Ab initio Studies of NH4+(H2O)1-5 and the Influence of Hydrogen-Bonding Nonadditivity on Geometries and Vibrations

Ab initio calculations were performed to investigate the structures, energetics, and vibrations of NH 4 + (H 2 O) n cluster ions at n = 0−5 . Equilibrium geometries of NH 4 + and NH 4 + −H 2 O are optimized at the MP2, MP4, CCD, QCISD, and B3LYP levels using the 6-31G*, 6-31G**, 6-31+G*, 6-31++G**, 6-311+G**, and 6-311++G** basis sets. The benchmark calculations indicate that using MP2 and B3LYP approaches with the 6-31+G* basis set is well suited for characterizing large NH 4 + (H 2 O) n clusters. The two approaches correspondingly find the existence of a number of structural isomers at n = 2−5, of which the isomer with a filled first solvation shell is lowest in energy at n = 4. The calculations further predict that, at n = 5, the lowest energy isomer contains a four-membered ring with the second-shell H 2 O acting as a double-proton acceptor (AA). The prediction is in good agreement with the observation of jet-cooled NH 4 + (H 2 O) 5, where a characteristic hydrogen-bonded-OH stretching absorption at ∼3550 cm -1 is identified for the AA−H 2 O molecule in the vibrational predissociation spectra (Wang, Y.-S.; Chang, H.-C.; Jiang, J. C.; Lin, S. H.; Lee, Y. T.; Chang, H.-C. J. Am. Chem. Soc. 1998, 10 2, 8777). In this study, in addition to energetics, how hydrogen-bonding nonadditivity influences the geometries and vibrations of these clusters is analyzed.