GAS-PHASE NUCLEOPHILIC REACTIONS IN TETRAALKOXYSILANES

The gas-phase reactions of F-, O--., OH-, MeO(-), EtO(-), NH2- and EtNH(-) with Si(OMe)(4) and Si(OEt)(4) were investigated by Fourier transform ion cyclotron resonance in order to probe the intrinsic reactivity of substrates used in plasma chemical vapour deposition. Isotopic data and the nature of the product ions suggest that reactions proceed primarily by attack of the nucleophile on the silicon centre. The ensuing chemistry can be explained by the fragmentation pathways available to the [XSi(OR)(4)](-) intermediate, which includes alkoxide displacement, internal nucleophilic displacement and cyclic elimination of ethylene in the ethoxy case. Pentacoordinated adducts are observed in the reactions with F- and in the symmetric alkoxide/tetraalkoxysilane reactions. The reaction of F- with Si(OMe)(4) is also shown to produce a perplexing ion of unknown structure and identified as [FSi(OMe)(2)(OCH2)](-).