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 $\bf {2 b}$

Chirality control of a single carbene molecule by tip-induced van der Waals interactions

Non-covalent interactions such as van der Waals interactions and hydrogen bonds are crucial for the chiral induction and control of molecules, but it remains difficult to study them at the single-molecule level. Here, we report a carbene molecule on a copper surface as a prototype of an anchored molecule with a facile chirality change. We examine the influence of the attractive van der Waals interactions on the chirality change by regulating the tip-molecule distance, resulting in an excess of a carbene enantiomer. Our model study provides insight into the change of molecular chirality controlled by van der Waals interactions, which is fundamental for understanding the mechanisms of chiral induction and amplification