Zhang et al. employed theoretical methods to predict that the degenerate Cope rearrangement of semibullvalene could proceed rapidly at temperatures as low as 40 K due to carbon tunneling. Their DFT calculations of rate constants, including small-curvature tunneling corrections, suggested a rate constant of 1.43 × 10 -3 s -1 at 40 K, leading to a half-life of approximately 8 minutes for the reaction (Figure ). Without tunneling, the process would be over ten orders of magnitude slower. The study proposed that an isotopically labeled semibullvalene could be a test case to experimentally confirm the effect. dimethylsemibullvalene isotopomers and observed their rearrangement at 3 K. As predicted, the less stable isotopomer converted to the more stable one, even in the dark, with a rate constant of about 10 -4 s -1 . Given the experimentally determined activation barrier of 4.8 kcal/mol, such a rate is too fast for classical thermal activation at these temperatures, thus confirming heavy-atom tunneling. The study further demonstrated that vibrational excitation via IR light could modulate the tunneling process, reinforcing the quantum mechanical nature of the reaction.