Examination of the Valence Tautomers Benzene Oxide and Oxepin and Two Derivative Systems by ab Initio Methods

The enthalpies of the valence tautomerism between benzene oxide 3 and oxepin 4 and of benzene sulfide 5 and thiepin 6 are estimated from post-Hartree−Fock ab initio calculations (QCISD(T)/6-31G*//MP2/6-31G*) to be 0.59 kJ mol-1 and 29.32 kJ mol-1, respectively. The latter value is larger due to a combination of greater stability of the sulfide relative to the oxide and of the relative instability of thiepin compared to oxepin. For the dimethyl analog of the benzene oxide/oxepin system (9 and 10) the ΔH at the same level is −6.73 kJ mol-1. The calculated molecular orbital energies are in linear relationship to those available from photoelectron spectra and suggest reassignment in some cases. The structures of the transition states for the conformational inversion of oxepin and of thiepin are shown to be planar, and the QCISD(T) enthalpies of inversion are 14.5 and 30.5 kJ mol-1, respectively. Barriers to tautomerization are estimated to be 29.4 and 85.7 kJ mol-1, respectively, for the oxide and the sulfide. Protonation stabilizes the oxide form versus the oxepin. The high level of facial selectivity seen in the Diels−Alder reactions of 3 is shown to be consistent with higher angular strain in syn addition.