Abstract
The mid-infrared predissociation spectra of Cl-·H2O·Arn clusters are dominated by an intense band that is due to excitation of the ionic H-bonded OH stretching vibration. The shape of this band is found to be quite sensitive to the number of attached argon atoms, becoming incrementally narrower until n = 11, the size at which a strong “magic number” appears in the cluster intensity profile. This observation is consistent with the formation of a capped icosahedral structure where the water molecule replaces one argon atom in the closed shell Cl-·Ar12 structure. This conjecture is supported by a theoretical study of the Cl-·H2O·Arn structures using a simulated annealing procedure. These calculations find only one isomeric form for n = 11 but identify several distinct local minima for n ≠ 11. We recover the qualitative features of the band envelopes by calculating the electrostatic effects of the various isomeric forms on the potential energy surface describing the motion of a hydrogen atom toward the chlori...
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