Fragmentation of Silaspiroalkanes in EI Mass Spectrometry.

Abstract

The fragmentations of six silaspiroalkanes combined with four-, five-, and six-membered rings in EI mass spectrometry have been studied by a B/E linked scanning technique and an ab initio MO method. The main fragmentation process from the molecular ion is the consecutive losses of two ethylene molecules which occur via the cleavage of an Si-C bond in each ring of silaspiroalkanes. From the ab initio MO calculation, we found that the initial bond fission occurred in an Si-C bond of the molecular ion. At that time, the cleavage of an Si-C bond occurs in the smaller ring more easily than in the larger one, since the strain on the smaller ring is larger. We found the metastable decomposition due to a CH3 loss from the molecular ions of 4-silaspiro[3.5]nonane, 5-silaspiro[4.4]nonane, and 4-silaspiro[3.4]octane.