Rate Constants for the Relaxation of CH(X2Π,ν=1) by CO and N2 at Temperatures from 23 to 584 K

Product Detection of the CH(X2Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

The reaction of the methylidyne radical (CH(X²Π)) with cyclopentadiene (c-C₅H₆) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C₆H₆ + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C₆H₆ isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C₆H₆ isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C₆H₇ potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

Product Detection of the CH Radical Reactions with Ammonia and Methyl-Substituted Amines

Reactions of the methylidyne (CH) radical with ammonia (NH₃), methylamine (CH₃NH₂), dimethylamine ((CH₃)₂NH), and trimethylamine ((CH₃)₃N) have been investigated under multiple collision conditions at 373 K and 4 Torr. The reaction products are detected by using soft photoionization coupled to orthogonal acceleration time-of-flight mass spectrometry at the Advanced Light Source (ALS) synchrotron. Kinetic traces are employed to discriminate between CH reaction products and products from secondary or slower reactions. Branching ratios for isomers produced at a given mass and formed by a single reaction are obtained by fitting the observed photoionization spectra to linear combinations of pure compound spectra. The reaction of the CH radical with ammonia is found to form mainly imine, HN═CH₂, in line with an addition-elimination mechanism. The singly methyl-substituted imine is detected for the CH reactions with methylamine, dimethylamine, and trimethylamine. Dimethylimine isomers are formed by the reaction of CH with dimethylamine, while trimethylimine is formed by the CH reaction with trimethylamine. Overall, the temporal profiles of the products are not consistent with the formation of aminocarbene products in the reaction flow tube. In the case of the reactions with methylamine and dimethylamine, product formation is assigned to an addition-elimination mechanism similar to that proposed for the CH reaction with ammonia. However, this mechanism cannot explain the products detected by the reaction with trimethylamine. A C-H insertion pathway may become more probable as the number of methyl groups increases.

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

Gas-phase radical reactions of CN and CH with small hydrocarbons are overviewed with emphasis on isomer-resolved product detection.

Formation of dimethylketene and methacrolein by reaction of the CH radical with acetone

The reaction of the methylidyne radical (CH) with acetone ((CH(3))(2)C[double bond, length as m-dash]O) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of bromoform and react with acetone in an excess of helium and nitrogen gas flow. The main observed reaction exit channel is elimination of a hydrogen atom to form C(4)H(6)O isomers. Analysis of photoionization spectra identifies dimethylketene and methacrolein as the only H-elimination products. The best fit to the data gives branching ratios of 0.68 ± 0.14 for methacrolein and 0.32 ± 0.07 for dimethylketene. A methylketene spectrum measured here is used to reanalyze the photoionization spectrum obtained at m/z = 56 for the CH + acetaldehyde reaction, (Goulay et al., J. Phys. Chem. A, 2012, 116, 6091) yielding new H-loss branching ratios of 0.61 ± 0.12 for acrolein and 0.39 ± 0.08 for methylketene. The contribution from methyleneoxirane to the reaction product distribution is revised to be negligible. Coupled with additional product detection for the CD + acetone reaction, these observations pave the way for development of general set of reaction mechanisms for the addition of CH to compounds containing an acetyl subgroup.

Long-range transition state theory

The implementation of variational transition state theory (VTST) for long-range asymptotic potential forms is considered, with particular emphasis on the energy and total angular momentum resolved (microJ-VTST) implementation. A long-range transition state approximation yields a remarkably simple and universal description of the kinetics of reactions governed by long-range interactions. The resulting (microJ-VTST) implementation is shown to yield capture-rate coefficients that compare favorably with those from trajectory simulations (deviating by less than 10%) for a wide variety of neutral and ionic long-range potential forms. Simple analytic results are derived for many of these cases. A brief comparison with a variety of low-temperature experimental studies illustrates the power of this approach as an analysis tool. The present VTST approach allows for a simple analysis of the applicability conditions for some related theoretical approaches. It also provides an estimate of the temperature or energy at which the "long-range transition state" moves to such short separations that short-range effects, such as chemical bonding, steric repulsion, and electronic state selectivity, must be considered.

CH radical production from 248nm photolysis or discharge-jet dissociation of CHBr3 probed by cavity ring-down absorption spectroscopy

The A-X bands of the CH radical, produced in a 248 nm two-photon photolysis or in a supersonic jet discharge of CHBr(3), have been observed via cavity ring-down absorption spectroscopy. Bromoform is a well-known photolytic source of CH radicals, though no quantitative measurement of the CH production efficiency has yet been reported. The aim of the present work is to quantify the CH production from both photolysis and discharge of CHBr(3). In the case of photolysis, the range of pressure and laser fluences was carefully chosen to avoid postphotolysis reactions with the highly reactive CH radical. The CH production efficiency at 248 nm has been measured to be Phi=N(CH)N(CHBr(3))=(5.0+/-2.5)10(-4) for a photolysis laser fluence of 44 mJ cm(-2) per pulse corresponding to a two-photon process only. In addition, the internal energy distribution of CH(X (2)Pi) has been obtained, and thermalized population distributions have been simulated, leading to an average vibrational temperature T(vib)=1800+/-50 K and a rotational temperature T(rot)=300+/-20 K. An alternative technique for producing the CH radical has been tested using discharge-induced dissociation of CHBr(3) in a supersonic expansion. The CH product was analyzed using the same cavity ring-down spectroscopy setup. The production of CH by discharge appears to be as efficient as the photolysis technique and leads to rotationally relaxed radicals.

