DYNAMIC REACTION-PATH STUDY OF SIH4--]SIH4F- AND THE BERRY PSEUDOROTATION WITH VALLEY-RIDGE INFLECTION(F)

Dynamical processes involving the reaction SiH4 + F- --> SiH4F- and th e relation between the incident path of F- and SiH4F- pseudorotation a re studied by employing a dynamic reaction path (DRP) method. The DRPs are analyzed in terms of a function which estimates the distance betw een any geometry and a reference geometry in configuration space. SiH4 F- has only one trigonal bipyramidal minimum, with F axial, and two di fferent transition states (first-order saddle points): a trigonal bipy ramid with F equatorial (TS2) and a square pyramid with F apical (TS1) . This gives rise to a valley-ridge inflection point on the reaction p ath between TS1 and TS2. To investigate dynamical effects of the valle y-ridge inflection on the behavior of SiH4F-, several directions of at tack have been considered. The DRP analyses illustrate that, in contra st to SiH5-, it is difficult to induce pseudorotations in SiH4F-; inst ead, scattering of F- is observed in most cases. Only for side attacks in which C-2v symmetry is preserved does pseudorotation between two e quivalent TS2 structures occur. Lf the initial position of F- deviates even very slightly from this C-2v path, the F- collides just once wit h SiH4, essentially along the C-2 axis. Then, due to the valley-ridge inflection, the system rapidly loses C-2v symmetry, entering the C-s s ubspace. in this case, several pseudorotations occur between two equiv alent axial minima. The S(N)2 reaction, SiH4 + F- --> SiH3F + H-, was not observed, but the abstraction reaction, F- + SiH4 --> HF + SiH3, d oes occur for a back attack. (C) 1998 Elsevier Science B.V. All rights reserved.