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Reaktionen von Molekülen in definierten Schwingungszuständen: VI. Energieverteilung in den Reaktionen CN(υ) + O( 3 P), O 2 *)
Abstract
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.
... O( 3 P) + CN(X 2 Σ + ) → CO(X 1 Σ + ) + N( 4 S), ΔH o = − 3.44 eV (R2) at room temperature 6,7 and under hyperthermal conditions. [8][9][10] These experiments measured the disappearance of the reactants, but it is estimated that at room temperature, 80% ± 10% of the O + CN reaction proceeds through (R1). ...
... [8][9][10] These experiments measured the disappearance of the reactants, but it is estimated that at room temperature, 80% ± 10% of the O + CN reaction proceeds through (R1). 6 We note in passing that the excited atomic nitrogen in (R1) is known to play a key role in the associative ionization (AI) reaction, N + O → NO + + e − , 11 which is responsible for plasma formation under hyperthermal conditions. 12 The AI rate of N( 2 D) is orders of magnitude larger than that of N( 4 S). ...
... The earlier dynamic/kinetic calculations were based on an empirical PES. 6 Later, Andersson et al. calculated the rate coefficients for (R1) from 5 to 5000 K on ab initio based doublet PESs using the quasi-classical trajectory (QCT) method. 27 At high temperatures, the rate coefficients show reasonable agreement with the results measured by Davidson et al. 10 However, at room temperature, their QCT results yield a value of 1.39 × 10 −10 cm 3 molecule −1 s −1 , which substantially deviates from the experimental findings reported by Schmatjko and Wolfrum 6 (1.7 ± 0.7 × 10 −11 cm 3 molecule −1 s −1 ) and Titarchuk and Halpern 7 (3.69 ± 0.75 × 10 −11 cm 3 molecule −1 s −1 ). ...
The barrierless exothermic reactions between atomic oxygen and the cyano radical, O(3P) + CN(X2Σ+) → CO(X1Σ+) + N(2D)/N(4S), play a significant role in combustion, astrochemistry, and hypersonic environments. In this work, their dynamics and kinetics are investigated using both wave packet (WP) and quasi-classical trajectory (QCT) methods on recently developed potential energy surfaces of the 12A′, 12A,″ and 14A″ states. The product state distributions in the doublet pathway obtained with the WP method for a few partial waves show extensive internal excitation in the CO product. This observation, combined with highly oscillatory reaction probabilities, signals a complex-forming mechanism. The statistical nature of the reaction is confirmed by comparing the WP results with those from phase space theory. The calculated rate coefficients using the WP (with a J-shifting approximation) and QCT methods exhibit agreement with each other near room temperature, 1.77 × 10−10 and 1.31 × 10−10 cm3 molecule−1 s−1, but both are higher than the existing experimental results. The contribution of the quartet pathway is small at room temperature due to a small entrance channel bottleneck. The QCT rate coefficients are further compared with experimental results above 3000 K, and the agreement is excellent.
... 5 Next, the O( 3 P) + CN reaction was investigated using flash photolysis in a discharge flow reactor and two different electronic states 4 S and 2 D of the nitrogen atom in a ratio of 20% and 80% at room temperature were found. 6 These results were also supported by quasiclassical trajectory (QCT) calculations on an empiri- cal London-Eyring-Polanyi-Sato (LEPS) type surface. 6 These findings suggest that both 2 A and 4 A electronic states of [CNO] participate in the dynamics at room temperature. ...
... 6 These results were also supported by quasiclassical trajectory (QCT) calculations on an empiri- cal London-Eyring-Polanyi-Sato (LEPS) type surface. 6 These findings suggest that both 2 A and 4 A electronic states of [CNO] participate in the dynamics at room temperature. A subsequent theoretical study suggested that the C + NO → O + CN reac- tion occurs significantly only on the 2 A electronic states of [CNO]. ...
The C + NO collision system is of interest in the area of high-temperature combustion and atmospheric chemistry. In this work, full dimensional potential energy surfaces for the ²A′, ²A″, and ⁴A″ electronic states of the [CNO] system have been constructed following a reproducing kernel Hilbert space approach. For this purpose, more than 50 000 ab initio energies are calculated at the MRCI+Q/aug-cc-pVTZ level of theory. The dynamical simulations for the C(³P) + NO(X²Π) → O(³P) + CN(X²Σ⁺), N(²D)/N(⁴S) + CO(X¹Σ⁺) reactive collisions are carried out on the newly generated surfaces using the quasiclassical trajectory (QCT) calculation method to obtain reaction probabilities, rate coefficients, and the distribution of product states. Preliminary quantum calculations are also carried out on the surfaces to obtain the reaction probabilities and compared with QCT results. The effect of nonadiabatic transitions on the dynamics for this title reaction is explored within the Landau-Zener framework. QCT simulations have been performed to simulate molecular beam experiment for the title reaction at 0.06 and 0.23 eV of relative collision energies. Results obtained from theoretical calculations are in good agreement with the available experimental as well as theoretical data reported in the literature. Finally, the reaction is studied at temperatures that are not practically achievable in the laboratory environment to provide insight into the reaction dynamics at temperatures relevant to hypersonic flight.
... The 2 C n N radicals are characterized by doublet ground states and are very reactive species. The CN radical is reactive with molecules (, Meads et al. 1993) and atoms (, Whyte & Phillips 1983, Schacke et al. 1973, Albers et al. 1975, Schmatjko & Wolfrum 1978). The C 2 N radical is also reactive with molecules (Zhu et al. 2003, Wang et al. 2005). ...
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.
The objective of this chapter is to summarize the current knowledge of energy disposal by chemical reactions. Emphasis will be upon reactions proceeding via a single potential energy surface, which for the most part limits the discussion to ground electronic state energy surfaces. Thus, the energy of interest is translational, rotational, and vibrational energies of the products from elementary chemical reactions. The work reviewed here is limited primarily to reagents with Boltzmann or near Boltzmann vibrational and rotational energies; cases with excess translational energy will be considered since much of the molecular-beam work is done with high translational energy reactants. Non-Boltzmann energy effects on reaction rates are considered by Smith in Chapter 1. This appears to be the first review devoted solely to energy disposal, although several reviews pertain to related subjects. Golde and Thrush(1) have discussed chemical reactions yielding electronically excited products; the infrared chemiluminescence results prior to 1972 were summarized by Carrington and Polanyi(2); recent chemical laser results were reviewed by Berry(3); and several articles covering molecular-beam work include discussion of translational energy measurements(4,5) Some general reviews(6-9) also have considered state distributions of reaction products. During the last few years, many experimental data have been published and this review attempts to catalogue these results. Both unimolecular and bimolecular reactions are considered because after passing the last energy barrier separating products from reactants release of the potential energy should be a common aspect of both types of reactions.
Our concern in this volume is with molecular systems that have been displaced from thermal equilibrium, often to a considerable extent. Lasers and chemical lasers in particular operate because such displacements are feasible and are used, in turn, as means for driving other systems away from equilibrium. The first task is thus to examine those concepts and theoretical constructs of chemical physics that provide the framework for a discussion of disequilibrium on the molecular level. This chapter will center attention on the means for describing the system, means for the characterization of the phenomena in a compact fashion. The need for such a chapter stems from the traditional concern of physical chemistry with the simpler case of systems in thermal equilibrium. When disequilibrium was introduced it was often discussed as a macroscopic phenomenon. An extension of the traditional conceptual machinery is now required. In particular, the two mainstays of physical chemistry, chemical kinetics and chemical thermodynamics, need to be discussed from a molecular or state-to-state point of view.
