Theoretical Studies of the Benzene Oxide-Oxepin Valence Tautomerism

We have calculated the geometry and energy of the valence tautomers benzene oxide and oxepin using the semiempirical AM1 model and the 6-31G and 6-31G* basis sets utilizing full geometry optimization. In the oxide the folding angle alpha, the angle between the epoxide ring and the adjacent plane containing four carbon atoms, is about 106 degrees, The carbon skeleton is almost planar, the folding angle beta, the angle between the two four-carbon atom planes being about 175 degrees. In contrast, oxepin is found to have a marked boat-shaped structure with the corresponding alpha and beta angles about 137 degrees and 159 degrees, respectively. The AM1, 6-31G, and 6-31G* calculations give -11.4, -10.8, and -2.9 kcal mol(-1) for the energy change that accompanies the valence tautomerism, oxide-oxepin, compared to an experimental value of about + 0.3 kcal mol(-1). Single point calculations of the energies at the 6-31 G* geometry using Moller-Plesset perturbation theory to second order (MP2/6-31G*) and third order (MP3/6-31G*) give Delta E-T = + 3.3 and + 0.8 kcal mol(-1). The values for the energy change in the transfer of epoxide oxygen from ethylene oxide to benzene using AM1, 6-31G, and 6-31G* are in good agreement, viz., + 31.1, +34.5, and +33.6 kcal mol(-1), respectively. A large positive energy change is to be expected in view of the loss of benzene aromaticity.