Metastable ion study of organosilicon compounds. Part III—trimethoxyphenylsilane

Abstract

The unimolecular decomposition of trimethoxyphenylsilane (1) was investigated by mass-analysed ion kinetic energy (MIKE) spectrometry, deuterium-labelling studies and from high resolution data. The characteristic fragmentations of metastable molecular ion of 1 were losses of C6H6 and C,H,' with rearrangements. Almost complete H/D scrambling occurred in the molecular ion prior to these decompositions. The other important fragmentation routes corresponded to expulsions of CH,O' and C6H,'. These fragmentations were followed by consecutive elimination of an H,CO molecule, as commonly observed in the mass spectra of alkoxysilanes. In these fragmentation processes, H/D scrambling increased as the internal energy of the molecular ion was lowered. The fragmentations of 1 were compared with those of its carbon analogue, a,a,a-trimethoxytoluene.

Gas‐Phase Ion Chemistry of Silicon‐Containing Molecules†This article is dedicated to Saul Patai.

“…It can be generated via electron-impact ionization of tetramethoxysilane 183 . The genesis of this distonic ion presumably takes place via a sequence of unimolecular hydrogen and formaldehyde losses a metastable behavior that has been observed previously in methoxy-substituted silane cations 184 . Neutralization of ž CH 2 OSi C leads to the stable ž CH 2 OSi ž diradical which, in accordance with ab initio calculations (MP2/6-31G ŁŁ ), corresponds to a local minimum on the [H 2 ,C,O,Si] hypersurface 184 .…”

“…These findings presumably reflect the well-documented inability of silicon to form -bonds with significant strength. Tajima, Okada and coworkers 247 reported the unimolecular metastable decompositions of alkoxysilanes in a number of recent studies 184,248,249 . These authors found significant differences in the ion fragmentation characteristics between the silanes and their carbon analogues.…”

Metastable ion study of organosilicon compounds. Part XIII: dimethoxydimethylsilane, (CH3)2Si(OCH3)2, and dimethoxymethylsilane, CH3SiH(OCH3)2

“…3(b) are generated by the loss of CD 2 O and C 2 H 2 D 2 respectively. The former reaction is a typical one in the mass spectra of methoxysilane,1, 2, 4–6 as described above, and the resulting product ion at m / z 59 in Fig. 4(a) is (CH 3 ) 2 SiH + , not CH 3 OSi + .…”

“…The loss of formaldehyde is commonly observed in the MIKE spectra for methoxysilanes,1, 2, 4–6 but was not observed in that of the ion at m / z 89, CH 3 C + (OCH 3 ) 2 , from the corresponding carbon analogue, 3 +· and 4 +· 8, 9. The ion at m / z 89 in the carbon analogue eliminates predominantly a CH 3 OCH 3 in the metastable time window 8, 9.…”

“…Unimolecular fragmentations of ionized organosilicon compounds upon electron impact have been studied extensively 1, 2. The fragmentations of several alkoxysilanes, including trimethoxy‐ and tetramethoxy‐silanes, have also been reported 3–6. To the authors' knowledge, however, the fragmentation pathway of the ionized dimethoxydimethylsilane ((CH 3 ) 2 Si(OCH 3 ) 2 , MW 120, 1 ), which is one of the simple methoxysilanes, was only reported three decades ago, without deuterium labeling 4.…”