Heavy-Atom Tunneling at Room Temperature

Quantum-mechanical tunneling enables molecules to react through high-energy barriers. In chemistry, the effects of tunneling are normally assumed to be negligible except at very low temperatures, especially when heavy atoms are involved. This assumption is typically justified using one-dimensional theories of tunneling based on strong simplifying approximations. In this work, we employ a more advanced and recently developed computational approach (called nonadiabatic instanton theory) to capture the true multidimensional tunneling pathway. Surprisingly, we find that the tunneling of heavy carbon atoms significantly speeds up the spin-forbidden ring closing of various strained organic radicals to form housanes and caged diones (which are direct precursors for cubanes) even at room temperature. This highlights the importance of tunneling in spin-forbidden reactions and opens up possibilities for tunneling control in the synthesis of strained molecules, not just at cryogenic temperatures, but under ordinary laboratory conditions.