Monte Carlo simulations of structural properties and infrared spectra of SF6–(Ar)n clusters

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Abstract
A method for predicting the perturbed infrared spectrum of a solvated chromophore such as SF6 in a mixed van der Waals cluster is developed and applied to a realistic model for SF6–(AR)n clusters with n=1–100. The dominant contribution to the band shift arises from the interaction of an induced dipole on the perturber with the oscillating instantaneous dipole of the vibrationally distorted SF6. Spectra generated by Monte Carlo averaging these frequency shifts over a canonical distribution of cluster configurations and over a plausible assumed cluster size distribution are in near-quantitative agreement with experiment. For the smaller clusters (n≲18), features of the simulations point to the presence of two distinct classes of cluster structure, one in which the chromophore is wrapped in a unimolecular layer of atoms, and a second in which the solvent atoms have multilayered structures. However, the question of whether the infrared spectra can provide evidence for the presence of the kind of phase coexistence identified in simulations for pure Ar clusters is not yet settled.