Full dimensional (15D) quantum-dynamical simulation of the protonated water-dimer II: infrared spectrum and vibrational dynamics

TL;DR

This study performs a comprehensive 15-dimensional quantum-dynamical simulation of the protonated water dimer's infrared spectrum, revealing detailed vibrational couplings and matching recent experimental results.

Contribution

First full-dimensional quantum simulation of H5O2+ IR spectrum using MCTDH, elucidating vibrational couplings and spectral assignments.

Findings

Spectrum agrees well with experimental data

Identifies key vibrational couplings

Assigns spectral lines to specific motions

Abstract

The infrared absorption spectrum of the protonated water dimer (H5O2+) is simulated in full dimensionality (15D) in the spectral range 0-4000 cm-1. The calculations are performed using the Multiconfiguration Time-Dependent Hartree (MCTDH) method for propagation of wavepackets. All the fundamentals and several overtones of the vibrational motion are computed. The spectrum of H5O2+ is shaped to a large extent by couplings of the proton-transfer motion to large amplitude fluxional motions of the water molecules, water bending and water-water stretch motions. These couplings are identified and discussed, and the corresponding spectral lines assigned. The large couplings featured by H5O2+ do not hinder, however, to describe the coupled vibrational motion by well defined simple types of vibration (stretching, bending, etc.) based on well defined modes of vibration, in terms of which the spectral lines are assigned. Comparison of our results to recent experiments and calculations on the system is given. The reported MCTDH IR-spectrum is in very good agreement to the recently measured spectrum by Hammer et al. [JCP, 122, 244301, (2005)].