A theoretical abinitio and Monte Carlo simulation study of the pyridine+CCl2 reaction kinetics in the gas phase and in carbon tetrachloride solution using canonical flexible transition state theory
Abstract
The potential energy surface for the pyridine+CCl2 reaction was studied at the abinitio MP4/6-311G(2df,p)//MP2/6-31G(*) level of theory. The MP4/6-311G(2df,p) energies were evaluated by the additivity approximation E[MP4/6-311G(2df,p)]≈E[MP4/6-31G(*)]+E[MP2/6-311G(2df,p)]-E[MP2/6-31G(*)]. The first step proceeds by the addition of CCl2 to pyridine forming a dipolar ylide structure without an activation barrier. Then this species rearranges to a more stable biradical like ylide on a picosecond time scale. The generalized transition state for dipolar ylide formation occurs at a large center of mass distance between the species, and to calculate the reaction rate constant we have used canonical flexible transition state theory. The configurational integral was solved by Monte Carlo simulation and statistical perturbation theory, and the potential of mean force in the gas phase was obtained. This procedure was extended to the liquid phase by including the solvent coordinates in the configurational integral. The activation free energy in the gas phase and in carbon tetrachloride solution was calculated as 1.44 and 2.62 kcal mol-1, respectively. The corresponding rate constants are 5.5×1011 and 7.5×1010 l mol-1 s-1. The last value is in reasonable agreement with the experimental result of 7×109 l mol-1 s-1 determined in isooctane solution.