Comparison of direct dynamics simulations with different electronic structure methods. F− + CH3I with MP2 and DFT/B97-1

Abstract

In previous work, ion imaging experiments and direct chemical dynamics simulations with DFT/B97-1 were performed to study the atomic-level dynamics of the F⁻ + CH₃I → FCH₃ + I⁻ S_N2 nucleophilic substitution reaction at different collision energies. Overall, the simulations are in quite good agreement with experiment at the low collision energy of 0.32 eV, however there are differences between experiment and simulation at the high collision energy of 1.53 eV. A recent CCSD(T) study of the potential energy surface for the F⁻ + CH₃I → FCH₃ + I⁻ S_N2 reaction shows that it has both a traditional C_3v and a hydrogen-bond entrance channel. They are represented by MP2 but not by B97-1, which has only the latter channel. On the other hand, B97-1 gives a reaction exothermicity in excellent agreement with experiment, while MP2 is in error by 24.3 kJ mol⁻¹. In the work presented here, direct dynamics simulations using MP2/aug-cc-pvdz/ECP/d were performed for the F⁻ + CH₃I → FCH₃ + I⁻ reaction at a 1.53 eV collision energy. The same direct rebound and stripping and indirect atomistic reaction mechanisms are found in the B97-1 and MP2 simulations. Both the B97-1 and MP2 simulations agree with the experimental fraction of the available product energy partitioned to CH₃F internal energy, i.e. f_int = 0.59 ± 0.08. However, the MP2 f_int distribution is broader and in better agreement with experiment than B97-1. The two simulations methods give the same product energy partitioning for the stripping mechanism, but different partitionings for the rebound and indirect mechanisms. Compared to experiment, the principal difference between the B97-1 and MP2 results is the differential cross section which is nearly isotropic for B97-1. For MP2 backward scattering is more important than forward, as found in the experiments. Though there is no overall barrier for the reaction, B97-1 gives a reaction cross section appreciably larger than that for MP2, i.e. 8.6 ± 2.2 Ųversus 1.8 ± 0.3 Ų. For B97-1 59% of the reaction consists of indirect mechanisms, while for MP2 the indirect mechanisms only comprise 11% of the reaction. The experimental differential cross section is more consistent with the atomistic mechanisms for MP2 than for B97-1.