The ionization of water plays a crucial role in many biological and chemical processes. Insight into this process can be found by studying the dynamics of water clusters of various sizes containing a hydronium ion. This study centers around the eigenion ((H2O)4H+), a tightly bound molecular cluster with a pyramidal structure centered around the hydronium ion. The infrared spectrum of the eigenion has been extensively studied experimentally and used to explain the infrared spectra of larger clusters and bulk systems. In this study we aim to provide a theoretical anharmonic spectrum of the eigenion. We present a detailed account of such an anharmonic calculation using an ab initio potential and vibrational structure theory. The potential energy and dipole surfaces were initially modeled using normal coordinates and a quartic force field. The coordinates were then optimized using a novel algorithm based on the vibrational self-consistent field method. The optimal coordinates were used to expand the strong coupling terms on a grid. All electronic structure calculations were done at the B3LYP/aug-cc-pVDZ level of theory. Finally a theoretical calculation of the infrared vibrational spectrum using vibrational quasi-degenerate perturbation theory in the range from 900-4000cm-1 was conducted. The calculated spectrum accounts for the positions and features of the experimental spectrum and the coefficients of the anharmonic wave functions are used to provide an assignment of the experimental spectrum.

Calculating the infrared spectra of the eigenion using anharmonic vibrational theory

249th National Meeting (2015)

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