Endgroup-assisted siloxane bond cleavage in the gas phase

Unimolecular dissociation of H2N(CH2)(3)SiOSi(CH2)(3)NH3+ generates SiC5H16NO+ and SiC5H14N+. The formation of SiC5H16NO+ involves dissociation of a Si-O bond and formation of an O-H bond through rearrangement. The fragmentation mechanism was investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination with hydrogen/deuterium (H/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) studies of the fully deuterated ion D2N(CH2)(3)SiOSi(CH2)(3)ND3+ provided convincing evidence for a backbiting mechanism which involves hydrogen transfer from the terminal amine group to the oxygen to form a silanol-containing species. Theoretical calculations indicated decomposition of H2N(CH2)(3)SiOSi(CH2)(3)NH3+ through a backbiting mechanism is the lowest energy decomposition channel, compared with other alternative routes. Two mechanisms were proposed for the fragmentation process which leads to the siloxane bond cleavage and the SORI-CID results of partially deuterated precursor ions suggest both mechanisms should be operative. Rearrangement to yield a silanol-containing product ion requires end groups possessing a labile hydrogen atom. Decomposition of disiloxane ions with end groups lacking labile hydrogen atoms yielded product ions from direct bond cleavages.