,2205 (1994). ' The unimolecular metastable ion and low-energy collision-induced fragmentation reactions of alkyl ammonium ions R,,H,.,,Nf (tz = 2 to4, R = tz-C3H7 and n-C,H9) have been studied and breakdown graphs expressing the % fragment ion abundances as a function of the centre-of-mass collision energy have been established. The dialkyl and trialkyl ammonium ions fragment primarily by olefin ([R -HI) elimination or by formation of the alkyl ion Rf (with rearrangement to the more stable secondary structure). By contrast the tetraalkyl ammonium ions show both elimination of [R -HI and elimination of an alkane, which is C,H12 for the propyl compound C7H,6 for the butyl compound. The results are interpreted in terms of competition between heterolytic bond cleavage to form a [Rf---NH,.,,R,,.,] complex and homolytic bond cleavage to form a [R,,-,H,.,,Nf.---'R] complex. The former complex fragments either to form R+ or undergoes internal proton transfer to form R,,-,H,.,,NH+ + [R -HI, while the latter complex fragments by alkane elimination resulting from attack of R' on the a-CC bond of the alkyl group. For the propyl ammonium ions plausible potential energy profiles are established which show that the former complex is favoured for n = 2, 3 while the two complexes have similar energies for n = 4. [Traduit par la rCdaction]

The unimolecular (metastable ion) and collision-induced fragmentation reactions of the trimethylsilyl cation adducts of six C6 ketones have been studied by tandem mass spectrometry. A major unimolecular fragmentation channel involves regeneration of the trimethylsilyl cation; other fragmentation channels involve elimination of (CH3)3SiOH, C5HIo, CJI, and C3H6 from the adduct ions. Some mechanistic detail has been obtained from a study of the adducts with deuterium-containing ketones, and the reactions observed are compared with the fragmentation reactions of the protonated ketones. Collision-induced dissociation (CID) of the adduct ion results in predominant formation of (CH3)$i+. However, in the high energy (6 keV) CID new minor reaction channels, involving elimination of alkanes, CHI to CJIlO, become accessible. It is concluded that tandem mass spectrometry of the trimethylsilyl adducts provides only limited ability to distinguish between isomeric ketones.Although the existence of silylium ions (or silyl cations) in the condensed phases is uncertain,' there is no doubt about their existence as stable entities in the gas phase. In particular, the chemistry of the trimethylsilyl cation has been extensively studied in the gas phase. It is well established that electron impact on pure tetramethylsilane,* tetramethylsilane in admixture with isobutane or nitrogen3 or tetramethylsilane/methane mixtures4 at pressures appropriate for chemical ionization (CI) leads to dominant formation of (CH3)$i+ and the reactions of this ion with a variety of substrates have been studied using CI techniques;'-" in addition, the reactions of (CH,),Si+ with a variety of organic molecules have been studied by Bowie and c o -~o r k e r s~~-'~ using ion cyclotron resonance (ICR) techniques. With compounds containing n-or n-electrons (Lewis bases), adduct [M + (CH3)3Si]+[M+73]+ ions frequently are observed as the major reaction products. Consequently, chemical ionization using the trimethylsilyl cation as reagent has become a useful technique for establishing the molecular masses of organic substrates. 16 However, in many cases few fragment ions are formed which would allow structural characterization. For example, both Blair and BowieI3 and Clemens and Munson4 have reported that the trimethylsilyl adducts of ketones undergo no fragmentation reactions. We report here a study of the unimolecular (metastable ion) and collision-induced fragmentation reactions of the trimethylsilyl adducts of C6 ketones using the more sensitive techniques of tandem mass ~pectrometry.'"~~ This study was aimed, in part, at determining whether the fragment-ion spectra observed would be useful in assigning molecular structure and, in particular, in distinguishing among isomeric molecules. This objective have only partly been achieved. A major fragmentation reaction of the adducts, particularly in CID experiments, was observed to involve regeneration of the trimethylsilyl cation, a fragmentation reaction 'Taken, in part, from the M.Sc. Thesis of N. L. Bosma, Univers...