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, 1 rearranges to 2 via tunneling with a rate constant of approximately 5.3×10(−5) s(−1). Thus, in a nonpolar environment, 2 is slightly more stable than 1, in agreement with calculations at the CCSD(T) level of theory. However, if the argon is doped with 1 % of H(2)O or CF(3)I as typical hydrogen or halogen bond donors, respectively, weak complexes of 1 and 2 are formed, and now 2 is tunneling back to form 1. Thus, by forming non‐covalent complexes, 1 becomes slightly more stable than 2 and the direction of the heavy‐atom tunneling is reversed.