Metastable and collision-induced fragmentation studies and thermochemistryof isomeric C4H11Si+ ions and their adducts with C4H12Si silanes

Metastable Ion (MI) and collision-induced dissociation (CID) mass spectra f or all isomeric even-electron [C4H12Si - H](+) ions were recorded and compa red. Deuterium labeling experiments indicated that most precursors give ris e to silylium ions. Silylium ions with two or more methyl groups are found to lose C2H4 after isomerization via a straightforward hydrogen transfer to the appropriate ethylsilylium ion. Similarly, all isomer ic propyl- and bu tyl-containing silylium ions are found to lose C2H4 by rearrangement preced ing dissociation. In the CI source of the mass spectrometer many of the sil ylium ions are found to cluster with the parent neutral silane present in t he source to give stable ([M - H](+) + M] adduct ions. The MI and CID spect ra of these adduct ions were also obtained. In the MI spectra of all adduct s, except i-BuSiH3, only the starting silylium ion is observed. Under CID c onditions generation of silylium ions dominates. Deuterium labeling studies show that this dissociation may be accompanied by some rearrangement, in p articular for the adducts from i-BuSiH3. High-pressure mass spectrometric c lustering equilibrium measurements were also carried out to determine the e nthalpies and entropies of binding of the silylium ions to the neutral sila nes, These measurements yield insight into the effects of various alkyl gro up substitutions on the association thermochemistry in these adducts, Copyr ight (C) 2000 John Wiley & Sons, Ltd.