In this review branching between various exit channels of a chemical reaction is considered. Exit channels are here defined by chemical species, electronic state and charge state, while the finer branching between “internal” states is generally excluded. Classic as well as recent examples are discussed, drawn from many different areas: Neutral and ionic bimolecular reactions, chemiluminescence, chemiionization, charge transfer, Penning and associative ionization, excimer formation, electronic energy transfer, photodissociation. Only those examples have been included for which an explanation, however tentative, has been offered in the literature, and this theoretical background is sketched briefly in each case. The determining factors considered by various authors include effects of mass, orientation, geometry of the heavy particles; electronic symmetries, Woodward-Hoffmann rules, alignment, non-adiabatic transition requirements; energetics; time scales; shapes of potential curves or surfaces; statistics.
This chapter focuses on the the distribution of energy released by chemical reactions. Reaction products are formed with the chemical energy, together with that initially possessed by the reagents, being channeled into the various accessible states. These may be electronic, vibrational or rotational, as well as recoil motion of the products. The resulting energy distributions for these modes of the products may be Boltzmann in form, being characterised by a temperature which may differ from that of the initial reagents. For some reactions, however, the distributions are non-Boltzmann, showing some population inversion. The subsequent reactivity of the products of a reaction in a complex kinetic scheme is often strongly dependent on their energy content. The rate coefficient of a reaction involving an energy-rich species may differ by many orders of magnitude from that for the corresponding room temperature reaction. Failure to recognise such a possibility can invalidate the results of many kinetic modelling schemes.
This paper gives a brief review of experimental methods used to obtain rate data for neutral-neutral reactions involving free radicals. The application of these methods to reactions of two types — radical-molecule and radical-radical reactions — is illustrated by reference to some recent studies in the author’s laboratory. As most radical-molecule reactions have significant activation energies they will be insignificantly slow at the temperatures of interstellar clouds. However, most radical-radical reactions are rapid and their rates increase as the temperature is lowered. The experimental data base for these reactions, which are the only neutral-neutral reactions likely to be important in most astrochemical environments, is still very limited and, in particular, measurements at low temperatures are badly needed.
Rate constants for the radical-radical reactions N + OH → NO + H (1), and O + OH → O 2 + H (2) have been measured for the first time by a direct method. In each experiment, a known concentration of N or O atoms is established in a discharge-flow system. OH radicals are then created by flash photolysis of H 2 O present in the flowing gas, and the disappearance of OH is monitored by time-resolved observations of its resonance fluorescence. The experiments yield K 1 = (5.0 = 1.2) × 10 −11 cm 3 molecule −1 s −1 and k 2 = (3.8 = 0.9) × 10 −11 cm 3 molecule −1 s −1 , for the reactions at 298 = 5 K.
The rate coefficients of the reactions of CN and NCO radicals with O2 and NO2 at 296 K: (1) CN + O2 → products; (2) CN + NO2 → products; (3) NCO + O2 → products and (4) NCO + NO2 → products have been measured with the laser photolysis-laser induced fluorescence technique. We obtained k1 = (2.1 ± 0.3) × 10−11 and k2 = (7.2 ± 1.0) × 10−11 cm3 molecule−t s−1 which agree well with published results. As no reaction was observed between NCO and O2 at 297 K, an upper limit of k3 < 4 × 10−17 cm3 molecule−1 S−1 was estimated. The reaction of NCO with NO2 has not been investigated previously. We measured k4 = (2.2 ± 0.3) × 10−11 cm3 molecule−1 s−1 at 296 K.
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.
The radical–radical reaction between CN (2Σ) and O2 has been theoretically investigated at the UCCSD(T)/6-311+G(d)//UB3LYP/6-31+G(d) level. This reaction proceeds most likely through the doublet CNO2 potential energy surface (PES) initiated by the carbon-to-oxygen attack leading to the linear NCOO radical, followed by the direct oxygen–oxygen single-bond cleavage leading to (A), or by the sequential three-centered isomerizations and final dissociations leading to (B) and (C). This study may be helpful for understanding the combustion chemistry of nitrogen-containing compounds.
The two-laser photoinitiation/probe technique has been used to obtain room-temperature second-order rate constants for the reaction of CN radicals with compounds key to atmospheric and combustion chemistry. CN was generated by 266 nm photolysis of ICN. Laser-induced flourescence probing via both CN(A ← X) and CN(B ← X) has been utilized. Values reported are: k(H2)=(4.9±0.4)×10−14, k(CH4)=(1.1±0.1)×10−12, k(C2H6)= (2.9±0.1)×10−11, k(C2H2)=(2.3±0.1) ×10−10, k(C2H4)=(2.7±0.1)×10−10, k(C3H6)=(2.3±0.3)×10−10, and k(O2)=(2.5±0.2)×10−11 cm3 molecule−1 s−1. A comparison with available literature data is made and mechanism are discussed.
Rate constants for the reaction of CN(ν=0) with Cl2, F2 and O2 have been measured. CN radicals were generated by photolysis of C2N2 with pulsed radiation at 193 nm. Laser-induced fluorescence (LIF) was used to monitor the decay profiles of CN radical concentrations. The values obtained are: k(CN/Cl2)=(6.0±0.3)×10−12 cm3s−1; k(CN/F2)=(6.4±0.8)×10−14 cm3s−1; and k(CN/O2)=(2.2±0.2)×10−11 cm3s−1.
Rate coefficients for the reactions N + OH → NO + H (1) and O + OH → O2+ H (2) have been determined, in direct experiments, from 250 to 515 K. A discharge-flow system is used to generate measured concentrations of N or O; then OH radicals are formed by flash photolysis of H2O and monitored by resonance fluoresence as they are removed by reaction (1) or (2) under pseudo-first-order conditions. The results are fitted to the rate expressions: k1=(2.21 ± 0.18)×10–10T–0.25 ± 0.17 cm3 molecule–1 s–1, k2=(6.65 ± 0.23)×10–10T–0.50 ± 0.12 cm3 molecule–1 s–1 where the errors represent 95% confidence intervals within the range of temperature covered in the experiments. The rate coefficient k2 is compared with a calculation assuming that the reaction proceeds via a bound state HO2 collision complex.
We have extended the highly precise laser photolysis/cw laser-induced fluorescence technique to the study of CN-radical reaction kinetics. We investigated the reaction of CN with O2 over the temperature range 295–710 K. The measured reaction-rate coefficients are well represented over this temperature range by the two-parameter Arrhenius expression, k=(1.24±0.04) × 10−11 exp[(1630±100) J mol−1/RT] cm3 molecule−1 s−1, where the quoted uncertainties represent ± 2σ estimates of the total experimental error. NCO forms as a direct product of the reaction.
Experimental techniques are described for determining the vibrational population distribution in nonequilibrium systems and measuring the rate constants governing the vibrational energy transfer processes which are important from the standpoint of their kinetic behavior. In particular, attention is given to infrared spectroscopy, laser techniques, and light-scattering techniques including Raman scattering-fluorescence and coherent anti-Stokes Raman scattering. Other techniques discussed include multiphoton ionization and velocity-modulated infrared laser spectroscopy. Some representative results are examined.
The reaction of CN with O2 has been studied through a photolysis‐probe laser experiment in a cell at a total pressure of 70 mTorr. Rotationally hot CN reagent was prepared by 193 nm photolysis of BrCN. NCO( 2Π) product in various vibronic levels was detected by laser fluorescence excitation in its 2Σ+– 2Π band system at variable delays after the photolysis laser. In order to monitor the CN collisional relaxation which is taking place simultaneously with the reaction, we have also derived the CN rotational state distribution as a function of the photolysis‐probe delay from laser fluorescence excitation spectra of the CN B 2Σ+–X 2Σ+ (0,0) band. From these observations, we deduce that rotationally hot CN reacts more slowly than thermalized CN. Moreover, reaction of the former yields NCO product with greater bending vibronic excitation. These results are compared with previous dynamical studies of this reaction, carried out with crossed beams and in cells.
