Dynamic reaction path study of SiH 4+F −→SiH 4F − and the Berry pseudorotation with valley–ridge inflection

Abstract

Dynamical processes involving the reaction SiH 4+F −→SiH 4F − and the relation between the incident path of F − and SiH 4F − pseudorotation are studied by employing a dynamic reaction path (DRP) method. The DRPs are analyzed in terms of a function which estimates the distance between any geometry and a reference geometry in configuration space. SiH 4F − has only one trigonal bipyramidal minimum, with F axial, and two different transition states (first-order saddle points): a trigonal bipyramid with F equatorial (TS 2) and a square pyramid with F apical (TS 1). This gives rise to a valley–ridge inflection point on the reaction path between TS 1 and TS 2. To investigate dynamical effects of the valley–ridge inflection on the behavior of SiH 4F −, several directions of attack have been considered. The DRP analyses illustrate that, in contrast to SiH − 5, it is difficult to induce pseudorotations in SiH 4F −; instead, scattering of F − is observed in most cases. Only for side attacks in which C 2 v symmetry is preserved does pseudorotation between two equivalent TS 2 structures occur. If the initial position of F − deviates even very slightly from this C 2 v path, the F − collides just once with SiH 4, essentially along the C 2 axis. Then, due to the valley–ridge inflection, the system rapidly loses C 2 v symmetry, entering the C s subspace. In this case, several pseudorotations occur between two equivalent axial minima. The S N2 reaction, SiH 4+F −→SiH 3F+H −, was not observed, but the abstraction reaction, F −+SiH 4→HF+SiH − 3, does occur for a back attack.