The Mass Spectra of Some Hexaalxyl Disiloxanes

The electron impact induced fragmentations of some hexaalkyl disiloxanes have been studied with respect to the effect of the size and the positions of different alkyl substituents at the Siatoms. Positional isomers of hexaalkyl disiloxanes differing by the distribution of alkyl groups over both Si-atoms can be easily distinguished by large intensity differences of fragment ions in the 70eV EI-mass spectra. D-labelling studies and MIKE-spectra of relevant ions show that this effect is due to an unusual preference for the loss of a small alkyl group flanked by two larger ones at the same Si-atom from molecular ions. The results also indicate that after the initial loss of an alkyl radical the structural identity of the fragment ions is lost probably by migrations of alkyl groups. INTRODUCTION Hexaalkyl disiloxanes are simple members of the large family of organ0 siloxanes. To establish the analytical application of mass spectrometry for these technical important compounds the mass spectra of some hexaalkyl disiloxanes and related compounds have been investigated before (ref. l-5). The most important fragmentation route corresponds to loss of an alkyl radical followed by consecutive eliminations of CnH2n in analogy to the mass spectra of aliphatic ethers (ref. 6). However, elimination of CnH2n in pentaalkyl disiloxane ions may occur either by a four membered transition state as proposed for triethyl silyl