Microsolvation of the Water Cation in Argon: II. Infrared Photodissociation Spectra of H2O-Arn (n ) 1-14)

Mid-infrared photodissociation spectra of H2O-Arn (n ) 1-14) complexes have been recorded in the vicinity of the OsH stretch vibrations of the water cation. The rovibrational structure of the transitions in the dimer spectrum (n ) 1) are consistent with a planar, proton (H)-bound HsOsHsAr equilibrium geometry. The slightly translinear intermolecular bond in the ground vibrational state is characterized by a bond angle æ0 ) 175(5)°, an interatomic HsAr separation R0 ) 1.929(15) Å, and an intermolecular stretching force constant ks ∼ 29 N/m. The assignment of the vibrational transitions is confirmed by spectra of partly deuterated species. The relaxation dynamics depend strongly on the excited vibrational state and do not obey statistical theories. Analysis of the spin-rotation constants indicates that the electromagnetic properties of the H2O cation in its B1 ground electronic state are not significantly affected by the formation of the intermolecular bond to Ar. The vibrational bands in the spectra of larger clusters (n ) 2-14) are assigned to OsH stretch fundamentals and their combination bands with the intermolecular HsAr stretch modes. The observed systematic band shifts as a function of cluster size provide information about the cluster’s geometries and the occurrence of structural isomers. The most stable trimer (n ) 2) geometry has two equivalent intermolecular H-bonds. This stable trimer core is further solvated by two Ar ligands attached to opposite sites of the 2py orbital of oxygen (n ) 3,4) and subsequently by less strongly bound Ar ligands (n ) 5-14) to form an Ar solvation shell, probably around an interior H2O ion.