Infrared Predissociation Spectroscopy of M+(C6F6)1-4(H2O)1-2Ar0-1 Cluster Ions, M = Li, Na

Infrared predissociation (IRPD) spectra of Li+-(C6H6)(1-4)(H2O)(1-2)Ar0-1 and Na+(C6H6)(2-4)(H2O)(1-2)Ar-1 are presented along with ab initio calculations. The results indicate that the global minimum energy structure for Li+-(C6H6)(2)(H2O)(2) has each water forming a pi-hydrogen bond with the same benzene molecule. This bonding motif is preserved in Li+(C6H6)(3-4)(H2O)(2)Ar0-1 with the additional benzene ligands binding to the available free OH groups. Argon tagging allows high-energy Li+(C6H6)(2-4)(H2O)(2)Ar isomers containing water-water hydrogen bonds to be trapped and detected. The monohydrated, Li+ containing clusters contain benzene-water interactions with varying strength as indicated by shifts in OH stretching frequencies. The IRPD spectra of M+(C6H6)(1-4)(H2O)(1-2)Ar are very different for lithium-bearing versus sodium-bearing cluster ions emphasizing the important role of ion size in determining the most favorable balance of competing noncovalent interactions.