Three facets of radical–radical reactions are examined: their kinetics, i.e. the rates at which such reactions proceed; their dynamics, i.e. the detailed nature of reactive collisions between radicals; and their mechanisms, i.e. the products which are given when more than one reactive channel is energetically accessible. The goal is to show how information about each of these attributes of radical–radical reactions helps to illuminate the fundamental character of radical–radical collisions. The connecting thread, in the spirit of Michael Polanyi's pioneering work, is the form of the potential-energy surface or surfaces that control the collisions. The general themes are illustrated by reference to studies of particular systems, especially those which have been, or are being, investigated in the author's laboratory. Every effort is made to assess the current level of understanding, to indicate areas where that understanding is deficient, and to suggest how our knowledge of radical–radical reactions might be improved.
Time-dependent wave packet calculations have been performed for the O+CN→CO+N reaction. The state-to-state and state-to-all reaction probabilities for total angular momentum J=0 have been calculated. The probabilities for J>0 have been estimated from the J=0 results by using J-shifting approximation based on a capture model. Then, the integral cross-sections and thermal rate constants have been calculated.
The CN+OCS reaction was studied using flash photolysis/time-resolved infrared laser spectroscopy. The reaction is fast, with a rate constant of (9.75±0.5)×10−11 cm3 molecule−1 s−1 at 296K and 2.2 Torr pressure. Detection of CO products indicates that the CO+NCS product channel dominates this reaction, and that the CO is produced vibrationally cold, with >98% in the vibrational ground state.
Electronic structure (CASPT2) calculations have been performed for the lowest 2Π and 4Σ− states of the NCO system. To create the potential energy surfaces, the generalized discrete variable representation (GDVR) method has been used. Wave packet calculations have been performed for the collinear O+CN reaction on both surfaces. These are the first reported quantum dynamics calculations on this reaction. State-to-state reaction probabilities are presented. On the 2Π surface, which has an almost 6eV deep well, we obtain structure in the reaction probabilities at low kinetic energies but at higher energies they are smooth. The 4Σ− surface is highly exoergic and vibrationally non-adiabatic dynamics is observed. The 4Σ− surface has an early barrier and as a result we find that translational energy more efficiently promotes the reaction than vibrational energy does. The wave packet results are compared with QCT results. Generally the agreement is good as would be expected but some notable differences are found, particularly for reaction out of the vibrational ground state.
The full version of the statistical adiabatic channel model has been employed to calculate rate constants for the title reaction. In very good agreement with available experimental data, the computed values increase by almost one order of magnitude between 300 and 5000 K.
The vibronic state distribution of the NCO product from the CN+O2 reaction has been extracted from a laser fluorescence experiment in a cell at a total pressure of 140 mTorr. The CN reagent was prepared by 193 nm photolysis of cyanogen, and individual NCO vibronic levels were interrogated by fluorescence excitation in its 2Σ+− 2Π band system after a variable delay. A finite induction time was observed for the formation of the NCO product in all detected vibronic levels, indicative of the necessity to moderate the translational and rotational energy of the CN photolysis fragment before appreciable reaction can occur. The NCO product was observed in 80 different vibronic levels, with energies up to 51.5 kJ/mol. A nascent vibronic state distribution among the various (v1,v2) Renner–Teller components was estimated from the relative intensities of the various bands. While it was not possible to determine the distribution in the v3 levels because of insufficiently accurate spectroscopic data, it is nevertheless clear that the NCO product from CN+O2 is formed with considerable vibrational excitation. These results are consistent with previous kinetic studies which suggest that the reaction occurs on an attractive potential energy surface, with no activation barrier.
A combination of ab initio electronic structure and variational statistical calculations are employed in a study of the kinetics of the CN+O2 reaction. Interaction energies for the transition state region of the CN+O2 reaction are evaluated within a multiconfiguration self‐consistent field framework. Optimized geometries and force fields are determined for six fixed CO separation distances (RCO) ranging from 1.7 to 3.0 Å and for the NCOO complex. The optimized NCO and COO bending angles are generally near 180° and 115°, respectively. A model analytical potential is fit to the ab initio data. This model potential is then used in variational statistical evaluations of the rate of complex formation employing a bond length reaction coordinate. A comparison between theoretical and experimental results indicates the importance of considering the deviations of the electronic interactions from those predicted by long‐range expansions. In particular, variational statistical calculations employing a realistic potential energy surface which fully incorporates the short‐range interactions are in quantitative agreement with the experimental data for temperatures ranging from 50 to 3000 K.
The rate coefficients of reactions that occur on potential energy surfaces without a barrier often exhibit a negative temperature dependence at low temperatures. Generally, this behavior is modeled with either the Harcourt–Essen equation, k(T) = AT−m, or a “negative” activation energy, k(T) = ATm exp{ΔE/kBT}. Neither of these expressions is consistent with the Wigner threshold law. The general expression k(T) = (1+T/TW)−m∑l = 0∞Al(1+T/TW)−l(T/TW)l is proposed where the relative angular momentum of the reacting species is l, TW and m are independent parameters to be extracted from the data, and the amplitude of each partial wave is Al. This expression may be approximated by k(T) = A0(1+T/TW)−m exp[(T/TW)/(1+T/TW)]. For CN+O2→ NCO+O and CO+NO the above expression reproduces the rate data, the branching ratio to the CO+NO channel, and the reactive cross section for the NCO+O channel. The rate coefficient for the NCO+O channel is given by k(cm3 s−1) = 1.79×10−10(+T/21.7)−1.38{exp[(T/21.7)/(1+T/21.7)]−1}+4.62×10−12 exp[(T/21.7)/(1+T/21.7)] while for CO+NO we obtain k(cm3 s−1) = 1.79×10−10(1+T/21.7)−1.38. An analytic form of the C–O bonding potential and the electric dipole–quadrupole interaction is used to show that the quantum threshold region extends up to 7 K. These results demonstrate the need of a complete quantum treatment for reactions that proceed on potential surfaces without a barrier. © 1999 American Institute of Physics.
The spectroscopy and dissociation dynamics of the NCO radical have been investigated by applying fast radical beam photodissociation spectroscopy to the 2Π← 2 Π electronic transition. Measurements of the photodissociation cross section as a function of dissociation wavelength show that even the lowest vibrational levels of the 2Π state predissociate. Analysis of fragment kinetic energy release reveals that the spin‐forbidden N(4S)+CO(1Σ+) products are produced exclusively until 20.3 kcal/mol above the origin, at which point, the spin‐allowed N(2D)+CO product channel becomes energetically accessible. The spin‐allowed channel dominates above this threshold. By determining the location of this threshold, we obtain a new ΔHf0 for NCO of 30.5±1 kcal/mol, several kcal/mol lower than the previously accepted value.
An entirely new experimental method is described which enables the rate constants of neutral–neutral gas‐phase reactions to be measured at ultralow temperatures. The measurements are made by applying the pulsed laser photolysis (PLP), laser‐induced fluorescence (LIF) technique of studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The experimental method is described in some detail and its application and limitations are discussed. Results are reported for the reactions of CN radicals with O2 and NH3. For reaction (1) between CN and O2 data are reported for the temperature range T=13–295 K and the rate constants are well‐matched by the expression k1(T)=(2.49±0.17)×10−11 (T/298)(−0.63±0.04) cm3 molecule−1 s−1. For reaction (2) between CN and NH3, rate constants in the temperature range T=25–295 K fit the expression k2(T)=(2.77±0.67)×10−11 (T/298)(−1.14±0.15) cm3 molecule−1 s−1. The kinetic data are discussed in terms of the latest quantum chemical and reaction rate theories for these systems.
