Fragmentation reactions of Si2Cl6+[rad] in the gas phase—A quantum-chemical and mass-spectrometric assessment

Abstract

The gas-phase ion chemistry of the hexachlorodisilane radical cation Si2Cl6+ has been reinvestigated by a combined electron ionization (EI) mass spectrometry (MS) and high-level quantum chemistry approach. The computational investigation of isomerization and fragmentation pathways of [Si2Cln]+() (n=1–6) species reveals (a) the coexistence of structures of the type Cl3Si–SiClm+1+() and chlorobridged SiCl4–SiClm+() isomers for m=1,2, and (b) a wealth of nearly isoenthalpic structures[Si2Cln]+() for each n with n=2–4, which can result from degenerate rearrangements. This structural diversity appears to be a general feature of the disilicon (radical) cations with low chlorine content. The ionization energy of Si2Cl6 determined by EI-MS (10.1±0.2stat±0.2systeV) with argon (15.759±0.001eV) used as reference, is slightly lower than previously reported experimental values, but in good agreement with the CBS-QB3 (9.87eV) and W1U (9.97eV) adiabatic ionization energy. Measured appearance energies for the lowest two fragmentation channels, which afford SiCl2+ and SiCl3+, are well in line with CBS-QB3 values and with the literature value determined for SiCl3+ in threshold photoelectron–photoion coincidence experiments. Using the experimental heats of formation (ΔfH°298) for SiCl4 and Cl· as anchor points, W1U theory was employed to derive ΔfH°298 values for Si2Cln+() with n=0–6 and for SiClm+() with m=1–4 based on the lowest energy isomer identified in our quantum chemical investigation of reaction pathways for all species investigated. W1U heats of formations obtained for select neutrals lead to a more consistent description of kinetic aspects previously inferred from metastable ion mass spectra.