The Mystery of the Benzene-Oxide/Oxepin Equilibrium-Heavy-Atom Tunneling Reversed by Solvent Interactions

The equilibrium between benzene oxide (1) and oxepin (2) is of large importance for understanding the degradation of benzene in biological systems and in the troposphere. Our studies reveal that at cryogenic temperatures, this equilibration is governed by rare heavy-atom tunneling. In solid argon at 3 K,1rearranges to2via tunneling with a rate constant of approximately 5.3x10(-5) s(-1). Thus, in a nonpolar environment,2is slightly more stable than1, in agreement with calculations at the CCSD(T) level of theory. However, if the argon is doped with 1 % of H2O or CF3I as typical hydrogen or halogen bond donors, respectively, weak complexes of1and2are formed, and now2is tunneling back to form1. Thus, by forming non-covalent complexes,1becomes slightly more stable than2and the direction of the heavy-atom tunneling is reversed.