The mystery of the benzene-oxide/oxepin equilibrium

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 H2O or CF3I 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

Heavy-atom tunneling in semibullvalenes

The Cope rearrangement of selectively deuterated isotopomers of 1,5-dimethylsemibullvalene $\bf {2 a}$ and 3,7-dicyano-1,5-dimethylsemibullvalene $\bf {2 b}$ were studied in cryogenic matrices. In both semibullvalenes the Cope rearrangement is governed by heavy-atom tunneling. The driving force for the rearrangements is the small difference in the zero-point vibrational energies of the isotopomers. To evaluate the effect of the driving force on the tunneling probability in $\bf {2 a}$ and $\bf {2 b}$, two different pairs of isotopomers were studied for each of the semibullvalenes. The reaction rates for the rearrangement of $\bf {2 b}$ in cryogenic matrices were found to be smaller than the ones of $\bf {2 a}$ under similar conditions, whereas differences in the driving force do not influence the rates. Small curvature tunneling (SCT) calculations suggest that the reduced tunneling rate of $\bf {2 b}$ compared to that of $\bf {2 a}$ results from a change in the shape of the potential energy barrier. The tunneling probability of the semibullvalenes strongly depends on the matrix environment; however, for $\bf {2 a}$ in a qualitatively different way than for $\bf {2 b}$