Towards complete assignment of the infrared spectrum of the protonated water cluster H+(H2O)21

Jinfeng Liu,Sotiris S Xantheas,Xiao Cheng Zeng,Jinrong Yang,Xiao He,Kiyoshi Yagi

doi:10.1038/s41467-021-26284-x

Abstract

The spectroscopic features of protonated water species in dilute acid solutions have been long sought after for understanding the microscopic behavior of the proton in water with gas-phase water clusters H+(H2O)n extensively studied as bottom-up model systems. We present a new protocol for the calculation of the infrared (IR) spectra of complex systems, which combines the fragment-based Coupled Cluster method and anharmonic vibrational quasi-degenerate perturbation theory, and demonstrate its accuracy towards the complete and accurate assignment of the IR spectrum of the H+(H2O)21 cluster. The site-specific IR spectral signatures reveal two distinct structures for the internal and surface four-coordinated water molecules, which are ice-like and liquid-like, respectively. The effect of inter-molecular interaction between water molecules is addressed, and the vibrational resonance is found between the O-H stretching fundamental and the bending overtone of the nearest neighboring water molecule. The revelation of the spectral signature of the excess proton offers deeper insight into the nature of charge accommodation in the extended hydrogen-bonding network underpinning this aqueous cluster.

Highlights

The spectroscopic features of protonated water species in dilute acid solutions have been long sought after for understanding the microscopic behavior of the proton in water with gasphase water clusters H+(H2O)n extensively studied as bottom-up model systems
The proton in water cluster has been conventionally considered to be in two accommodation motifs, the Eigen form[21] (i.e., a hydrated hydronium cation, H3O+(H2O)3) and the Zundel form[16], which would induce dramatically different vibrational features near 1000 and 2660 cm−1, respectively, in the IR spectrum[12]
The Eigen-type hydronium ion, integrating three hydrogen-bonded DDA water molecules, is located on the surface of the cage

Summary

Introduction

The spectroscopic features of protonated water species in dilute acid solutions have been long sought after for understanding the microscopic behavior of the proton in water with gasphase water clusters H+(H2O)n extensively studied as bottom-up model systems. In this study we introduce a new protocol based on the combination of high level electronic structure theory and the inclusion of anharmonicity, and demonstrate that it is able to obtain the nearly complete assignment of the infrared (IR) spectrum of the magic number H+(H2O)[21] cluster in excellent agreement with experiment. Torrent-Sucarrat and Anglada[30] have shown that the anharmonic coupling plays a crucial role in the characterization of the IR spectrum of the H+(H2O)[21] cluster Their calculation by the second-order vibrational perturbation theory (VPT2) predicted a strong asymmetric O-H stretching band of the H3O+ around 2000 cm−1, outside the region measured in the experiment. Theoretical calculations that account for both the electronic and vibrational structures of the full H+(H2O)[21] cluster remain a challenge

Methods

Results

Conclusion