational studies, chemical simulations, quantum calculations, and laboratory experiments, and discusses open questions and how future facilities may answer them.
Context . Electron–molecule interaction is a fundamental process in radiation-driven chemistry in space, from the interstellar medium to comets. Therefore, knowledge of interaction cross sections is key. There have been a plethora of both theoretical and experimental studies of total ionization cross sections spanning from diatomics to complex organics. However, the data are often spread over many sources or are not public or readily available.
Aims . We introduce the Astrochemistry Low-energy Electron cross-section (ALeCS) database. This is a public database for electron interaction cross sections and ionization rates for molecules of astrochemical interest. In particular, we present here the first data release, comprising total ionization cross sections and ionization rates for over 200 neutral molecules.
Methods . We include optimized geometries and molecular orbital energies at various levels of quantum chemistry theory. Furthermore, for a subset of the molecules, we have calculated ionization potentials. We computed the total ionization cross sections using the binary-encounter Bethe model and screening-corrected additivity rule, and we computed ionization rates and reaction network coefficients for molecular cloud environments.
Results . We present the cross sections and reaction rates for >200 neutral molecules ranging from diatomics to complex organics, with the largest being C 14 H 10 . We find that the screening-corrected additivity rule cross sections generally significantly overestimate experimental total ionization cross sections. We demonstrate that our binary-encounter Bethe cross sections agree well with experimental data. We show that the ionization rates scale roughly linearly with the number of constituent atoms in the molecule.
Conclusions . We introduce and describe the public ALeCS database. For the initial release, we include total ionization cross sections for >200 neutral molecules and several cations and anions calculated with different levels of quantum chemistry theory, the chemical reaction rates for the ionization, and network files in the formats of the two most popular astrochemical networks: the Kinetic Database for Astrochemistry, and UMIST. The database will be continuously updated for more molecules and interactions.
Recent detections of aromatic species in dark molecular clouds suggest that formation pathways may be efficient at very low temperatures and pressures, yet current astrochemical models are unable to account for their derived abundances, which can often deviate from model predictions by several orders of magnitude. The propargyl radical, a highly abundant species in the dark molecular cloud TMC-1, is an important aromatic precursor in combustion flames and possibly interstellar environments. We performed astrochemical modeling of TMC-1 using the three-phase gas-grain code NAUTILUS and an updated chemical network, focused on refining the chemistry of the propargyl radical and related species. The abundance of the propargyl radical has been increased by half an order of magnitude compared to the previous GOTHAM network. This brings it closer in line with observations, but it remains underestimated by 2 orders of magnitude compared to its observed value. Predicted abundances for the chemically related C 4 H 3 N isomers within an order of magnitude of observed values corroborate the high efficiency of CN addition to closed-shell hydrocarbons under dark molecular cloud conditions. The results of our modeling provide insight into the chemical processes of the propargyl radical in dark molecular clouds and highlight the importance of resonance-stabilized radicals in polycyclic aromatic hydrocarbon formation.
Context. Protonated hydrogen cyanide, HCNH ⁺ , plays a fundamental role in astrochemistry because it is an intermediary in gas-phase ion-neutral reactions within cold molecular clouds. However, the impact of the environment on the chemistry of HCNH ⁺ remains poorly understood.
Aims. We aim to study HCNH ⁺ , HCN, and HNC, as well as two other chemically related ions, HCO ⁺ and N 2 H ⁺ , in different star formation regions in order to investigate how the environment influences the chemistry of HCNH ⁺ .
Methods. With the IRAM 30 m and APEX 12 m telescopes, we carried out HCNH ⁺ , H ¹³ CN, HN ¹³ C, H ¹³ CO ⁺ , and N 2 H ⁺ imaging observations toward two dark clouds, the Serpens filament and Serpens South, both of which harbor sites of star formation that include protostellar objects and regions that are quiescent.
Results. We report the first robust distribution of HCNH ⁺ in the Serpens filament and in Serpens South. Our data suggest that HCNH ⁺ is abundant in cold and quiescent regions but is deficient in active star-forming regions. The observed HCNH ⁺ fractional abundances relative to H 2 range from 3.1 × 10 ⁻¹¹ in protostellar cores to 5.9 × 10 ⁻¹⁰ in prestellar cores, and the HCNH ⁺ abundance generally decreases with increasing H 2 column density, which suggests that HCNH ⁺ coevolves with cloud cores. Our observations and modeling results suggest that the abundance of HCNH ⁺ in cold molecular clouds is strongly dependent on the H 2 number density. The decrease in the abundance of HCNH ⁺ is caused by the fact that its main precursors (e.g., HCN and HNC) undergo freeze-out as the number density of H 2 increases. However, current chemical models cannot explain other observed trends, such as the fact that the abundance of HCNH ⁺ shows an anticorrelation with that of HCN and HNC but a positive correlation with that of N 2 H ⁺ in the southern part of Serpens South’s northern clump. This indicates that additional chemical pathways have to be invoked for the formation of HCNH ⁺ via molecules such as N 2 in regions in which HCN and HNC freeze out.
