Revisiting the H5O2+ IR Spectrum with VSCF/VCI and the Influence of Mark Johnson's Experiments in Advancing the Theory of Protonated Water Clusters.

Abstract

The interplay between experiment and theory is widely appreciated in the fields of chemical physics and physical chemistry. Indeed, some experiments actually push the frontiers of theory. This is the case for protonated water clusters and, in particular, the experiments of the Mark Johnson group on the Zundel and Eigen cations, H₅O₂⁺ and H₉O₄⁺, respectively, and the challenging "in-between" cation H₇O₃⁺. In this perspective, we demonstrate this with a focus on H₅O₂⁺ and, specifically, the strong doublet feature of the proton stretch, uncovered experimentally by Johnson's group. This proved to be a major challenge for theory, from developing "gold standard" potential energy surfaces to quantum dynamics. Full-dimensional multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) calculations using an accurate potential and dipole moment surface were the first quantum ones to capture this feature, as well as the full IR spectrum. Earlier vibrational self-consistent field and virtual state configuration interaction (VSCF/VCI) calculations, using the code MULTIMODE, were unable to describe this owing to a lack of convergence. We show here that pushing that approach does recover the doublet and overall an IR spectrum, in agreement with the earlier MCTDH and recent time-dependent tree tensor network states (td-TTNS) and experiment. VSCF/VCI and subsequent very computationally intensive ML-MCTDH calculations of the IR spectrum of the Eigen isomer of H₉O₄⁺ produce very good agreement with Johnson's experimental one. These calculations were performed with an ab initio many-body potential and dipole moment surface. The VSCF/VCI MULTIMODE approach has been extended to the larger clusters, and this is shown for two isomers of H₁₃O₆⁺, with very good agreement with experimental spectra.