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Vibrations and thermodynamics of clusters of polycyclic aromatic hydrocarbon molecules: the role of internal modes.

Rapacioli M, Calvo F, Joblin C, Parneix P, Spiegelman F

Institut für Physikalische Chemie und Elektrochemie, TU Dresden, Mommenstrassse 13, D-01062 Dresden, Germany.

The vibrational spectra of clusters of coronene molecules are theoretically calculated using a mixed quantum/classical scheme, each molecule being described by a tight-binding Hamiltonian, the intermolecular forces being provided by explicit Lennard-Jones and point charge sites. The normal modes of vibrations are shown to exhibit significant variations upon clustering. In particular, for large clusters intra- and intermolecular modes tend to mix and fill the mid-infrared range. We also calculate the heat capacity of the (C24H12)8 cluster as a function of temperature, emphasizing the isomerizations that take place during melting. Quantum delocalization effects, as obtained from the Pitzer-Gwinn semiclassical approximation, are important enough to wash out all signatures of the structural transitions on the caloric curve. On the basis of a simple two-state model we estimate that clusters containing about 300 molecules are required for melting to be detected on the caloric curve.

Published 19 April 2007 in J Phys Chem A, 111(16): 2999-3009.
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