IDENTIFYING 2-COORDINATED AND 3-COORDINATED H2O IN PROTONATED ION WATER CLUSTERS BY VIBRATIONAL PRE-DISSOCIATION SPECTROSCOPY AND AB-INITIOCALCULATIONS

Clustering of water on protonated molecular ions has been investigated by vibrational predissociation spectroscopy. Systematic measurements at different cluster sizes reveal a close resemblance of the OH stretc h spectra between NH4+(H2O)(n), CH3NH3+(H2O)(n), and H3O+(H2O)(n). par ticularly at n greater than or equal to 6, a sharp feature, identical to that found on ice and water surfaces, emerges at 3690 cm(-1) for fr ee-OH stretching. The feature is distinguished from the other free-OH absorption, commonly observed for small-and medium-sized (H2O)(n) clus ters at 3715 cm(-1). The results, in conjunction with ab initio calcul ations, provide compelling evidence for 2- and 3-coordinated H2O in th e protonated ion-water clusters. (C) 1997 American Institute of Physic s.