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Tim Schleif, Jörg Tatchen, Julien F. Rowen, Frederike Beyer, Elsa Sanchez-Garcia, Wolfram Sander

• 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 \(\bf {2 b}\).