The [M − 1 ]+ quasi-molecular ion in chemical ionization mass spectrometry. Fragmentation of bis(benzyloxy)silanes by intramolecular reactions

Abstract

The unusual chemical ionization mass spectrometric behavior of bis(benzyloxy)silanes showing heavy fragmentation and an [M − 1]+ fragment, instead of the expected [M + 1]+ ion with isobutane as the reactant gas, was investigated by means of (chloromethyl)bis(benzyloxy)methylsilane and model compounds. It could be shown that probably an intramolecular hydride transfer to an adjacent proton via a six-membered transition state gives rise to the uncommon [M − 1]+ quasi-molecular ion. Similar intramolecular reactions transferring a hydride, a chloro-methyl, a phenyl or a benzyl group via cyclic transition states to a neighboring electrophile are held responsible for five of the six additional major fragments. The results demonstrate that fragmentation reactions become important in chemical ionization mass spectrometry if a molecule or an initially formed cluster ion possesses reactive groups which are in close proximity to each other.