The reaction of CN radicals with O2 was studied using infrared diode laser absorption spectroscopy. CO and CO2 products were detected directly, while the yield of NCO products was inferred by measuring the N2O yield upon addition of excess NO. Experiments and kinetic modeling calculations were performed to examine the extent of secondary chemistry in this system. The following branching ratios of the CN + O2 reaction at 296 K were determined: φ(N+CO2) = 0.02 ± 0.01 and φ(CO+NO) = 0.22 ± 0.02. The branching ratio into the CO + NO channel has a strong negative temperature dependence over the range 239−643 K.
Thermal rate coefficients for the reactions C(3P) + NO(X2Π) → CN(X2Σ+) + O(3P), C(3P) + NO(X2Π) → CO(X1Σ+) + N(2D), and O(3P) + CN(X2Σ+) → CO(X1Σ+) + N(2D) in the temperature range from 5 to 5000 K have been obtained using quasiclassical trajectory calculations. Results are reported for two ab initio potential energy surfaces corresponding to states of 2A‘ and 2A‘ ‘ symmetry. Good agreement between calculated and experimental rate coefficients are obtained for the C + NO reactions for all temperatures, whereas the rate coefficient for the O + CN reaction at room temperature is larger than that found experimentally. The dynamics is considerably different on the two potential energy surfaces with the 2A‘ ‘ giving rate coefficients in better agreement with experiments. The quality of the potential energy surfaces are discussed in the light of new electronic structure calculations including spin−orbit coupling.
The 193-nm photolysis of the NCO radical has been investigated. NCO was generated from the reaction of CN + O2, where the CN was produced by 193-nm photolysis of C2N2 close to the nozzle of a pulsed jet. A second 193-nm photon dissociated the NCO radical during the same laser pulse. At this photon energy both the N−CO and the NC−O bonds may break. N(2D,2P) and CO products have been detected using vacuum ultraviolet laser induced fluorescence. A direct measurement of the N(2D):N(2P) branching ratio yielded an upper limit of 72 ± 18. The CO vibrational distribution was modeled with prior distributions for each of the contributing channels with coproducts N(4S,2D,2P). Combination of the results from the prior model and the direct measurement yielded a branching ratio of N(4S):N(2D):N(2P) of (5.1 ± 1.8):(93.6 ± 4.8):(1.3 ± 0.3). For the N(2D) + CO product channel, the average energy disposal into product relative translation (8%) and CO vibration (24%) was determined, leaving 68% of the available energy to appear as CO rotation. This observation suggests that the geometry of the dissociating state of NCO is likely bent.
In this review, the production and detection of a wide range of unstable free-radical species under molecular beam conditions are described. The use of such molecular beam methods to study the photodissociation and inelastic and reactive scattering of free radicals in recent years is reviewed. A selection of the many experiments on the photodissociation of radicals that have been performed recently using molecular beam conditions is described to illustrate the range and scope of such experiments. For the comparatively smaller fields of free-radical inelastic and reactive scattering studied using molecular beam techniques, a comprehensive review is presented. In all cases, the experimental results are interpreted and discussed with reference to recent related theoretical calculations on the electronic structure and dynamics for the systems. A particular aim is to illustrate how the observed features of the experiments are related to information on the topography of and the couplings between the appropriate potential energy surfaces and the associated dynamics.
A theoretical treatment is given for fast, multiple bond-switching reactions, such as NO + NH2 → N2 + H2O. These reactions are characterized by all or most of the bonds being broken. The collision complex involved (whether long or short lived) is shown to be extremely anharmonic. Consideration of the master equation describing the competing processes of complex formation, internal rearrangement and collisional deactivation yields easily applied sufficient conditions for the recombination rate coefficient being independent of pressure.
The rate coefficients of the reactions
and
were determined in a series of shock tube experiments from CN time histories recorded using a narrow-linewidth cw laser absorption technique. The ring dye laser source generated 388.44 nm radiation corresponding to the CN B2Σ+(v = 0) ← X2Σ+(v = 0) P-branch bandhead, enabling 0.1 ppm detection sensitivity.
Reaction (1) was measured in shock-heated gas mixtures of typically 200 ppm N2O and 10 ppm C2N2 in argon in the temperature range 3000 to 4500 K and at pressures between 0.45 and 0.90 atm. k1 was determined using pseudo-first order kinetics and was found to be 7.7 × 1013 (±20%) [cm3 mol−1 s−1]. This value is significantly higher than reported by earlier workers. Reaction (2) was measured in two regimes. In the first, nominal gas mixtures of 500 ppm O2 and 10 ppm C2N2 in argon were shock heated in the temperature range 2700 K to 3800 K and at pressures between 0.62 and 1.05 atm. k2 was determined by fitting the measured CN profiles with a detailed mechanism. In the second regime, gas mixtures of 500 ppm O2 and 1000 ppm C2N2 in argon were shock heated in the temperature range 1550 to 1950 K and at pressures between 1.19 and 1.57 atm. Using pulsed radiation from an ArF excimer laser at 193 nm, a fraction of the C2N2 was photolyzed to produce CN. Pseudo-first order kinetics were used to determine k2. Combining the results from both regimes, k2 was found to be 1.0 × 1013 (±20%) [cm3 mol−1 s−1].
Experiments of two kinds have been performed to determine the vibronic state distribution in NCO(X̃ 2Π) produced in the radical—radical reaction CN+O2→NCO+O; ΔH00=−29±6 kJ mol−1. In both experiments, CN radicals are produced by photolysis of NCNO using a frequency-doubled Nd:YAG laser. NCO radicals are observed as products of the reaction with O2, either by recording laser excitation spectra at short times after the initiation of reaction, or by fixing the probe laser frequency and recording the variation of laser-induced fluorescence with the time delay between photolysis and probe lasers. NCO is produced in a wide range of vibronic levels. Insufficient information about band intensities is available to transform the spectra into relative populations. However, the ν2 bending vibration is highly excited and this mode absorbs ∼ 50–60% of the energy available to the products. The stretching modes absorb ≈ 15–20% of the energy. The implications of these results for the dynamics of reactive collisions between CN and O2 are discussed.
CN radicals were generated by 193 nm ArF laser flash photolysis of C2N2 in a fast-flow system, and their decay was monitored by dye-laser-induc
IntroductionReactions of O(3Pj) AtomsRelaxation and Reactions of O(1D2) AtomsRelaxation of O(1S0) AtomsConcluding Remarks
Recent progress in reactive molecular collision calculations is reviewed. The following topics are discussed: classification of theories of reactive scattering, the accuracy of ab initio potential energy surface calculations, global and local methods for fitting ab initio potential energy surfaces in a form useful for scattering calculations, and recent developments in exact and approximate quantum, semiclassical and classical theories of chemical reactions. The reactions F + H2 → FH + H and X + F2 → XF + F (X = Mu, H, D, T) are discussed as examples of recent theoretical studies of exoergic chemical reactions.