Conclusions. Both the fact that HCNH ⁺ is most abundant in molecular cores prior to gravitational collapse and the fact that low-J HCNH ⁺ transitions have very low H 2 critical densities make this molecular ion an excellent probe of pristine molecular gas.
Accurate quantum chemical studies at the CCSD(T)/CBS//CCSD/cc-pVTZ level predicted the depletion reaction of C3O by both singlet and triplet O-atoms to be barrierless, leading to the astrophysically very abundant CO plus triplet CCO. The barrierless nature of the reaction fully complied with the long conjecture, whereas the product differed significantly. Our kinetic calculations indicated that the reaction possesses significant negative temperature effect below 30 K. The calculations should be useful for understanding the astrophysical recycling for both the carbon and oxygen species.
The temperature dependences of the reactions of the methylidine radical (CH) with several hydrocarbons were studied in the temperature range 23-295 K using the Rennes CRESU apparatus coupled with the pulsed laser photolysis (PLP), laser induced fluorescence (LIF) technique. All the reactions remain very fast at low temperature. For the reactions of CH with olefins, a maximum rate is obtained at about 70 K and then the rate coefficients slightly decrease at lower temperatures. A similar effect is observed in the data for CH+CH4 and CH+C2H6 with a maximum rate being obtained at about 40 K. The results are compared with previous studies where available, and the nature of the possible products is also discussed, although this study does not enable their detection. Consequences of these new results for the chemistry of planetary atmospheres and interstellar clouds are stressed.
Rate coefficients for reactions of the C-2 radical in its a(3)Pi(u) electronic state with H, N and O atoms and with C3O2, C4F6, H-2, NO, N2O, C2H2, C2H4, C3H4 and C6H6 were determined. This work represents the first study of reactions of C-2(a(3)Pi(u)) radicals with the atoms investigated at room temperature and 4 Torr: total pressure. The bimolecular rate constants obtained for the atom reactions were k(C2+O) = (9.8 +/- 1.0) x 10(-11), k(C2+N) = (2.8 +/- 1.0) x 10(-11) and k(C2+N) < 6 x 10(-14), in units of cm(3) s(-1). In addition, the reaction C-2 + O was found to be independent of total pressure in the range 2-60 Torr. For the reaction C-2(a(3)Pi(u)) + ethene (C2H4) a temperature and pressure independent rate constant of (9.5 +/- 1.2) x 10(-11) cm(3) s(-1) was obtained in the temperature range 298-1000 K at 100 Torr total pressure and in the pressure range 5-100 Torr at 298 K. The following rate constants were determined at room temperature and a total pressure of 4 Torr for the reactions of C-2(a(3)Pi(u)) radicals with benzene (C6H6), acetylene (C2H2) and allene (C3H4): k(C2+C6H6) = (4.9 +/- 0.1) x 10(-10), k(C2+C2H2) = (1.0 +/- 0.1) x 10(-10) and k(C2+C3H4) = (1.9 +/- 0.3) x 10(-10), in units of cm(3) s(-1). The reaction C-2 + NO was investigated at room temperature and 100 Torr total pressure, a rate constant k(C2+NO) = (6.8 +/- 0.3) x 10(-11) cm(3) s(-1) was obtained. The reaction C-2 + N2O was studied at 4 Torr total pressure in the temperature range 300-700 K for which a temperature independent rate constant k(C2+N2O) = (3.1 +/- 0.4) x 10(-14) cm(3) s(-1) was determined.
Die Energieverteilung in den Produkten der Reaktionen (1) CN( v ) + O( ³ P) und (2) CN( v ) + O 2 wurde an Hand der Besetzung der Schwingungszustände des Produkts CO( v′ ) untersucht. CN‐Radikale wurden dabei durch Blitzphotolyse aus Dicyan erzeugt und absolute CO( v′ )‐Konzentrationen durch zeitaufgelöste Resonanz‐Absorptionsspektroskopie im Infraroten mit Hilfe eines kontinuierlichen CO‐Lasers bestimmt. – Für den Elementarschritt (1) zeigt die gemessene Verteilung CO( v′ = 0,1, …, 13), daß die Reaktion gleichzeitig über die Wege
mit der Geschwindigkeitskonstanten k 1 = (1,0 ± 0,4) · 10 ¹³ cm ³ /mol · s bei 298 K abläuft. Eine primäre Schwingungsanregung des Reaktanden CN( v = 0, 1, …, 6) wird auf dem Reaktionsweg (1a) mit einem Anteil f′ v = 0,5 auf die Schwingungsanregung des Produkts CO( v′ ) übertragen. – In der Reaktion
beobachtet man im Reaktionsweg (2a) eine CO( v′ )‐Bildung in den Zuständen v′ = 0,1, …, 4.
