On the Search for H5O2+centered Water Clusters in the Gas Phase

Infrared spectrum of NH4+(H2O): Evidence for mode specific fragmentation

The gas phase infrared spectrum (3250-3810 cm-1) of the singly hydrated ammonium ion, NH4+(H2O), has been recorded by action spectroscopy of mass selected and isolated ions. The four bands obtained are assigned to N-H stretching modes and to O-H stretching modes. The N-H stretching modes observed are blueshifted with respect to the corresponding modes of the free NH4+ ion, whereas a redshift is observed with respect to the modes of the free NH3 molecule. The O-H stretching modes observed are redshifted when compared to the free H2O molecule. The asymmetric stretching modes give rise to rotationally resolved perpendicular transitions. The K-type equidistant rotational spacings of 11.1(2) cm-1 (NH4+) and 29(3) cm-1 (H2O) deviate systematically from the corresponding values of the free molecules, a fact which is rationalized in terms of a symmetric top analysis. The relative band intensities recorded compare favorably with predictions of high level ab initio calculations, except on the nu3(H2O) band for which the observed value is about 20 times weaker than the calculated one. The nu3(H2O)/nu1(H2O) intensity ratios from other published action spectra in other cationic complexes vary such that the nu3(H2O) intensities become smaller the stronger the complexes are bound. The recorded ratios vary, in particular, among the data collected from action spectra that were recorded with and without rare gas tagging. The calculated anharmonic coupling constants in NH4+(H2O) further suggest that the coupling of the nu3(H2O) and nu1(H2O) modes to other cluster modes indeed varies by orders of magnitude. These findings together render a picture of a mode specific fragmentation dynamic that modulates band intensities in action spectra with respect to absorption spectra. Additional high level electronic structure calculations at the coupled-cluster singles and doubles with a perturbative treatment of triple excitations [CCSD(T)] level of theory with large basis sets allow for the determination of an accurate binding energy and enthalpy of the NH4+(H2O) cluster. The authors' extrapolated values at the CCSD(T) complete basis set limit are De [NH4+-(H2O)]=-85.40(+/-0.24) kJ/mol and DeltaH(298 K) [NH4+-(H2O)]=-78.3(+/-0.3) kJ/mol (CC2), in which double standard deviations are indicated in parentheses.

Structure of protonated water clusters: Low-energy structures and finite temperature behavior

The structure of protonated water clusters H+(H2O)n (n=5-22) are examined by two Monte Carlo methods in conjunction with the OSS2 potential [L. Ojamae, I. Shavitt, and S. J. Singer J. Chem. Phys. 109, 5547 (1998)]. The basin-hopping method is employed to explore the OSS2 potential energy surface and to locate low-energy structures. The topology of the "global minimum," the most stable low-energy structure, changes from single ring to multiple ring to polyhedral cage as the cluster size grows. The temperature dependence of the cluster geometry is examined by carrying out parallel tempering Monte Carlo simulations. Over the temperature range we studied (25-330 K), all water clusters undergo significant structural changes. The trends are treelike structures dominating at high temperature and single-ring structures appearing in slightly lower temperatures. For n> or =7, an additional transition from single ring to multiple rings appears as the temperature decreases. Only for n> or =16 do polyhedral structures dominate the lowest temperature range. Our results indicate very dynamic structural changes at temperature range relevant to atmospheric chemistry and current experiments. The structures and properties of medium-sized protonated clusters in this temperature range are far from their global minimum cousins. The relevance of these findings to recent experiments and theoretical simulations is also discussed.