Reaction of silicon ion (2P) with silane (SiH4, SiD4). Heats of formation of SiH/sub n/, SiH/sub n/+ (n = 1, 2, 3), and SiH/sub n/+ (n = 0, 1, 2, 3). Remarkable isotope exchange reaction involving four hydrogen shifts

The reaction of ground-state silicon ion with silane is investigated by using a guided ion beam tandem mass spectrometer. Reaction cross sections of all possible fragments, Si/sub m/H/sub n/+ (m = 1, 2; n = 0, 1, 2, 3), as a function of relative kinetic energy are determined. All thermal energies, the major product is Si2H2+. One remarkable reaction, the interchange of the projectile silicon ion with the target silicon atom, is observed at near zero kinetic energy. Labeling experiments employing 30Si+ for the beam or SiD4 for the reactant indicate the intermediacy of the disilicon hydrides for the formation of the observed products. From the endothermicities of several reactions, the 298 K heats of formation for several ionic and neutral silicon hydrides are derived: ΔH/sub f/0(SiH) = 90.0 +/- 1.7, ΔH/sub f/0(SiH2) = 69.0 +/- 2, ΔH/sub f/0(SiH3) = 48.5 +/- 1.6, ΔH/sub f/0(SiH2+) = 276.1 +/- 1.7, ΔH/sub f/0(SiH3+) = 237.1 +/- 2, ΔH/sub f/0(Si2+) ≤ 328.0 +/- 2, ΔH/sub f/0(Si2H+) ≤ 304.4 +/- 1.6, ΔH/sub f/0(Si2H2) ≤ 268.0 +/- 2.6, ΔH/sub f/0(Si2H3+) = 266 +/- 2, all in kcal/mol. From an evaluation of these and other experiments, values of ΔH/sub f/0(SiH2+) and ΔH/sub f/0(SiH2) of 278.0 +/- 1.4 and 68.5 +/- 1.5 kcal/mol, respectively, are recommended