Gas phase bimoleuclar rate constants at 294 ± 3 K (ambient temperature) for the reaction of CN radicals in the ν″ = 0 and 1 levels with a number of aliphatic organic compounds were measured using laser-induced fluorescence (LIF). Most of the rate constants are very large. The rate constants for CN(ν″ = 0) reactions with the saturated hydrocarbons increases from 5.6 × 10−13 cm3 s−1 molecule−1 for CH4 to 2.6 × 10−10 cm3 s−1 molecule−1 for C7H16. Except for CH4 the vibrational energy of the CN radical has little or no effect on the observed rate constants, indicating that any barriers occur early in the reaction coordinate. Theoretical model calculations using a long-range attractive potential suggest that many of these reactions are driven by long-range forces. These long-range forces may enhance the probability for the reaction by increasing the interaction time between the two molecules.
The temperature dependence of the reaction rates for CN radicals with C2H4 and C2H4 and C2H2 has been measured from room temperature to 700 K. The two laser photoionization/LIF-probe technique was used by photolyzing ICN at 266 nm and monitoring CN depletion via B ↔ X LIF at 388 nm. A resistively heated slow-flow gas reactor was employed at 50 Torr total pressure for the temperature dependence study. Both reactions were found to have rate constants that decreased with temperature, fitting kC2H4 = (4.72±0.25)×10−11exp[(509±20)/T] and kC2H2 = (3.49±10−11exp[(571±23)/T] cm3 molecule−1 s−1, indicating that both reactions occur by addition—elimination mechanism. No pressure dependence was observed within experimental errors.
We present a new reaction path without significant barriers for the C + NO reaction, forming ground state N((4)S) and CO. Electronic structure (CASPT2) calculations have been performed for the two lowest (4)A'' states of the CNO system. The lowest of these states shows no significant barriers against reaction in the C + NO or O + CN channels. This surface has been fitted to an analytical function using a many-body expansion. Using this surface, and the previously published (2)A' and (2)A'' surfaces [Andersson et al., Phys. Chem. Chem. Phys., 2000, 2, 613; Andersson et al., Chem. Phys., 2000, 259, 99], we have performed quasiclassical trajectory (QCT) calculations, obtaining rate coefficients for the C((3)P) + NO(X(2)Pi) --> CO(X(1)Sigma(+)) + N((4)S,(2)D) and C((3)P) + NO(X(2)Pi) --> O((3)P) + CN(X(2)Sigma(+)) reactions. We have also simulated the crossed molecular beam experiments of Naulin et al. [Chem. Phys., 1991, 153, 519] The inclusion of the (4)A'' surface in the QCT calculations gives excellent agreement with experiments. This is the first time an adiabatic pathway from C((3)P) + NO(X(2)Pi) to CO(X(1)Sigma(+))+N((4)S) has been reported.
Spin-orbit coupling between the two collinear (2)Pi and (4)Sigma(-) potential energy surfaces for the NCO system are calculated using the RASSI method with CASSCF wave functions as basis set. The GDVR method has been used to interpolate a spin-orbit coupling surface. Wave packet and quasi-classical trajectory surface hopping calculations have been performed and compared for both the O((3)P) + CN(X(2)Sigma(+)) --> N((4)S) + CO(X(1)Sigma(+)) reaction and for electronically inelastic scattering in the N + CO channels. The O + CN nonadiabatic reaction probabilities are small. The wavepacket study gives a resonance structure. Also for the N + CO electronically inelastic scattering the wave packet calculations give a distinct resonance structure with peak transition probabilities up to around 10%, which is somewhat lower than the trajectory surface hopping results.
The dynamics of the H-displacement channel in the reaction N((2)D) + CH(4) has been investigated by the crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies with the total available energy. From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios of the competing reaction channels leading to the two isomers have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the CH(4)N ground state doublet potential energy surface. Differently from previous theoretical studies, both insertion and H-abstraction pathways have been found to be barrierless at all levels of theory employed in this work. A comparison between experimental results on the two isomer branching ratio and RRKM estimates, based on the new electronic structure calculations, confirms the highly nonstatistical nature of the N((2)D) + CH(4) reaction, with the production of the CH(3)N isomer dominated by dynamical effects. The implications for the chemical models of the atmosphere of Titan are discussed.
The rate coefficients for the CN + NCO --> NCN + CO reaction have been measured by a laser-photolysis/laser-induced fluorescence technique in the temperature range of 254-353 K and the He pressures of 123-566 Torr. The CN radical was produced from the photolysis of BrCN at 193 nm, and the NCO radical from the CN + O(2) reaction. The NCN radical was monitored by laser-induced fluorescence with a dye laser at 329.01 nm. The rate constants derived from kinetic modeling, with a negative temperature dependence but no pressure effect, can be expressed by k = (2.15 +/- 0.70) x 10(-11) exp[(155 +/- 92)/T] cm(3) molecule(-1) s(-1), where the quoted errors are two standard deviations. The reaction mechanism and rate constant have also been theoretically predicted for the temperature range of 200-3000 K at He pressures ranging from 10(-4) Torr to 1000 atm based on dual channel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations with the potential energy surface evaluated at the G2M and CCSD(T) levels. The rate constant calculated by variational RRKM theory agrees reasonably with experimental data.
The reaction of the CN radical with O(2) was studied using infrared diode laser absorption spectroscopy. Detection of NO and secondary N(2)O products was used to directly measure the product branching ratio. After consideration of possible secondary chemistry and comparison to kinetic modeling simulations, the branching ratio of the CN + O(2) reaction into the NO + CO channel was determined to be phi (NO + CO) = 0.20 +/- 0.02, with little or no temperature dependence over the range 296-475 K.
Sensitive and precise measurements of rate coefficients, branching fractions, and energy disposal from gas-phase radical reactions provide information about the mechanism of elementary reactions as well as furnish modelers of complicated chemical systems with rate data. This chapter describes the use of time-resolved infrared laser absorption as a tool for investigating gas-phase radical reactions, emphasizing the exploitation of the particular advantages of the technique. The reaction of Cl atoms with HD illustrates the complementarity of thermal kinetic measurements with molecular beam data. Measurements of second-order reactions, such as the self-reactions of SiH3 and C3H3 radicals, and determinations of product branching fractions in reactions such as CN + O2 rely on the wide applicability of infrared absorption and on the straightforward relationship of absorption to absolute concentration. Finally, investigations of product vibrational distributions, as in the CN + H2 reaction, provide additional insight into the details of reaction mechanisms.
The geometries, the harmonic vibrational frequencies, and the Renner-Teller parameter have been reported for the NCO(+)(X (3)Sigma(-)), NCO(X (2)Pi,A (2)Sigma(+),B (2)Pi,2 (2)Sigma(+)), NCO(-)(X (1)Sigma(+)), CNO(+)(X), CNO(X (2)Pi,A (2)Sigma(+),B (2)Pi,2 (2)Sigma(+)), and CNO(-)(X (1)Sigma(+)) systems at the full valence-complete active space self-consistent-field (fv-CASSCF) level of theory. The (2)Pi electronic states of the NCO and CNO radicals have two distinct real vibrational frequencies for the bending modes and these states are subject to the type A Renner-Teller effect. The total energy of CNO(+) without zero point energy correction of the linear geometry is approximately 31 cm(-1) higher than the bent geometry at the fv-CASSCF level and the inversion barrier vanishes after the zero point energy correction; therefore, the ground state of the CNO(+) may possess a quasilinear geometry. The spin-orbit coupling constants estimated using atomic mean field Hamiltonian at the fv-CASSCF level of theory are in better agreement with the experimental values. The excitation energies, the electron affinity, and the ionization potential have been computed at the complete active space second order perturbation theory (CASPT2) and the multireference singles and doubles configuration (MRSD-CI) levels of theory. The computed values of the electric hyperfine coupling constants for the (14)N atom in the ground state of the NCO radical agree well with the experimental data. The magnetic hyperfine coupling constants (HFCC's) have been estimated employing the configuration selected MRSD-CI and the multireference singles configuration interaction (MRS-CI) methods using iterative natural orbitals (ino) as one particle basis. Sufficiently accurate value of the isotropic contribution to the HFCC's can be obtained using an MRS-CI-ino procedure.
