Tunneling above the crossover temperature.

Quantum mechanical tunneling of atoms plays a significant role in many chemical reactions. The crossover temperature between classical and quantum movement is a convenient preliminary indication of the importance of tunneling for a particular reaction. Here we show, using instanton theory, that quantum tunneling is possible significantly above this crossover temperature for specific forms of the potential energy surface. We demonstrate the effect on an analytic potential as well as a chemical system. While protons move asynchronously along a Grotthuss chain in the classical high-temperature range, the onset of tunneling results in a synchronization of their movement.