Infrared spectroscopy of water cluster anions, (H2O) n=3-24- in the HOH bending region:: Persistence of the double H-bond acceptor (AA) water molecule in the excess electron binding site of the class I isomers

We report vibrational predissociation spectra of water cluster anions, (H2O)(n=3-24)(-) in the HOH bending region to explore whether the characteristic red-shifted feature associated with electron binding onto a double H-bond acceptor (AA) water molecule survives into the intermediate cluster size regime. The spectra of the tagged (H2O)n(-.)Ar clusters indeed exhibit the signature AA band, but assignment of this motif to a particular isomer is complicated by the fact that argon attachment produces significant population of three isomeric forms (as evidenced by their photoelectron spectra). We therefore also investigated the bare clusters since they can be prepared exclusively in the high binding (isomer class I) form. Because the energy required to dissociate a water molecule from the bare complexes is much larger than the transition energies in the bending region, the resulting ( linear) action spectroscopy selectively explores the properties of clusters with most internal energy content. The (H2O)(15)(-) predissociation spectrum obtained under these conditions displays a more intense AA feature than was found in the spectra of the Ar tagged species. This observation implies that not only is the AA motif present in the class I isomer, but also that it persists when the clusters contain considerable internal energy.