Vibrational energy transfer and a chemical reaction between nitric oxide and the cyanogen radical have been studied by flash photolysing cyanogen and cyanogen bromide in the presence of nitric oxide. The product of the chemical reaction is, at least in part, the unstable compound nitrosyl cyanide NOCN and the rate constant is 2 x 10$^{12}$ ml. mole$^{-1}$ s$^{-1}$ or 1 x 10$^{17}$ ml.$^2$ mole$^{-2}$ s$^{-1}$ with nitrogen as third body. The compound has a continuous absorption in the ultra-violet and yields vibrationally excited nitric oxide on photolysis. Vibrationally excited cyanogen radicals produced by means of electronic excitation of the radical produce vibrational excitation of the nitric oxide through near resonance energy exchange. Vibrational equilibrium is reached by nitric oxide through further resonance exchanges: $CN + NO \rightarrow NOCN,$ $NOCN + h\nu \rightarrow NO (v > 0) + CN$, $NO (v = 0) + CN (v = n) \rightarrow NO (v =1) + CN (v = n - 1)$, $NO(v = 1) + CN (v = m) \rightarrow NO (v = 2) + CN (v = m = 1)$, $ 2NO (v = 1) \rightarrow NO (v = 2) + NO (v = 0)$, $NO (v = 2) + NO (v = 1) \rightarrow NO (v = 3) + NO (vn = 0)$, etc.
Two systems of absorption bands have been observed in the visible and ultra-violet regions of the spectrum during the flash photolysis of several organic cyanates, and have been photographed under high resolution with long absorbing paths. Extensive vibrational and rotational analyses have been carried out for the bands of one system and show that the spectrum is due to an electronic transition $\Lambda $($^{2}\Sigma ^{+}$) $\leftarrow $ X($^{2}\Pi \_{i}$) of the free NCO radical, which is linear in both states. All three vibrational frequencies and the first-order anharmonic constants have been obtained for the upper state, $\Lambda $($^{2}\Sigma ^{+}$), and give a close fit to the term values of 21 observed vibrational levels. A Fermi resonance has been observed between $\nu \_{1}^{\prime}$ and 2$\nu \_{2}^{\prime}$. In addition, the rotational constants B$^{\prime}$ and D$^{\prime}$ and their variations with all three fundamental vibrations have been obtained for this state. Transitions have been observed from four excited levels of the bending vibration in the lower state, X($^{2}\Pi \_{i}$), and the rotational constants have been determined for some of these levels. Interaction between the electronic and vibrational motions (Renner effect) complicates the vibrational structure of this state. The state belongs to Hund's coupling case (a), and the spin-orbit coupling gives a splitting A$^{\prime \prime}$ = -95.6 cm$^{-1}$. In a $^{2}\Sigma ^{+}$ vibronic level of this state (arising from l = 1 and A = 1) the spin splitting is proportional to N + $\frac{1}{2}$, but the spin-splitting constant $\gamma $ is unusually large, and amounts to 30% of the B value. The electronic states of NCO are correlated with those of its dissociation products. This shows that the bond dissociation energy of the CO bond is slightly greater than that of the CN bond in the three known states.
Configuration interaction calculations verified an earlier hypothesis that the experimentally observed apparently anomalous behavior of the O++N2→NO++N reaction is due to the symmetry and spin restrictions which prohibit the ground state reactants or products from correlating directly with the ground state intermediate. Complex symmetry adapted functions were used, and all single and double excitations were included for the ground and first excited 2Πi states, the lowest and second 2Σ+ states, the 4Π state and the 4Σ‐ state of the intermediate NNO+ for two cuts through the potential surface close to the reaction path. The number of configurations included were: 2Πi (123), 2Σ+ (177), 4Π (1349) and 4Σ‐ (1310). Analyses of the first order CI density matrices at large distances in terms of their ``fragmental'' MO's (or AO's) verified that the 4Σ‐ NNO+ state was formed from the ground state reactants O+(4Su)+N2(1Σg+) and that it separated into ground state products NO+(1Σ+)+N(4Su). The positions of other electronic states of NNO+ were well substantiated in agreement with photoelectron spectroscopic measurements. The calculated potential energy curves show an even greater wealth of structure than would have been anticipated prior to such a detailed computation. The most significant conclusion of the present research is the unambiguous confirmation of the overriding role that spin and symmetry restrictions play in any type of collisional process.
cw single‐line laser oscillation has been achieved on the P(9), P(10), and P(11) transitions in the v = 1 → v = 0 band of CO. The observations were made on a conventional electrical discharge slow‐flow device filled with a mixture of He, N2, Xe, and CO.
A cw probe laser is used to measure the time-dependent gain on vibrational transitions of CO following a weak, pulse discharge at 100, 300, and 500 K. From the time-dependent gain data, vibrational transfer rates are determined for endothermic processes CO(v)+CO(v=0) ↠CO(v−1)+CO(v=1) over the range v=2–7 at 100 K, v=2–11 at 300 K, and v=2–12 at 500 K. The measured rates exhibit a simple functional dependence on energy defect and temperature which is predicted by theories based on long-range interactions.
In this paper we give details of the laser action on rotational transitions of 10-9, 9-8, 8-7, 7-6, and 6-5 vibrational bands belonging to the ground electronic state (X1Σ+) of CO. Laser action is produced in a low-pressure CO pulsed discharge. A comparison between the measured laser wavelengths (accuracy of ±0.5 Å at 5.0-5.4 μ) and wavelengths calculated from available molecular constants of CO shows that a small correction in the vibrational constants may be necessary. A generalized treatment of optical gain on vibrational-rotational transitions is given and it is seen that it is advantageous to operate these lasers at as low a temperature as possible for production of maximum gain. An attempt is made to analyze the excitation mechanisms responsible for these laser transitions. It is seen that the excitation processes, under pulsed operation, have to be complicated and time-dependent in order to be able to duplicate theoretically the time dependence observed for the laser power output. It is shown that under the conditions of very selective excitation of a particular vibrational level, it should be possible to obtain cw laser oscillation on some P-branch vibrational-rotational transitions.
The kinetics of the reaction between oxygen atoms and cyanogen have been studied in a capacity flow reactor at temperatures between 570 and 687 $^\circ$K. The concentration of CN radicals was measured by electronic absorption spectroscopy. This work confirms the previously proposed mechanism (part II). The initial step has a rate constant of $k\_1 = 2.5 (\pm 0.3) x 10^{13}\exp(-11 000 \pm 2000/RT) cm^3 mole^{-1} s^{-1}.$ CN radicals are removed mainly by reactions (4) and (2) for which $k\_4 = 6.3 (\pm 3.5) x 10^{13}\exp(-2400 \pm 700/RT) cm^3 mole^{-1} s^{-1}$ and $k\_2 = 4.4 (\pm 2.0) x 10^{12} cm^3 mole^{-1} s^{-1}.$ \begin{equation*}\tag{4}CN + O = CO + N,\end{equation*} \begin{equation*}\tag{2}CN + O\_2 = NCO + O.\end{equation*} The rates of reaction of CN with NO and NH$\_3$ were also measured; for CH$\_4$ and H$_2$ limiting values were obtained.
