Ab initio calculations of the transition-state geometry and vibrational frequencies of the S_N2 reaction of Cl^- with CH₃Cl

We present ab initio geometries, energies, and frequencies at the MP2/6-31G** level for three distinct stationary points on the Cr^- + CH₃Cl′ → ClCH₃ + Cl′^- gas-phase reaction path. We compare the saddle point geometries and frequencies for several basis sets, with and without correlation energy, and we compare geometries, charge distributions, and energy differences for the two most reliable method/basis set combinations. Finally, we use the MP2/6-31G** ab initio properties to evaluate the conventional transition-state theory rate constant at 300 K. Comparison with a classical calculation is used to test the validity of classical simulation techniques for this reaction. Comparison with the experimental rate constant yields a semiempirical estimate of the barrier height of 3 kcal/mol, in reasonable agreement with our best ab initio value of 4.5 kcal/mol.