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(OC 2 H 5 ) 3 , and tetraethoxysilane, Si(OC 2 H 5 ) 4 . This follows from tandem mass spectrometric experiments and CBS‐QB3 model chemistry calculations. The calculations predict that 1a + (Δ H f (298 K) = 205 kJ mol −1 ) is separated by high barriers from its isomers HOSiOH 2 + , 1b + and HSi(O)OH 2 + , 1c + . Low‐energy (metastable) ions 1a + dissociate by loss of H 2 O via the pathway 1a + → 1b + → SiOH + + H 2 O. 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 SiO···H···OH 2 + , 1d + . This dimer ion is much lower in energy than its counterpart OSi···H···OH 2 + , 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 (Δ H 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.