The 300 K reactions of O 2 with C 2(X
1Σ +g), C 2(a
3 Π u), C 3(X¯
1Σ +g) and CN(X
2Σ +), which are generated via IR multiple
photon dissociation (MPD), are reported. From the spectrally resolved
chemiluminescence produced via the IR MPD of C 2H
3CN in the presence of O 2, CO molecules in the a
3Σ +, d 3Δ i,
and e 3Σ - states were identified, as well
as CH(A 2Δ) and CN(B 2Σ +)
radicals. Observation of time resolved chemiluminescence reveals that
the electronically excited CO molecules are formed via the single-step
reactions C 2(X 1Σ +g,
a 3Π u) + O 2 → CO(X
1Σ + + CO(T), where T denotes are
electronically excited triplet state of CO. The rate coefficients for
the removal of C 2(X 1Σ
+g) and C 2(a 3Π
u) by O 2 were determined both from laser induced
fluorescence of C 2(X 1Σ
+g) and C 2(a 3Π
u), and from the time resolved chemiluminescence from excited
CO molecules, and are both (3.0 ± 0.2)10 -12 cm
3 molec -1 s -1. The rate coefficient
of the reaction of C 3 with O 2, which was
determined using the IR MPD of allene as the source of C 3
molecules, is <2 × 10 -14 cm 3 molec
-1 s -1. In addition, we find that rate
coefficients for C 3 reactions with N 2, NO, CH
4, and C 3H 6 are all < × 10
-14 cm 3 molec -1 s -1.
Excited CH molecules are produced in a reaction which proceeds with a
rate coefficient of (2.6 ± 0.2)10 -11 cm 3
molec -1 s -1. Possible reactions which may be the
source of these radicals are discussed. The reaction of CN with O
2 produces NCO in vibrationally excited states. Radiative
lifetime of the Ā 2Σ state of NCo and the Ā
1Π u(000) state of C 3 are reported.
An accurate knowledge of the long-range interaction potential for the
ground and first few excited electronic states is needed for
quantitative prediction of the rate coefficients for astrochemical
reactions at low temperatures. Some reactions important for
astrochemical modeling include an open-shell atom as one of the
fragments. Due to the interplay between the spin-orbit and quadrupole
interactions such reactions require a special treatment. In this paper
we derive the general expressions for the energy levels for such
systems, apply them to the
C2H(2Σ+)+O(3P)
reaction, and compare the results with ab initio calculations.
CH(X 2R) was produced by multiple infrared photon dissociation of CH3OH in the presence of excess atomic oxygen or nitrogen. Time‐resolved measurements of relative CH concentrations were made at 298 K with a tunable dye laser. Rate constants deduced from the dependence of CH decay rate on atom concentration are (9.5±1.4)×10−11 cm3 s−1 for CH+O and (2.1±.5)×10−11 cm3 s−1 for CH+N.
Experimental and theoretical developments in the field of dissociative recombination research over the last 15 years are reviewed. Rate coefficients and branching ratios for final channels of the reaction are tabulated.
The doublet potential-energy surface for the reaction of C2H with O, including three minimum isomers and three transition states, is explored theoretically using the coupled cluster and density functional theory. The initial association between C2H and O is confirmed to be a barrierless process forming a low-lying adduct named as 1 (HCCO), followed by C–C bond rupture leading to product P1(CH + CO), which might be the most abundant considering form both energetic and entropic factors. Less competitively, 1 can lead to P2(CCO + H) directly via C–H bond cleavage or undergo H-shift and ring-closure to 2(c-COC–H), and then take H-shift and ring-opening to 3(HOCC) followed by dissociation to P2(CCO + H). Because the intermediates, transition states and products involved in the feasible pathways all lie below the reactants, the C2H + O reaction is expected to be rapid, as is confirmed by experiment. The present results can lead us to deeply understand the mechanism of the title reaction and may be helpful for the modeling of ethynyl–oxygen combustion chemistry.
We present a kinetic study of the collisional behaviour of ground state atomic carbon, C(2p2(3PJ) with the molecules C3O2, C2H4 and C2H2. Atomic carbon was generated by the repetitive pulsed irradiation (λ>ca. 160 nm of C3O2 in a slow flow system and then monitored photoelectrically by time-resolved atomic resonance absorption in vacuum ultraviolet (λ= 166 nm, 33PJ←23PJ) with direct computer interfacing for data capture and analysis. The following absolute second-order rate constants for the reactions of C(23PJ) with the molecules C3O2, C2H4 and C2H2 were obtained: kR/cm3 molecule−1 s−1(300 K); C3O2 (1.8 ±0.1) x 10−10, C2H4(2.0±0.1) x 10−10 and C2H4(2.1±0.1) x 10−10. The result for C3O2 is in accord with earlier measurements. Those for C2H4 and C2H2 yield values at least six orders of magnitude faster than initial estimates from earlier indirect measurements on a flow system.