Analysis of intramolecular modes of liquid water in two-dimensional spectroscopy: a classical hierarchical equations of motion approach

TL;DR

This paper introduces a classical hierarchical equations of motion approach to simulate 2D vibrational spectra of water, capturing complex intramolecular modes and their interactions more efficiently than quantum calculations.

Contribution

The authors develop a classical method based on an anharmonic multimode Brownian oscillator model to simulate 2D spectra including multiple vibrational modes simultaneously.

Findings

Classical approach reproduces key spectral features of water's vibrational modes.

Separation of stretching modes improves peak profile descriptions.

Method offers a computationally feasible alternative to quantum calculations for complex spectra.

Abstract

Two-dimensional (2D) vibrational spectroscopy is a powerful means of investigating the structure and dynamics of complex molecules in condensed phases. However, even in theory, analysis of 2D spectra resulting from complex inter- and intra-molecular motions using only molecular dynamics methods is not easy. This is because molecular motions comprise complex multiple modes, and peaks broaden and overlap owing to various relaxation processes and inhomogeneous broadening. On the basis of an anharmonic multimode Brownian oscillator model with nonlinear system-bath coupling, we have developed an approach that simulates 2D spectra, taking into account arbitrary modes of intermolecular and intramolecular vibrations simultaneously. Although only two-mode quantum calculations are feasible with this model, owing to high computational costs, here we restrict ourselves to the classical case and perform three-mode calculations. We demonstrate the applicability of our method by calculating 2D correlation infrared spectra of water for symmetric stretching, antisymmetric stretching, and bending modes. The quantum effects of these results are deduced by comparing 2D quantum spectra previously obtained for two intramolecular modes with those obtained using our classical approach under the same physical conditions. The results show that the 2D spectra calculated by separating the stretching modes into symmetric and asymmetric modes provide better descriptions of peak profiles, such as the splitting of cross-peaks.