Photodissociation dynamics of the HCNN radical

The photodissociation dynamics of the diazomethyl (HCNN) radical have been studied using fast radical beam photofragment translational spectroscopy. A photofragment yield spectrum was obtained for the range of 25,510-40,820 cm(-1), and photodissociation was shown to occur for energies above 25,600 cm(-1). The only product channel observed was the formation of CH and N2. Fragment translational energy and angular distributions were obtained at several energies in the range covered by the photofragment yield spectrum. The fragment translational energy distributions showed at least two distinct features at energies up to 4.59 eV, and were not well fit by phase space theory at any of the excitation energies studied. A revised C-N bond dissociation energy and heat of formation for HCNN, D0(HC-NN)=1.139+/-0.019 eV and DeltafH0(HCNN)=5.010+/-0.023 eV, were determined.

Reactions at Very Low Temperatures: Gas Kinetics at a New Frontier

Advances in experimental techniques, especially the development of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method, allow many gas-phase molecular processes to be studied at very low temperatures. This Review focuses on the reactions of molecular and atomic radicals with neutral molecules. Rate constants for almost 50 such reactions have been measured at temperatures as low as 13 K by using the CRESU method. The surprising demonstration that so many reactions between electrically neutral species can be extremely rapid at these very low temperatures has excited interest both from theoreticians and from those seeking to understand the chemistry that gives rise to the 135 or so molecules that are present in low-temperature molecular clouds in the interstellar medium. Theoretical treatments of these reactions are based on the idea that a reaction occurs when the long-range potential between the reagent species brings them into close contact. The astrochemical context, theoretical studies, and the determination of the rate constants of these low-temperature reactions are critically discussed.

Determination of the Rate Coefficients for the Reactions IO + NO2 + M (Air) → IONO2 + M and O(3P) + NO2 → O2 + NO Using Laser-Induced Fluorescence Spectroscopy

Laser-induced fluorescence spectroscopy via excitation of the A2pi(3/2) <-- X2pi(3/2) (2,0) band at 445 nm was used to monitor IO in the presence of NO2 following its generation in the reactions O(3P) + CF3I and O(3P) + I2. Both photolysis of O3 (248 nm) and NO2 (351 nm) were used to initiate the production of IO. The rate coefficients for the thermolecular reaction IO + NO2 + M --> IONO2 + M were measured in air, N2, and O2 over the range P = 18-760 Torr, covering typical tropospheric conditions, and were found to be in the falloff region. No dependence of k1 upon bath gas identity was observed, and in general, the results are in good agreement with recent determinations. Using a Troe broadening factor of F(B) = 0.4, the falloff parameters k0(1) = (9.5 +/- 1.6) x 10(-31) cm6 molecule(-2) s(-1) and k(infinity)(1) = (1.7 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) were determined at 294 K. The temporal profile of IO at elevated temperatures was used to investigate the thermal stability of the product, IONO2, but no evidence was observed for the regeneration of IO, consistent with recent calculations for the IO-NO2 bond strength being approximately 100 kJ mol(-1). Previous modeling studies of iodine chemistry in the marine boundary layer that utilize values of k1 measured in N2 are hence validated by these results conducted in air. The rate coefficient for the reaction O(3P) + NO2 --> O2 + NO at 294 K and in 100 Torr of air was determined to be k2 = (9.3 +/- 0.9) x 10(-12) cm3 molecule(-1) s(-1), in good agreement with recommended values. All uncertainties are quoted at the 95% confidence limit.

Reaction of Anthracene with CH Radicals: An Experimental Study of the Kinetics between 58 and 470 K

The rate coefficient of the reaction of the methylidine radical CH with anthracene has been studied over a wide temperature range (58-470 K) in a dedicated "Cinétique de Réaction en Ecoulement Supersonique Uniforme" (Reaction Kinetics in Uniform Supersonic Flow) apparatus. The reaction exhibits a slight positive temperature dependence, which can be fitted to the expression k(T) = (3.32 +/- 1.00) x 10(-10)(T/298)((0.46+/-0.14)) cm3 molecule(-1) s(-1). Even at the lowest temperature, the reaction remains very fast indicating that the kinetics are probably driven by a capture process.

The CH + N2 Association Reaction at Low Temperatures: Ab Initio MO/VRRKM-Theory Analysis of Temperature and Pressure Effects

The microcanonical variational RRKM formalism combined with high level