Dipole-bound anions of molecules and clusters: An ab initio study

Abstract

The quantum mechanical investigation of molecular dimers, trimers and, to a greater extent, higher clusters, constitutes a legitimate attempt towards linking the gas and condensed phases of matter. The interaction of gas-phase clusters with ionizing media can result in anion formation, a special class of which, the dipole-bound anions is the main focus of this dissertation. By applying special techniques of ab initio calculations to model systems, we have addressed the question of the extent to which certain molecular species can form dipole-bound anions. We have shown the effects of attaching a dipole-bound electron on charge distribution, geometrical distortions, and mechanistic fates of certain reactive clusters. Our investigations include cases of homodimer anions where we reported the feasibility of formation of a dipole-bound anion for the dimer agglomerates to the exclusion of the single molecules (ethylene glycol and hydrogen fluoride). The hydrogen cyanide-water anion is a heterodimer system where our results demonstrate the effect of linkage isomerism on an isomer's ability to form dipole-bound states. Only the [H₂O...HCN]⁻ anion was experimentally detected and it corresponds to the higher EA we reported. In addition, the formation of meta-stable trimer and tetramer dipole-bound anions in the water cluster system is supported by our calculation. The water cluster study demonstrates the utility of ab initio calculation techniques for investigations of reaction mechanisms in the gas phase. For instance, both the mass spectrum intensity pattern and general mechanism of formation of the cluster anions can be inferred from the relative stabilities and geometries that we obtained for the anion and neutral systems. Finally, we point out the potential utility of an IR analysis in distinguishing neutral from anion systems, based on shifts of certain IR bands generated in our harmonic frequency analysis. We hope that our results for electron affinities, dipole moments, and vertical detachment energies will serve as a useful resource for the researchers in the field. In conjunction with the corresponding values measured by experimentalists, both results should provide a self-consistent system for identifying and overcoming shortcomings of both experimental and theoretical approaches of solving chemical and physical problems involving anion formation.