PHOTOIONIZATION MASS-SPECTROMETRIC STUDIES OF THE METHYLSILANES SI(CH3)NH4-N (N = 0-3)

Abstract

Photoionization efficiency curves for the low energy fragment ions (M - H)+, (M - H2)+, (M - CH3)+, and (M - CH4)+ for the series of methyl substituted silanes Si(CH3)nH4-n (n = 0-3) are reported. The molecular ions were undetectable except SiH4+. (M - H2)+ and (M - CH4)+ ions show sharp appearance onsets compared with (M - H)+ ions, which have distinct threshold curvature. (M - H2)+ ions are ascribed to silylene positive ions, SiR2+. The alternative silaethylene positive ion structure, CH2SiHR+, is unlikely because the fragmentation process yielding CH2SiD2+ with loss of HD is not observed in the photoionization mass spectrum of CH3SiD3. Thresholds are interpreted in terms of the thermochemistry of the various ionic and neutral silicon species and afford accurate calculation of hydride affinities of the silylene positive ions. The calculated hydride affinities for silylene positive ions are 263.4, 244.3 and 230.6 kcal mol-1 for SiH2+, SiMeH+ and SiMe2+, respectively. Within experimental error, the hydride affinities of the silylene positive ions are identical to those of the silicenium ions with the same number of methyl groups. The present results, combined with other available thermochemical data, lead to the critical assessment of the Si-H and Si-CH3 bond energies of the neutral and ionic fragments of methylsilanes, as well as ionization potentials of the silyl radicals and silylenes.