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Abstract
The mass spectra of hexamethylcyclotrisiloxane, octamethylcylo-tetrasiloxane, decamethylcyclopentasiloxane and dodecamethylcyclohexasiloxane vary with ring size. These compounds are occasionally seen to elute unexpectedly from GLC columns under temperature programmed conditions. The two larger ring compounds exhibit intense M-103 (Me + SiMe4) and m/e 73 (SiMe3 +) ions, whereas the two smaller rings yield intense doubly-charged M-30 ions (loss of two Me). A transannular mechanism is proposed to account for the behavior of the larger ring methylcyclosiloxanes. This mechanism is sterically precluded with the smaller rings which follow the alternate pathway. Parallels are drawn to the contrasting behavior of TMSi derivatives of vicinally and non-vicinally disubstituted aromatics.