Condensed phase isomerization through tunneling gateways – Nature

Quantum-mechanical tunneling describes transmission of matter-waves through a barrier whose height is larger than the wave’s energy.¹ Tunneling becomes important when the particle’s de Broglie wavelength exceeds the barrier thickness; since wavelength increases with decreasing mass, lighter particles tunnel more efficiently than heavier ones. How can we then explain examples in condensed-phase chemistry where increasing mass leads to increased tunneling rates?² In contrast to the textbook approach, which considers transitions between continuum states, condensed-phase reactions involve transitions between bound states of reactants and products. Here, this conceptual distinction is highlighted by experimental measurements of isotopomer-specific tunneling rates for CO rotational isomerization at a NaCl surface,^3,4 showing non-monotonic mass-dependence. A quantum rate-theory of isomerization is developed wherein transitions between sub-barrier reactant/product states occur through interaction with the environment. Tunneling is fastest for specific pairs of states (gateways), whose quantum-mechanical details lead to enhanced cross-barrier coupling; the energies of these gateways arise non-systematically, giving an erratic mass-dependence. Gateways also accelerate ground-state isomerization, acting as leaky holes through the reaction barrier. This simple model provides a new way to account for tunneling in condensed-phase chemistry, and indicates that heavy-atom tunneling may be more important than typically assumed.