Quantum dynamics of the complex-forming SN2 reaction Cl− + CD3Cl′ → ClCD3 + Cl′− on a four-dimensional coupled-cluster potential surface

Abstract

Time-independent quantum scattering calculations have been carried out on the gas-phase S_N2 reaction Cl⁻ + CD₃Cl′ (υ₁,υ₂,υ₃) → ClCD₃ (υ^′₁,υ^′₂,υ^′₃) + Cl′⁻. The two C–Cl stretching modes (quantum numbers υ₃ and υ^′₃) and the totally symmetric modes of the methyl group (C–D symmetric stretching vibration, υ₁ and υ^′₁, and symmetric or umbrella bending vibration, υ₂ and υ^′₂) are treated explicitly, making use of a four-dimensional coupled-cluster potential energy surface. Converged state-selected reaction probabilities and product distributions have been calculated up to 4380 cm⁻¹ above the vibrational ground state of CD₃Cl, i.e. up to initial vibrational excitation (2,0,0). In order to extract all scattering resonances, a fine energetic grid had to be chosen. Excitation of the umbrella bending mode leads to a significant enhancement of the reaction probability, which, owing to the absence of the ν₂ ≈ 2ν₃ near-degeneracy, is smaller than in the Cl⁻ + CH₃Cl system, however. Exciting the high–frequency symmetric C–D stretching vibration has a considerable influence that is much larger than in the non-deuterated system. For small translational energies, reactants excited with one quantum in ν₁ are more reactive than those with one quantum in either ν₂ or ν₃. This leads to the conclusion that the C–D stretching mode should not be treated as a spectator. The calculated state-selected reaction probabilities nicely confirm the inverse kinetic isotope effect found experimentally and reproduced earlier through variational transition state computations.