Vibrational predissociation spectroscopy of the (H2O)6–21− clusters in the OH stretching region: Evolution of the excess electron-binding signature into the intermediate cluster size regime

Abstract

We report vibrational predissociation spectra of the (H2O)n- cluster ions in the OH stretching region to determine whether the spectral signature of the electron-binding motif identified in the smaller clusters [Hammer et al. Science 306, 675 (2004)] continues to be important in the intermediate size regime (n = 7-21). This signature consists of a redshifted doublet that dominates the OH stretching region, and has been traced primarily to the excitation of a single water molecule residing in a double H-bond acceptor (AA) binding site, oriented with both of its H atoms pointing toward the excess electron cloud. Strong absorption near the characteristic AA doublet is found to persist in the spectra of the larger clusters, but the pattern evolves into a broadened triplet around n = 11. A single free OH feature associated with dangling hydrogen atoms on the cluster surface is observed to emerge for n > or = 15, in sharp contrast to the multiplet pattern of unbonded OH stretches displayed by the H+(H2O)n clusters throughout the n = 2-29 range. We also explore the vibration-electronic coupling associated with normal-mode displacements of the AA molecule that most strongly interact with the excess electron. Specifically, electronic structure calculations on the hexamer anion indicate that displacement along the -OH2 symmetric stretching mode dramatically distorts the excess electron cloud, thus accounting for the anomalously large oscillator strength of the AA water stretching vibrations. We also discuss these vibronic interactions in the context of a possible relaxation mechanism for the excited electronic states involving the excess electron.

Excess electrons in methanol clusters: Beyond the one-electron picture

“…This structural pattern is significantly different from that of the aqueous case where acceptor-acceptor (AA) type water molecules with two dangling hydrogen atoms form the dominant electron binding motif. 63,64 For methanol, however, only one hydroxyl hydrogen atom is available for each methanol unit to bind directly to the electron. This results in much weaker electron binding and more diffuse excess electron states in surface state methanol cluster anions than in surface state hydrated electron clusters.…”

Perspective: Water cluster mediated atmospheric chemistry

“…145,146 Elegant experiments made it possible to follow the evolution of vibrational spectra and of reactivity with cluster size, allowing for molecular level descriptions of hydrated clusters. [147][148][149][150] Important and general messages emerged from these studies about the role of the hydration structure around the ionic core in determining spectroscopic and chemical properties of these species. 151 Computational studies predict red shifts in the vibrational spectra of hydrates and emphasize the importance and necessity for anharmonic effects to be treated appropriately to describe their properties.…”

On the applicability of one- and many-electron quantum chemistry models for hydrated electron clusters

“…The MD generated configurations can also be compared, at least qualitatively, with the electron binding motifs found experimentally in vibrational predissociation spectra by the Johnson group. 40,41 They identified two different classes for the electron binding sites in ( ) − 6 2 O H clusters, one with a double hydrogen-bonding acceptor (AA) water molecule pointing in the excess electron distribution with two hydrogen atoms, and the other with several, hydrogen-bond acceptor-donor (AD) water molecules with oriented dangling hydrogen atoms toward the electron. Of the two isomers, the one with the AA type water molecule binds the electron more strongly.…”