Investigation of fragmentation pattern of bis(trimethylsilyl) ester of 3-mercaptopropionic acid

The routes of fragmentation of the parent molecule resulting from electron impact ionization have been studied in a quadrupole ion trap using collision induced dissociation (CID) of fragments. Fragmentation of the parent molecular ion m/z 250 results in major ions 235, 178, 163, 135, 129, 75, 73, and 55. While some fragments are easily assigned according to literature - m/z 235 to [M - CH₃]⁺, m/z 73 to [Si(CH₃)₃] ⁺, m/z 75 to [Si(CH₃)₂OH]⁺, and m/z 55 to [CH₂CHCO]⁺ - other ions required CID to obtain their structure. Fragment m/z 178 was found to emerge from breaking of Si-S bond and hydrogen rearrangement in the molecular ion. Fragment m/z 163 is formed by elimination of CH₃ group from ion m/z 178. Elimination of H ₂S from ion m/z 163 leads to formation of the ion m/z 129. Another route is elimination of molecule CO from ion m/z 163 to form ion m/z 135. Fragmentation of ion m/z 129 by elimination of neutral molecule CO₂ leads to emerging of ion m/z 85. For supporting the assumed structure of ions m/z 178, 163, 135, quantum chemical calculations were performed using methods PM3 and DFT. The calculations demonstrated that these ions have TMS group attached to the carboxyl group. The stability of the ions is associated with forming cyclic structures containing new bonds S-O (m/z 178), O-Si (m/z 163) and S-Si (m/z 129). Molecular dynamics calculations confirmed that these structures are the most stable conformations of the ions.