Identifying 2- and 3-coordinated H2O in protonated ion–water clusters by vibrational pre-dissociation spectroscopy and ab initio calculations

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 stretch spectra between NH4+(H2O)n, CH3NH3+(H2O)n, and H3O+(H2O)n. Particularly at n⩾6, a sharp feature, identical to that found on ice and water surfaces, emerges at 3690 cm−1 for free-OH stretching. The feature is distinguished from the other free-OH absorption, commonly observed for small- and medium-sized (H2O)n clusters at 3715 cm−1. The results, in conjunction with ab initio calculations, provide compelling evidence for 2- and 3-coordinated H2O in the protonated ion–water clusters.