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The electron-impact-induced decomposition of the title compounds has been investigated in detail. Deuterium labelling was used to study the mechanisms of fragmentation and the structures of ions generated. Hydrogen, methyl and phenyl group migrations to one of the silicon atoms were observed as significant secondary processes. These processes are usually y-transitions, from which it is inferred that these rearrangements occur via 4-membered cyclic transition complexes. I N T R O D U C T I O NDESPITE the fact that over the past ten years the mass spectra of numerous compounds containing Si-N bonds, mainly those of trimethylsilylated amines (i.e. compounds having one TMS group in each amino group) have been published, very little has been reported on the mass spectrometric behaviour of disilazanes.2-8The interesting fragmentation characteristics of these compounds and their great ability to rearrange could have been deduced from the mass spectra of hexamethyldisilazane (1)2-5 and N-methylhexamethyldisilazane (II)?.' but the mechanisms of the rearrangements and the structure of the product ions could not be defined unambiguously except by isotope labelling or comparative investigations of various derivatives. In the present paper the results of a detailed study of the electronimpact-induced fragmentation of hexamethyldisilazane, its several N-alkyl derivatives (I to IV) and D-labelled analogues, together with other N-substituted derivatives (V to IX), are presented.

The phosphabicyclohexane derivatives (I), obtained by treatment of dihydrophosphole oxides with dichlorocarbene, react with water in the presence of silver nitrate, yielding mixtures of the diastereomeric tetrahydrophosphorins (II) and (III) and their regioisomers (IV) and (V).

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