Selective alkane transformations via radicals and radical cations: insights into the activation step from experiment and theory.

The Laser Photolysis-Shock Tube technique coupled with H-atom atomic resonance absorption spectrometry has been used to study the reaction, H + CH{sub 4} {yields} CH{sub 3} + H{sub 2}, over the temperature range, 928-1697 K. Shock-tube studies on the reverse of this reaction, CH{sub 3} + H{sub 2} {yields} H + CH{sub 4}, using CH{sub 3}I dissociation in the presence of H{sub 2} yielded H-atom formation rates and rate constants for the reverse process over the temperature range, 1269-1806 K. These results were transformed (using well-established equilibrium constants) to the forward direction. The combined results for H + CH{sub 4} can be represented by an experimental three parameter expression, K = 6.78 x 10{sup -21} T{sup 3.156} exp(-4406 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (348-1950 K) that was evaluated from the present work and seven previous studies. Using this evaluation, disagreements between previously reported values for the dissociation of CH{sub 4} could be reconciled. The thermal decomposition of CH{sub 4} was then studied in Kr bath gas. The dissociation results agreed with the earlier studies and were theoretically modeled with the Troe formalism. The energy transfer parameter necessary to explain both the present results and those of Kiefer andmore » Kumaran (J Phys Chem 1993, 97, 414) is, {l_angle}{Delta}E{r_angle}{sub all}/cm{sup -1} = 0.3323 T{sup 0.7}. The low temperature data on the reverse reaction, H + CH{sub 3} (in He) from Brouard et al. (J Phys Chem 1989, 93, 4047) were also modeled with the Troe formalism. Lastly, the rate constant for H + CH{sub 4} was theoretically calculated using conventional transition state theory with Eckart tunneling corrections. The potential energy surface used was from Kraka et al. (J Chem Phys 1993, 99, 5306) and the derived T-dependence with this method agreed almost perfectly with the experimental evaluation.« less

Rate constants for H + CH4, CH3 + H2, and CH4 dissociation at high temperature

Kinetic modeling of the decomposition and flames of hydrazine

Ab initio electronic structure calculations have been carried out on six hydrogen abstraction reactions of the form, X−H + Y• → X• + H−Y, where X, Y = CH3, NH2, and OH Geometric structures for the reactants, reactant complexes, transition states, product complexes, and products of each reaction have been gradient optimized at both the UMP2 and DFT(B3LYP) theory levels using the 6-311++G(2d,p) basis set The character of each stationary state as a minimum or saddle point was determined by a harmonic force field calculation PMP2 and CCSD(T) energies were also calculated at the UMP2 optimized geometries CCSD(T) geometry optimizations were carried out for selected cases to resolve differences in results between UMP2 and DFT The calculated reaction energies, barrier heights, and weak complex stabilities are compared to experiment, where possible, and among the different theory levels The geometric structures and wave function properties are compared between the UMP2, CCSD(T), and DFT(B3LYP) methods In ge

Ab Initio Study of Hydrogen Abstraction Reactions

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm has been used to study the reactions OH + CH4 → CH3 + H2O and CH3 + NO2 → CH3O + NO. Over the temperature range 840−2025 K, the rate constants for the first reaction can be represented by the Arrhenius expression k = (9.52 ± 1.62) × 10-11 exp[(−4134 ± 222 K)/T] cm3 molecule-1 s-1. Since this reaction is important in both combustion and atmospheric chemistry, there have been many prior investigations with a variety of techniques. The present results extend the temperature range by 500 K and have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range 195−2025 K. A three-parameter expression describes the rate behavior over this temperature range, k = (1.66 × 10-18)T2.182 exp[(−1231 K)/T] cm3 molecule-1 s-1. Previous theoretical studies are discussed, and the present evaluation is compared to earlier theoretical estimates. Since CH3 radicals a...

Reflected shock tube studies of high-temperature rate constants for OH + CH4 --> CH3 + H2O and CH3 + NO2 --> CH3O + NO.

Hydrogen abstraction reactions with R, X≡H, CH3, NH2,OH, and F have been studied at the abinitio 6-31G – UHF level. However, energetic properties were computed at the CI level. Rate constants and A...

Anne Tinant

Papers

Étude théorique des réactions d'abstraction d'hydrogène, avec R, X≡H, CH3, NH2,OH et F