EXAMINATION OF THE VALENCE TAUTOMERS BENZENE OXIDE AND OXEPIN AND 2 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 pos t-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 val ue is larger due to a combination of greater stability of the sulfide relative to the oxide and of the relative instability of thiepin compa red to oxepin. For the dimethyl analog of the benzene oxide/oxepin sys tem (9 and 10) the Delta H at the same level is -6.73 kT mol(-1). The calculated molecular orbital energies are in linear relationship to th ose available from photoelectron spectra and suggest reassignment in s ome cases. The structures of the transition states for the conformatio nal inversion of oxepin and of thiepin are shown to be planar, and the QCISD(T) enthalpies of inversion are 14.5 and 30.5 kT mol(-1), respec tively. Barriers to tautomerization are estimated to be 29.4 and 85.7 kJ mol(-1), respectively, for the oxide and the sulfide. Protonation s tabilizes the oxide form versus the oxepin. The high level of facial s electivity seen in the Diels-Alder reactions of 3 is shown to be consi stent with higher angular strain in syn addition.