Theoretical study on the unimolecular decomposition of thiophenol

The potential energy surface for the unimolecular decomposition of thiophenol (C ₆H ₅SH) is mapped out at two theoretical levels; BB1K/GTlarge and QCISD(T)/6-311+G(2d,p)//MP2/6-31G(d,p). Calculated reaction rate constants at the high pressure limit indicate that the major initial channel is the formation of C ₆H ₆S at all temperatures. Above 1000 K, the contribution from direct fission of the Si-H bond becomes important. Other decomposition channels, including expulsion of H ₂ and H ₂S are of negligible importance. The formation of C ₆H ₆S is predicted to be strong-pressure dependent above 900 K. Further decomposition of C ₆H ₆S produces CS and C ₅H ₆. Overall, despite the significant difference in bond dissociation, i.e., 8-9 kcal/mol between the Si-H bond in thiophenol and the Oi-H bond in phenol, H migration at the ortho position in the two molecules represents the most accessible initial channel.