The reaction between cyanogen radicals and oxygen has been studied and the rate constant for the reaction at room temperature found to be 4.6 x 1012 ml. mole-1 s-1. The reaction is predominantly CN + O_2 -> NCO - O, with a contribution of CO + NO The nitric oxide produced in this reaction is vibrationally excited. In the presence of excess oxygen, ozone is produced in the reaction O + O_2 + M -> O_3 + M, for which the rate constant is found to be 1\cdot 1 ± 0\cdot 6 x 10^8 l\cdot^2 mole-2 s-1. A new spectrum has been observed and is assigned to the molecules N_2C_2O or NCO_2CN produced in the reaction CN + NCO -> N_2 C_2 O or N + NCO_2 -> NCO_2 CN.
An infrared laser double resonance technique has been used to study the rate constants of the V-V exchange processes: CO(υ) + CO(υ′) → CO(υ−1) + CO(υ′+1) ± ΔE, with both υ and υ′ having large values (≈ 10). Experimental results are consistent with a theoretical model of Sharma and Brau. Transitions with |Δυ| ⪢ 1 are suggested.
The flash photolysis of cyanogen, cyanogen bromide and cyanogen iodide has been studied under isothermal conditions. Vibrationally excited ($v'' \leqslant$ 6) cyanogen radicals were produced and observed spectroscopically in absorption, in the $\Delta v$ = 0, $\pm$ 1 and -2 sequences of the violet $(B^2\Sigma \leftarrow X^2\Sigma)$ system. The CN radical produced in the reaction $\mathrm{CN} R + hv \rightarrow \mathrm{CN}(X^2\Sigma,v = 0) + R$ is excited electronically, $\mathrm{CN}(X^2\Sigma, v = 0) + hv \rightarrow \mathrm{CN} (B^2\Sigma, v = 0, 1, 2, \ldots),$ and then returns to various vibrational levels of the ground state by fluorescence or collision $\mathrm{CN}(B^2\Sigma, v = 0, 1, 2, \ldots) \rightarrow \mathrm{CN}(X^2\Sigma, v = 0, 1, 2, \ldots) + hv,\\ \mathrm{CN}(B^2\Sigma, v = 0, 1, 2, \ldots) + M \rightarrow \mathrm{CN}(X^2\Sigma, v = 0, 1, 2, \ldots) + M.$ Frequent repetition of this type of excitation in the absence of relaxation would lead to a vibrational `temperature' which may be described as virtually infinite, and in any case is extremely high when relaxation is relatively slow. The alternative reactions by which vibrationally excited radicals could be produced, namely $\mathrm{CN}R + hv \rightarrow \mathrm{CN}(X^2\Sigma, v \leqslant 6) + R$ and $\mathrm{CN} R + hv \rightarrow \mathrm{CN}(A^2\Pi, v > 0)+R,$ followed by $\mathrm{CN}(A^2\Pi, v > 0) \rightarrow \mathrm{CN}(X^2\Sigma, v > 0),$ were shown to account for < 6% of the vibrationally excited radicals observed and may be entirely inoperative. The probability of energy transfer to CNBr from the fourth vibrational level of CN, P$_{4-3}$, was found to be approximately 1 x 10$^{-2}$. The rate constant for the recombination of cyanogen radicals at room temperature was found to be $\sim$ 6 x 10$^{11}$ ml. mole$^{-1}$ s$^{-1}$ or $\sim$ 1.7 x 10$^{16}$ ml.$^2$ mole$^{-2}$ s$^{-1}$ with nitrogen as third body.
The vapor phase absorption spectra of cyanogen, cyanoacetylene, dicyanoacetylene, and dicyanodiacetylene have been recorded between 2000 and 1050 Å. Substantial perturbations in the intensity and positions of particular bands upon the addition of helium at 1500 psi pressure served to identify several Rydberg series. These series are assigned on the basis of their quantum defects and symmetry considerations. CNDO/2-CI calculations were used to interpret the valence shell transitions. A very intense intravalence shell band, assigned as a 1Sigmau+ pi* configuration, dominates the spectra of all the molecules. A moderately intense intravalence shell band, assigned as a 1Piu pi* configuration further characterizes the spectra.
Collinear quasiclassical trajectories are examined for two realistic potential energy surfaces for atom−diatomic molecule reactions for two reaction attributes: (1) vibrational energy of the products of a thermal−energy exothermic reaction; (2) threshold energy for endothermic reaction of ground−state reagents. Eight different mass combinations are studied. The potential energy surfaces differ primarily in the amount of potential energy released in an exothermic reaction before and in the region of large curvature of the minimum−energy path and in the curvature of the repulsive potential energy contours when all three atoms are close. For attribute (1), we find the results are qualitatively correlated by the theory of Hofacker and Levine although, contrary to previous work, one potential energy surface shows high mixed energy release (in the language of Polanyi and co−workers) but low excitation to product vibration for five different mass combinations. For reaction attribute (2), we find one surface has a high translational threshold (or no reaction at any energy) for six mass combinations, while the other surface shows this behavior in only three cases. Thus, this type of surface provides an exception to previous generalizations that extra vibrational energy is required for very endothermic reactions with late barriers. This demonstrates the importance of the location of the curvature of the reaction channel for such reaction attributes. Very accurate determinations of potential energy surfaces will be required to make reliable predictions of reaction attributes such as (1) and (2) for real systems. Analysis of the details of the trajectories shows that the high threshold can generally be attributed to reflection before the saddle point of the surface rather than to recrossing the saddle point region. The vibrational excitation of reagents in nonreactive collisions is also strongly effected by curvature of the minimum−energy path.
The infrared absorption of 12C214N2 and 12C215N2 has been studied with a resolution of 0.08 to 0.04 cm−1. The vibration—rotation band constants were determined for eight vibrational transitions of 12C214N2 and three vibrational transitions of 12C215N2. Many unresolved hot‐band Q branches were also measured. The values for many vibrational and rotational constants are given. The bond distances determined from the rotational analysis, and their three‐standard‐deviation error limits, are r0 (C☒C) = 1.389±0.030 Å and r0 (C☒N) = 1.154±0.017 Å. The N☒N distance and corresponding error limits are 3.697±0.010 Å.
The Ba+HF→BaF+H reaction has been studied under single‐collision conditions as a function of the reagent and product vibration in order to obtain state‐to‐state reaction rates. Using a beam + gas arrangement, the HF is pumped by a pulsed HF laser and the BaF product distribution is determined from its laser‐induced excitation spectrum. The BaF product is found to retain an average of 64% of the initial reactant vibrational excitation, and the distribution of product states for Ba+HF(v=1) has a broad maximum shifted to v=6 from the value v=1 for Ba+HF(v=0). The implications of these results on various proposed reaction surfaces are discussed.
A series of two‐dimensional classical kinematic computer calculations have been made on the hypothetical exothermic exchange reaction A+BC→AB+C, −ΔH=48.5 kcal mole−1. Product energy distribution (vibration, rotation, and translation; Evib, Erot, Etrans) was obtained as a function of initial position, impact parameter, and kinetic energy (αi, b, and Ekini). All eight different combinations of light (L = 1 amu) and heavy (H = 80 amu) masses were examined. Eight different potential‐energy hypersurfaces were explored. All were obtained from an empirical extension of the London—Eyring—Polanyi—Sato (L.E.P.S.) method. The hypersurfaces were categorized in terms of the percentage attraction, A⊥, and percentage repulsion R⊥, read off the collinear three‐dimensional surface. This categorization was shown to be helpful in arriving at a qualitative understanding of the product energy distribution to be expected from all the mass combinations reacting on these extended L.E.P.S. hypersurfaces. However, it was also shown that the A⊥, R⊥ categorization could only be of (approximate) quantitative value in predicting product energy distribution for the case that the atomic masses satisfy the inequality mA≪mB, mC. For all other mass combinations three types of energy release must be distinguished; attractive energy release (A approaching BC, at normal BC separation), mixed energy release (A still approaching BC, while BC extends), and repulsive energy release (AB at normal separation retreating from C). These three types of energy release, symbolized A, M, and R were defined. The value of the concept was tested by obtaining % A, M, and R for each mass combination on several energy surfaces of markedly different characteristics, from part of a single collinear trajectory, and then plotting A+M against the mean vibrational excitation, % 〈Evib〉, obtained from a representative group of trajectories on the full hypersurface. For all mass combinations on all the surfaces examined it was found that % (A+M)∼% 〈Evib〉.
