Generation and characterization of protonated silicic acid Si(OH)3+ and its neutral counterpart by tandem mass spectrometry and computational chemistry

Dissociative electron ionization of triethoxysilane, HSi(OC2H5)3, and tetraethoxysilane, Si(OC2H5)4, generates m/z 79 ions of composition [Si, O3, H3]+. From tandem mass spectrometry experiments and CBS-QB3 model chemistry calculations, it is concluded that these ions have the structure of protonated silicic acid, Si(OH)3+ (1a+), for which ΔHf(298K)=−36.3kcalmol−1 (for the anti-isomer). Low energy (metastable) ions 1a+ dissociate via loss of H2O and SiO2 to yield m/z 61 (HOSiO+) and m/z 19 (H3O+) product ions, respectively. These reactions involve isomerization of 1a+ into the stable isomers HOSi(O)OH2+ (1b+), and [H2O⋯H⋯OSiO]+ (1c+), the proton-bound dimer of SiO2 and H2O. The dimer ion lies 77kcalmol−1 higher in energy than 1a+, in contrast to the carbon analogues C(OH)3+ and [H2O⋯H⋯OCO]+ which have comparable heats of formation. From neutralization–reionization experiments it is concluded that the Si(OH)3 radical is a stable species in the gas phase, having a calculated ΔHf(298K)=−191.1kcalmol−1 (for the anti-isomer).