Heavy-Atom Tunneling at Room Temperature Revealed by Instanton Theory

Quantum-mechanical tunneling enables molecules to react even when their energy is below the activation barrier. Nonetheless, it is not normally expected that the dominant mechanism of chemical reactions relies on tunneling except at very low temperatures, especially when heavy atoms are involved. This assumption is valid for most adiabatic reactions proceeding on a single potential energy surface. However, we argue that heavy-atom tunneling at room temperature may be more important than previously supposed in the case of nonadiabatic processes, such as spin-forbidden reactions. In this work, we employ a recently developed computational approach (called nonadiabatic instanton theory) to capture the optimal multidimensional tunneling pathway for the ring closing of various strained organic triplet diradicals to form housane and a cage dione (which is a precursor for cubane). Surprisingly, we find that these reactions are dominated by the tunneling of heavy carbon atoms even at room temperature. This highlights the role of tunneling in spin-forbidden reactions and may open up the possibility for tunneling control in the synthesis of strained molecules, not just at cryogenic temperatures, but under ordinary laboratory conditions.