Dynamics of the reaction C(P3)+SiH4: Experiments and calculations

Abstract

We conducted the reaction C((3)P)+SiH(4) at a collision energy of 4.0 kcal mol(-1) in a crossed molecular-beam apparatus measuring time-of-flight mass spectra and selective photoionization. Product ions with m/z=41-43 are associated with two product channels, H(2)SiCH/HSiCH(2)/SiCH(3)+H and H(2)SiC/HSiCH/SiCH(2)+H(2). Apart from daughter ions and isotopic variants of reaction products, the species observed at m/z=43 is assigned to product H(2)SiCH/HSiCH(2)/SiCH(3) and that at m/z=42 to product H(2)SiC/HSiCH/SiCH(2). The signals observed at m/z=41 are due to dissociative ionization of silicon-carbon hydrides of these two types. We report time-of-flight spectra of products at specific laboratory angles and theoretical simulations, from which both kinetic-energy and angular distributions of products in the center-of-mass frame were derived. The release of kinetic energy is weakly dependent on the scattering angle for these two reactions. The channels for loss of H and H(2) release average translational energies of 10.5 and 16.7 kcal mol(-1), respectively. As hydrogen transfer before decomposition is facile, products H(2)SiCH/HSiCH(2)/SiCH(3) and H(2)SiC/HSiCH/SiCH(2) exhibit mildly forward/backward preferred and isotropic angular distributions, respectively. We estimate the branching ratios of these channels for loss of H and H(2) to be roughly 6:4. The measurements of release of kinetic energy and ionization thresholds of products indicate that SiCH(3)((2)A(")) and SiCH(2)((3)A(2)) are dominant among isomeric products. To explore the reaction mechanism, we computed the potential-energy surfaces for the reaction C((3)P)+SiH(4). The most likely mechanism is that atom C (3)P inserts into bond Si-H of SiH(4) in the entrance channel, and the reaction complex H(3)SiCH subsequently isomerizes to HSiCH(3) followed by decomposition to SiCH(3)((2)A("))+H and SiCH(2)((3)A(2))+H(2). We observed no significant evidence for the reaction C((1)D)+SiH(4).