The laser fluorescence method has been used to measure CO(ν=1) collisional transfer rates in several binary and ternary gas mixtures at 296°K. Excitation of the CO(ν=1) level was achieved using pulsed 4.6 μm radiation from a frequency doubled CO2 laser. The relative inertness of CO molecules towards V→T deactivation greatly facilitates the study of vibrational relaxation rates of the additive species in selected cases. In this paper, intermolecular V→V transfer rates at 296°K are reported for CO(ν=1) with N2O, OCS, SO2, CS2, C2N2, and for CO(ν=2) with CO. In addition, the additive deactivation rates were determined for the following collisional processes: N2O(001) with N2O, CO, Ar; OCS(001) with OCS, CO, Ar; CS2(001) with CS2, CO; and C2N2(00100) with C2N2 and CO.
Infrared chemiluminescence has been observed from vibrationally excited CO, formed in the reaction, O + CS → CO + S. The quenching of the CO overtone spectrum has been studied as a function of the concentration of each of a number of added gases. A steady-state treatment is developed which allows rates to be determined for the de-excitation of individual vibrational levels of CO (4 ≤ v ≤ 13) by He, CO (v = 0), NO, N2, O2, OCS, N2O, and CO2. The experimental results are compared with theoretically predicted rates. Finally, the importance of the results for interpreting the behavior of CO vibrational lasers is considered.
For the collisional reactions F + HCl -> HF + Cl, F + D2 -> DF + D, and H + Cl -> HCl + Cl, the effects of the reagent translation (DT) and reagent vibration (DV) enhancements in excess of those required to cross the potential-energy barriers were studied exptl. by the ir chemiluminescence method, and theor. by the classical-trajectory (Monte-Carlo) method. Both DT and DV enhanced the reaction rate consts.; the former was more effective for these exothermic reactions. On the av., DT -> DT' + DR' (the primed energies refer to the reaction products; DR' = product rotational-energy enhancement), and DV -> DV'. The effect of collision-energy increase is related to a contribution from the \"induced repulsive-energy release\" (IRER), and the effect of increased reagent vibration to both IRER and (more important) \"induced attractive-energy release\". For the reaction F + HCl, the increase in rate const. (k) with reagent vibrational quantum no., and the rotational dependence of k were detd. [on SciFinder (R)]
Anschließend an experimentelle Untersuchungen (Teil I) der Reaktion CN(X ² Σ ⁺ , ν″) + O( ³ P) wurden dreidimensionale klassische Tra‐jektorienberechnungen auf zwei verschiedenen adiabatischen Potentialhyperflächen ausgeführt: 1. für den Reaktionsweg nach N( ² D) + CO(X ¹ Σ ⁺ ) über den Zwischenzustand NCO(X ² II), 2. für den Reaktionsweg nach N( ⁴ S) + CO(X ¹ Σ ⁺ ).
Für den Weg (1) findet man eine Abnahme des Reaktionsquerschnitts mit steigender Schwingungsanregung des CN(ν″). Die Reaktions‐energie wird dabei auf die Schwingungszustände des Produktmoleküls CO und die relative Translation der Produkte gleich verteilt. Auf dem Weg (2) erhält man einen langsamen Anstieg des reaktiven Querschnitts für CN(ν″) und eine Inversion in den Schwingungszuständen des CO. Nur die Ergebnisse für das Potential (2) sind in Übereinstimmung mit dem experimentell gefundenen Verhalten.
The dynamics of exchange reactions A+BC→AB+C have been examined on two types of potentialenergy hypersurfaces that differed in the location of the energy barrier along the reaction coordinate. On "surface I" the barrier was in the entry valley of the energy surface, along the approach coordinate. On "surface II" the barrier was in the exit valley of the energy surface, along the retreat coordinate. The classical barrier height was Ec= 7.0 kcal mole-1 on both surfaces, and was displaced from the corner of the energy surface by the same amount; on surface I, r 1‡;= 1.20 Å, r2‡=0.80 Å; on surface II, r1‡=0.80 Å, r2‡=l-20 Å(r1≡=rAB, r2≡rBC, and the superscript Irefers to the location of the crest of the barrier). Three-dimensional (3D) classical trajectory calculations were performed for the mass combination mA= mB=mC at several reagent energies. The reagent energy took the form of translation, vibration or an equilibrium distribution of the two. The main findings were that translation was markedly more effective than vibration in promoting reaction on surface I, and vibration markedly more effective than translation in promoting reaction on surface II. The total reactive cross section with the entire reagent energy vested in translation was symbolized ST, with the reagent energy (but for 1.5 kcal) in vibration, Sv, and with an equilibrium distribution over reagent translation and vibration, Seq. On surface I S T>>SV: on surface II SV>>S T. Close to the threshold for ST on surface I, S T/Seq∼10; close to the threshold for 5V, on surface II, SV/Seq∼10. At high reagent energies (2×threshold) on surface I ST/Seq fell to 2, whereas on surface II SV/Seq increased to extremely large values. Product energy and angular distributions were recorded for two reagent energies. On surface I with low translational energy in the reagents a major part of the available energy appeared as vibration in the molecular product. At higher collision energy this fraction decreased. On surface II with low vibrational energy in the reagents only a small part of the available energy appeared as vibration in the product. At higher vibrational energy this fraction increased. The product angular distribution at low reagent translational energy on surfaces I and II corresponded to backwardpeaked scattering of the molecular product. At increased reagent energy on both surfaces the distribution shifted forward (this is a novel phenomenon in the case of increased reagent vibration; surface II).
Citation
J. D. Barry and J. E. Brandelik, "CO Laser Gas Temperature," Appl. Opt. 12, 2809-2810 (1973)
http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-12-12-2809
The principles of use of gratings as laser wavelength-selective end reflectors are reviewed. A useful output beam can be derived from a grating's zeroth-order reflection. This beam moves when the grating is rotated to select various laser wavelengths, but can be made stationary by the addition of auxiliary mirrors. The grating-mirror combination has been applied to a CO(2) laser in the in and to a dye laser in the visible.
Examination of the effect of the inclusion of a small but significant amount of rotational energy in the reagents, and of a change in reagent masses in a previous study of the effect of barrier location on the dynamics of thermonuclear reaction A + BC yields AB + C. The qualitative generalizations introduced in the previous study are found to remain valid despite the introduction of the variables. Of these generalizations the most important is that reagent translational energy favors reaction on surface I, whereas reagent vibration is the most favorable to reaction on surface II.
The paper describes absorption calculations and measurements at selected infrared CO laser wavelengths which are nearly coincident with absorption lines in the fundamental vibration-rotation band of NO near 5.3 microns. Initial work was directed towards establishing the optimal CO laser-NO absorption line coincidence for high temperature applications. Measurements of the absorption coefficient at this optimal laser wavelength were carried out, first using a room-temperature absorption cell for high-temperature calculations and then using a shock tube, for the temperature range 630-4000 K, to validate the high temperature calculations.
The Spectra and Structures of Simple Free Radicals Ithaca-London
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