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 SN2 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.