Quantum scattering on S(N)2 reactions: Influence of azimuthal rotations

Time independent quantum scattering calculations have been carried out on the SN2 Walden inversion reaction Cl-+CH3Cl(v,k)→ClCH3(v ′,k′)+Cl-. The two C-Cl stretching degrees of freedom (quantum numbers v and v′) and the azimuthal angle describing the rotation of the CH3 group (quantum numbers k and k′) are treated explicitly. An infinite order sudden approximation has been introduced using Radau coordinates for the stretching modes. The potential energy surface of Vande Linde and Hase is used. The scattering problem is formulated in hyperspherical coordinates. For the reaction (k=0→k′=0) substitution is observed for initial vibrational excitation with v≥2. If the system departs from the collinear reaction pathway (initial rotational excitation) the substitution cross sections are strongly decreased. The state-to-state cross sections σvk→v′k′ are large only for transitions with Δk=0. The total reaction cross sections σvk for given v vary only slightly at low values of the azimuthal quantum number k and rise for larger values of k. This is explained by multiple (avoided) crossings of the hyperspherical adiabats. © 1998 American Institute of Physics