APCI low energy collision-induced dissociation fragmentation of protonated ortho silicates: McLafferty or ion-neutral complex rearrangement?

The fragmentation mechanism of tetraethyl ortho silicate and tetrapropyl ortho silicate was studied to determine if the consecutive alkene losses observed in their MS/MS daughter ion spectra were produced via a McLafferty rearrangement or by ion-neutral rearrangement mechanisms. The experiments were carried out using atmospheric pressure chemical ionization and low energy collision-induced dissociation. Deuterium labeling of the γ-position provided evidence mat the rearrangement mechanism of the successive alkene losses proceeds predominantly via the ion-neutral complex mechanism. Because the energies imparted to the ions in this experiment are of the same order of magnitude as solution phase reactions, this mechanism may shed light on the formation of silica gels (e.g., aerogels) in that similar structures have been proposed as reaction intermediates in the polymerization of SiO₂.