Gas‐Phase Infrared Spectrum of the Protonated Water Dimer: Molecular Dynamics Simulation and Accuracy of the Potential Energy Surface

Abstract

Dedicated to Professor Michele Parrinello on the occasion of his 60th birthday.The long-awaited observation of the infrared multiphoton dissociation (IRMPD) spectrum of the protonated water dimer [a] Prof.

The vibrational spectra of protonated water clusters: A benchmark for self-consistent-charge density-functional tight binding

“…The peaks around 700-1900 cm −1 correspond to the out-of-phase bending vibrations of the flanking water molecules. 37,54,55 For the protonated water clusters, the OH stretching peaks have a lower intensity in the FT-DAC spectrum compared to the FT-VAC spectrum, which has also been observed by Iyengar et al 40,42 The FT-VAC spectra show a peak around 1450 cm −1 , which is also present in NMA with a relatively low intensity. The component FT-VAC analyses indicate that this peak is mainly due to the oscillation of the shared proton perpendicular to the O-O axis.…”

“…The component FT-VAC analyses indicate that this peak is mainly due to the oscillation of the shared proton perpendicular to the O-O axis. However, this band is nearly absent in both the FT-DAC and experimental spectra, 36,37,54,55 indicating that the corresponding mode does not significantly change the dipole moment, i.e., not IR active.…”

Full dimensional (15-dimensional) quantum-dynamical simulation of the protonated water dimer. II. Infrared spectrum and vibrational dynamics

“…This doublet structure was not fully understood until recently, although the highest-energy component had been already assigned by Bowman and collaborators to the asymmetric proton-stretch fundamental ([O-H-O ]) [5]. A similar argument was made by Sauer and collaborators based on classical-trajectories calculations [12]. In this respect, it is known from recent classical-…”

“…Hence, more sophisticated theoretical approaches are required. Several theoretical studies have been conducted over the last years in order to understand and assign the IR spectrum of the cation [5,9,10,11,12,13,14,15].…”

“…Three main absorptions were discussed and assigned, based on a previous quantum-dynamical simulation of the IR absorption spectrum on a 4D model of the hydrogen-bond (O-H-O) fragment [9]. Those assignments were revisited in the context of newer IRMPD experiments and calculations, producing somewhat disparate results [5,7,12,13]. Recent measurements of the IR predissociation spectrum of the H 5 O + 2 cation in argon-solvate [8] and neon-and argon-solvate [5] conditions present spectra with a simpler structure than the multiphoton IRMPD ones.…”