potential energy surface ͉ reaction dynamics T he AϩBC type reactions have long served as benchmarks in the development of the kinetics and dynamics theories of chemical reactions (1-5). However, behavior of polyatomic reactions may be qualitatively different from what has been deduced from studies of atom-diatom dynamics. For example, a major reaction path, which does not follow the intrinsic reaction coordinate, has recently been found by Hase et al. (6) in their quasiclassical trajectory (QCT) study on the F Ϫ ϩCH 3 OOH reaction. Another example is the reaction of HϩCH 4 (and its isotopic variants), for which recent investigations (7) have focused on the new stripping mechanism observed by Zare and coworkers (8-9). The rebound mechanism is well-known for the HϩD 2 reaction (10-11), and a lot of polyatomic H abstraction reactions, including HϩCD 4 , have long been considered to proceed through this mechanism. However, in the recent combined experimental and theoretical studies on the reaction of HϩCD 4 (9, 12), the stripping mechanism was proposed, in which the velocity of incoming H atom is perpendicular to the C-D bond and the HD product is carried into the forward hemisphere. The stripping mechanism was further confirmed by Bowman and coworkers (13) using the QCT method.The HϩSiH 4 reaction is an analogue to HϩCH 4 , and both abstraction and exchange reactions can happen; i.e., Unlike the HϩCH 4 abstraction reaction, which is nearly thermoneutral, the HϩSiH 4 abstraction reaction R1 is exothermic by Ϸ13 kcal/mol, and regarded as a prototype of exothermic polyatomic H abstraction reactions. Also, as we will demonstrate, both reactions 1 (R1) and 2 (R2) could happen at collision energies above Ϸ12.5 kcal/mol, therefore the HϩSiH 4 reaction is actually a better candidate for studying the competition between abstraction and exchange than HϩCH 4 , for which the exchange channel is not open at collision energies Ͻ35 kcal/mol (noninversion exchange not open at Ͻ60 kcal/mol) (14). AbstractionIn this work, a global 12-dimensional potential energy surface (PES) that describes both abstraction and exchange reactions for the SiH 5 system is constructed, and detailed QCT calculations for both reactions R1 and R2 on this ab initio PES are performed, which yield insights into the reaction mechanism. The computed product angular distributions indicate that the abstraction reaction is a combination of rebound and stripping. More importantly, here we propose that the HϩSiH 4 exchange reaction is a combination of three mechanisms, inversion, torsion (more exactly, torsion-tilt), and side-inversion as demonstrated in Fig. 1. The interesting dynamical features for exchange can be explained with these atomic-level mechanisms. These findings are helpful for our understanding of the nature of polyatomic reactions, which goes beyond the atom-diatom pictures, and would have implications for a number of fields ranging from fundamental reaction dynamics to atmospheric and organic chemistry.The HϩSiH 4 reaction is important in the the...

The SiH 4 +H→ SiH 3 +H 2 reaction has been investigated by the quasiclassical trajectory ͑QCT͒ method on a recent global ab initio potential energy surface ͓M. Wang et al., J. Chem. Phys. 124, 234311 ͑2006͔͒. The integral cross section as a function of collision energy and thermal rate coefficient for the temperature range of 300-1600 K have been obtained. At the collision energy of 9.41 kcal/ mol, product energy distributions and rovibrational populations are explored in detail, and H 2 rotational state distributions show a clear evidence of two reaction mechanisms. One is the conventional rebound mechanism and the other is the stripping mechanism similar to what has recently been found in the reaction of CD 4 +H ͓J. P. Camden et al., J. Am. Chem. Soc. 127, 11898 ͑2005͔͒. The computed rate coefficients with the zero-point energy correction are in good agreement with the available experimental data.