The raid-infrared predissociation spectra of (Cl-.H2OArn)-Ar-., 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--(H2OAr12)-Ar-. structures using a simulated annealing procedure. These calculations find only one isomeric form for n = 11 but identify several distinct local minima for n not equal 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 chloride ion.