MONTE-CARLO STUDY OF SMALL BENZENE CLUSTERS .2. TRANSITION FROM RIGID TO FLUXIONAL FORMS

Simulations were carried out on clusters composed of 12, 13, and 14 molecules interacting with realistic intermolecular forces. Cold, nearly rigid clusters were heated until they began to fragment. According to indices of melting, including the Lindemann delta, heat capacity, coordination number, and self-diffusion, clusters began to undergo a melting-like transition at about 140 K. The 13-membered cluster remained in its low-energy configuration somewhat longer than the others, consistent with its closed-shell icosahedral structure. The transition was gradual, taking place over a 50 deg range, as indicated by criteria depending upon translational and out-of-plane tilting motions. Rotational diffusion about the 6-fold axis began at far lower temperatures. The activation energy for this molecular rotation to take place on the surface of the solid-like cluster is an order of magnitude lower than it is in the bulk crystal. Results of the present Monte Carlo computations are in qualitative agreement with spectroscopic observations of(C6H6)(C6D6)(12) clusters undergoing a transition from a liquid-like to a solid-like structure in a cooling supersonic jet. Our results provide additional information about the temperatures of clusters during the transition and about the mechanism underlying the cooling which induces the transition. Factors responsible for the isotope effect strongly favoring the migration of protiated species to the cluster center during solidification are also discussed briefly.