Photoionization mass spectrometric studies of the methylsilanes Si(CH3)nH4−n (n=0-3)

Size effects on cation heats of formation. III. Methyl and ethyl substitutions in group IV XH₄, X = C, Si, Ge, Sn, Pb

The heats of formation of alkyl-substituted XH4 (X = C, Si, Ge, Sn, Pb) ΔfH(M(+)) = IEad(M) + ΔfH (M) molecular ions have often never been measured or are not known to better than 10 or 20 kJ/mol. The present study on these Main Group IV compounds adopts a graphical approach that relates cation heats of formation to cation size. A study of methyl substitution in XH4 is followed by a similar examination of heat of formation data on ethyl substitution in methane, silane, germane, stannane, and plumbane. The results provide tests of the validity of this graphical method as well as its use as a predictive tool for determining cation and neutral heats of formation. Suggestions are made of the need to investigate or reinvestigate the ionization energies and/or the heats of formation of several of the molecules studied. Results and conclusions are tabulated.

Metastable and collision-induced fragmentation studies and thermochemistry of isomeric C(4)H(11)Si(+) ions and their adducts with C(4)H(12)Si silanes

Metastable Ion (MI) and collision-induced dissociation (CID) mass spectra for all isomeric even-electron [C(4)H(12)Si - H](+) ions were recorded and compared. Deuterium labeling experiments indicated that most precursors give rise to silylium ions. Silylium ions with two or more methyl groups are found to lose C(2)H(4) after isomerization via a straightforward hydrogen transfer to the appropriate ethylsilylium ion. Similarly, all isomeric propyl- and butyl-containing silylium ions are found to lose C(2)H(4) by rearrangement preceding dissociation. In the CI source of the mass spectrometer many of the silylium ions are found to cluster with the parent neutral silane present in the source to give stable [M - H](+)+M adduct ions. The MI and CID spectra of these adduct ions were also obtained. In the MI spectra of all adducts, except i-BuSiH(3), only the starting silylium ion is observed. Under CID conditions generation of silylium ions dominates. Deuterium labeling studies show that this dissociation may be accompanied by some rearrangement, in particular for the adducts from i-BuSiH(3). High-pressure mass spectrometric clustering equilibrium measurements were also carried out to determine the enthalpies and entropies of binding of the silylium ions to the neutral silanes. These measurements yield insight into the effects of various alkyl group substitutions on the association thermochemistry in these adducts. Copyright 2000 John Wiley & Sons, Ltd.