Spreading probability distribution and increase of entropy due to coherent quantum dynamics: Proof-of-principle for a model molecular boron rotor

Abstract

Spreading of probability distributions and the corresponding increases of entropy are a familiar effect of everyday life. Here we show that the phenomenon can also be achieved by coherent quantum dynamics of a small molecular system, specifically the oriented boron rotor 11B10B@11B10B8-. Its molecular wheel 11B10B rotates in the pseudo-rotating bearing 11B10B8 so that it can snap in eighteen different positions which correspond to eighteen global minimum structures (GMs). After initial preparation of the rotor in a single GM, the subsequent quantum dynamics populates all other GMs. Accordingly, the distribution of the probabilities of occupying the GMs spreads, and the corresponding entropy increases. It does not climb to the maximum value, however, due to a quantum dynamical constraint imposed by molecular symmetry which prevents perfect equidistribution of all GMs.