Abstract:

The mechanism of the CF₃CH₂CF₂CH₃+Cl reaction was investigated by the M06-2X method combining with the 6-31+G(d,p) basis set[M06-2X/6-31+G(d,p)]. There were two distinguishable stable conformers(RC1 and RC2) for the reactant CF₃CH₂CF₂CH₃, and eight H-abstraction channels as well as two substitution channels were located associated with them. The rate constants for each of the H-abstraction channels were evaluated by the improved canonical variational transition state theory(ICVT) with the small-curvature tunneling(SCT) approximation at the M06-2X/6-31+G(d,p) level. The overall rate constant(k_T) was obtained by considering the weight factor of each conformer from the Boltzmann distribution function, and the calculated values agree well with the available experimental values. Moreover, the contribution of the two conformers to the whole reaction as well as the site selectivity for each of the conformers were discussed. A three-parameter rate constant-temperature expression for the total reaction within 200—1000 K was fitted to k_T=1.88×10^-22T^3.76exp(-1780.69/T). In addition, because of the lack of available experimental data for the reactant as well as the corresponding product radicals involved in the reactions, their enthalpies of the formation(ΔH_{f,298 K}) were predicted via isodesmic reaction at the M06-2X/6-31+G(d,p) level.

Key words: Density functional theory, Direct dynamics, Rate constant, Improved canonical variational transition state theory, CF₃CH₂CF₂CH₃