Combined quantum chemical and mass spectrometric study of [Si,C,H3,O]+ isomers

Abstract

The potential energy surface of [Si,C,H₃,O]₊ has been explored by means of ab initio MO calculations at the G₂ level of theory as well as by mass spectrometric techniques. The silicon–methoxide cation H₃COSi₊ has been identified as the global minimum (Δ_fH= 151 kcal mol^–1), followed by the silaacetyl cations H₃SiCO⁺(Δ_fH= 172 kcal mol^–1) and H₃CSiO⁺(Δ_fH= 180 kcal mor^–1). A number of other intermediates, the transition structures associated with the mutual interconversion reactions and the energetics of possible fragmentation channels are explored computationally. It turns out that the energy demands for the unimolecular isomerizations of most of the isomers are substantially lower than those of fragmentations. Consequently, an identification of the different isomers based solely on mass spectrometric information is made difficult by intramolecular rearrangements which precede the structure-indicative fragmentation reactions, even though H₃SiCO⁺ is clearly distinguishable from H₃CSiO⁺ and H₃COSi⁺. However, when the experimental findings are combined with the computationally predicted [Si,C,H₃,O]⁺ potential-energy surface and with thermochemical information, this difficulty can be overcome, and a coherent picture of this complex system emerges. In addition, the existence of the neutral radicals H₃COSi˙, H₃CSiO˙, as well as H₃SiCO˙ and/or H₃SiOC˙ is explored by means of neutralization–reionization mass spectrometry.