IR spectral assignments for the hydrated excess proton in liquid water

Abstract

The local environmental sensitivity of infrared (IR) spectroscopy to a hydrogen-bonding structure makes it a powerful tool for investigating the structure and dynamics of excess protons in water. Although of significant interest, the line broadening that results from the ultrafast evolution of different solvated proton-water structures makes the assignment of liquid-phase IR spectra a challenging task. In this work, we apply a normal mode analysis using density functional theory of thousands of proton-water clusters taken from reactive molecular dynamics trajectories of the latest generation multistate empirical valence bond proton model (MS-EVB 3.2). These calculations are used to obtain a vibrational density of states and IR spectral density, which are decomposed on the basis of solvated proton structure and the frequency dependent mode character. Decompositions are presented on the basis of the proton sharing parameter δ, often used to distinguish Eigen and Zundel species, the stretch and bend character of the modes, the mode delocalization, and the vibrational mode symmetry. We find there is a wide distribution of vibrational frequencies spanning 1200-3000 cm for every local proton configuration, with the region 2000-2600 cm being mostly governed by the distorted Eigen-like configuration. We find a continuous red shift of the special-pair O⋯H⋯O stretching frequency, and an increase in the flanking water bending intensity with decreasing δ. Also, we find that the flanking water stretch mode of the Zundel-like species is strongly mixed with the flanking water bend, and the special pair proton oscillation band is strongly coupled with the bend modes of the central HO2+moiety.

Slow Proton Transfer in Nanoconfined Water

“…The broad featureless absorption at 2000–3000 cm –1 has been assigned to the OH-stretch vibrations of different proton hydration structures, whereas the absorption band observed at 1750 cm –1 is usually assigned to the bending vibrations of these structures. 31 − 33 …”

Decoding the spectroscopic features and time scales of aqueous proton defects

“…Specifically, recent twodimensional (2D) IR experiments on hydrochloric (HCl) acid solutions probed a stretch-bend cross-peak and observed a timescale of 480 fs, which was interpreted as a lower bound on the Zundel complex lifetime 8 . While a) Electronic mail: tmarkland@stanford.edu this result suggests that certain proton defect structures interconvert over timescales longer than previously observed 7 , the interpretation of the cross-peak decay rested largely on assigning frequencies to the Zundel gas-phase structure within a cluster-centric paradigm 17,20,24,[27][28][29] . However, it remains unclear whether an interpretation of the liquid spectra based on the limiting Eigen and Zundel gas-phase cluster structures provides a complete picture of the structural motifs of condensed-phase proton defects and their interconversion timescales.…”

Ab initio spectroscopy and ionic conductivity of water under Earth mantle conditions

“…42. We note that a broad, mid-IR continuum absorbance was also observed in a number of aqueous systems at ambient conditions, from protonated clusters (43) to bulk acidic solutions (44)(45)(46). An ultrafast 2D IR study of the excess protons in aqueous hydrochloric solutions (44) ascribed the broad-band intensity to vibrational modes of the Zundel and Eigen ions.…”