Protonated silanoic acid HSi(OH)2+ and its neutral counterpart: a tandem mass spectrometric and CBS-QB3 computational study

Protonated silanoic acid, HSi(OH)₂⁺, 1a ⁺, is cleanly generated by the dissociative electron ionization of triethoxysilane, HSi(OC₂H₅)₃, and tetraethoxysilane, Si(OC₂H₅)₄. This follows from tandem mass spectrometric experiments and CBS-QB3 model chemistry calculations. The calculations predict that 1a ⁺ (ΔH_f(298 K) = 205 kJ mol^?1) is separated by high barriers from its isomers HOSiOH₂⁺, 1b ⁺ and HSi(?O)OH₂⁺, 1c ⁺. Low-energy (metastable) ions 1a ⁺ dissociate by loss of H₂O via the pathway 1a ⁺ → 1b ⁺ → SiOH⁺ + H₂O. Analysis of the metastable peak for this process confirms that the isomerization step 1a ⁺ → 1b ⁺ is rate determining. The calculations further predict that the incipient ions 1b ⁺ communicate via a low barrier with the proton-bound dimer Si?O···H···OH₂⁺, 1d ⁺. This dimer ion is much lower in energy than its counterpart O?Si···H···OH₂⁺, 1e ⁺, which is calculated to be only marginally stable. A comparison of the potential energy diagram for the silicon-containing ions 1a ⁺– 1e ⁺ with that of their carbon analogues reveals that the dissociation chemistries of HSi(OH)₂⁺ and HC(OH)₂⁺ are only superficially similar. Neutralization–reionization experiments confirm the theoretical prediction that the HSi(OH)₂^? radical (ΔH_f(298 K) = ?455 kJ mol^?1) is a stable species in the rarefied gas phase. However, owing to a mismatch of Franck–Condon factors a large fraction of the neutralized ions dissociates by loss of H^? yielding Si(OH)₂. Copyright © 2004 John Wiley & Sons, Ltd.