The [(CO2)n(H2O)]- cluster anions are studied using infrared photodissociation (IPD) spectroscopy in the 2800－3800 cm-1 range. The observed IPD spectra display a drastic change in the vibrational band features at n = 4, indicating a sharp discontinuity in the structural evolution of the monohydrated cluster anions. The n = 2 and 3 spectra are composed of a series of sharp bands around 3600 cm-1, which are assignable to the stretching vibrations of H2O bound to C2O4- in a double ionic hydrogen-bonding (DIHB) configuration, as was previously discussed [J. Chem. Phys., 122, 094303 (2005)]. In the n ≥ 4 spectrum, a pair of intense bands additionally appears at ≈3300 cm-1. With the aid of ab initio calculations at the MP2/6-31+G* level, the 3300 cm-1 bands are assigned to the bending overtone and the hydrogen-bonded OH vibration of H2O bound to CO2- via a single O－H・・・O linkage. Thus, the structures of [(CO2)n(H2O)]- evolve with cluster size such that DIHB to C2O4- is favored in the smaller clusters with n = 2 and 3 whereas CO2- is preferentially stabilized via the formation of a single ionic hydrogen-bonding (SIHB) configuration in the larger clusters with n ≥ 4.

This is a preprint of an article published by American Chemical Society in Journal of Physical Chemistry A, 2009, available online: http://pubs.acs.org/doi/abs/10.1021/jp903578e.