Quantum chemical calculations of the Cl ^- + CH ₃i → CH ₃Cl + i ^- potential energy surface

Electronic structure theory calculations, using MP2 theory and the DFT functionals OPBE, OLYP, HCTH407, BhandH, and B97-1, were performed to characterize the structures, vibrational frequencies, and energies for stationary points on the Cl ^- + CH ₃I → C1CH ₃ + I ^- potential energy surface. The aug-cc-pVDZ and aug-cc-pVTZ basis sets, with an effective core potential (ECP) for iodine, were employed. Single-point CCSD(T) calculations were performed to obtain the complete basis set (CBS) limit for the reaction energies. DFT was found to give significantly longer halide ion/carbon atom bond lengths for the ion-dipole complexes and central barrier transition state than MP2. BhandH, with either the aug-cc-pVDZ or aug-cc-pVTZ basis sets, gives good agreement with the experimental structures for both CH ₃I and CH ₃C1. The frequencies of CH ₃I and CH ₃CI, obtained with the different levels of theory and basis sets, are in excellent agreement with experiment. The major difference between the MP2 and DFT frequencies is for the imaginary frequency of the central barrier. Using the aug-cc-pVTZ basis the MP2 value for this frequency ranges from 1.26 to 1.59 times larger than those for the DFT functionals. Thus, the MP2 and DFT theories have different PES shapes in the vicinity of the [CI-CH ₃-I] ^- central barrier. The CCSD(T)/CBS energies are in good agreement with experiments for the complexation energies and reaction exothermicity, with a small 1 kcal/mol difference for the latter. The CCSD(T)/CBS central barrier height is lower than values deduced by using statistical theoretical models to fit the Cl ^- + CH ₃I → C1CH ₃ + I ^- experimental rate constant, which is consistent with the expected nonstatistical dynamics for the reaction. The BhandH energies are in overall best agreement with the CCSD(T) values, with a largest difference of only 0.7 kcal/mol.