Kinetics of Hydrogen Abstraction Reactions from Fluoromethanes and Fluoroethanes

Abstract

Hydrogen abstraction reactions from hydrofluorocarbons are important reaction steps in both atmospheric and combustion chemistry. In this study, kinetics of the H-abstraction reactions from methane, ethane, fluoromethanes, and fluoroethanes by H, O, and OH radicals are investigated using the CBS-QB3//ωB97X-D quantum chemical method and transition state theory, and the site-specific reactivity of the fluoroalkanes systematically examined. The calculated rate constants for these reactions are found to accurately reproduce available experimental data for a wide temperature range, with only minor differences between experimental and theoretical barrier heights. Analysis of site-specific reactivity indicates that fluorine substitution at the β-carbon site of the fluoroalkanes systematically increases the barrier heights of H-abstraction, and the substitution effect at the α-carbon site can be interpreted by electron-withdrawing and steric effects. Fluorine substitution also decreases the pre-exponential factor for the H-abstraction reactions by O and OH radicals due to steric and electrostatic interactions between these radicals and the F atoms.

Dissociation of 1,1,1-Trifluoroethane Is an Intrinsic RRKM Process: Classical Trajectories and Successful Master Equation Modeling

“…The classical density and sum of states were calculated for zero total angular momentum and assuming harmonic vibrations. A frequency scaling factor of 0.95 33,34 was applied to the ωB97X-D/6-311G++(d,p) harmonic frequencies in order to partially account for anharmonic effects. There is excellent agreement between the trajectory and RRKM rate constants, with a The Journal of Physical Chemistry A Article deviation of at most 5%.…”

“…This value is close to the anharmonic correction to the density of state of C 2 H 6 at its bond dissociation energy, 2.20, calculated by Nguyen and Barker. 69 This indicates that the scaling factor of 0.95, which was derived to estimate fundamental frequencies, 33,34 can reasonably reproduce densities of states at high energies.…”

Gas-phase reaction mechanism in chemical dry etching using NF3and remotely discharged NH3/N2mixture

“…For the calculations of (NH 3 ) n (HF) m clusters, the structures and frequencies were calculated using the same method as above, uB97X-D/6-311++G(d,p), but the coupled cluster calculations with the triple-z basis sets were not affordable for large clusters; therefore, the energies of the clusters were calculated using the CBS-QB3//uB97X-D method. 12,31 The thermodynamic properties of the clusters were calculated using rigid-rotor and harmonic-oscillator approximations.…”

Computational Study of the Rate Coefficients for the Reactions of NO2 with CH3NHNH, CH3NNH2, and CH2NHNH2

“…The scale factors for the zero-point correction and partition function evaluations for the ωB97X-D/6-311++G(d,p) calculations were 0.975 and 0.950, respectively. 18 Those for the CASPT2/cc-pVDZ calculations were estimated from the above values and the harmonic frequencies calculated at 50.0 Å separated geometry, excluding loose intermolecular modes. The optimized geometries, rotational constants, and harmonic frequencies of the most stable isomers for the reactants, products, and the transition states are listed in Tables S1 and S2 in the Supporting Information.…”