Exploring the correlation between network structure and electron binding energy in the $(H2O)7−$ cluster through isomer-photoselected vibrational predissociation spectroscopy and ab initio calculations: Addressing complexity beyond types I-III

We report a combined photoelectron and vibrational spectroscopy study of the $(H2O)7−$ cluster anions in order to correlate structural changes with the observed differences in electron binding energies of the various isomers. Photoelectron spectra of the $(H2O)7−⋅Arm$ clusters are obtained over the range of $m=0–10$⁠. These spectra reveal the formation of a new isomer $(I′)$ for $m>5$⁠, the electron binding energy of which is about $0.15eV$ higher than that of the type I form previously reported to be the highest binding energy species [Coe et al, J. Chem. Phys. 92, 3980 (1990)]. Isomer-selective vibrational predissociation spectra are obtained using both the Ar dependence of the isomer distribution and photochemical depopulation of the more weakly (electron) binding isomers. The likely structures of the isomers at play are identified with the aid of electronic structure calculations, and the electron binding energies, as well as harmonic vibrational spectra, are calculated for 28 low-lying forms for comparison with the experimental results. The HOH bending spectrum of the low binding type II form is dominated by a band that is moderately redshifted relative to the bending origin of the bare water molecule. Calculations trace this feature primarily to the bending vibration localized on a water molecule in which a dangling H atom points toward the electron cloud. Both higher binding forms (I and $I′$⁠) display the characteristic patterns in the bending and OH stretching regions signaling electron attachment primarily to a water molecule in an AA binding site, a persistent motif found in non-isomer-selective spectra of the clusters up to $(H2O)50−$⁠.