Isotope Effects in 2D correlation infrared Spectra of Water: HEOM Analysis of Molecular Dynamics-Based Machine Learning Models

Abstract

We model, simulate, and analyze the intramolecular modes of liquid H2O and D2O to elucidate how energy excitation, relaxation, and vibrational dephasing interplay through anharmonic mode-mode coupling. Our analysis employs two-dimensional (2D) correlation spectra, a representative observable in nonlinear infrared vibrational spectroscopy. Accurate reproduction of these 2D spectral profiles requires not only a precise dynamical description of intramolecular vibrations but also an appropriate treatment of thermal environmental effects arising from strong interactions with surrounding molecules, which act as thermal baths. Capturing the essential features of the 2D spectra further demands a non-Markovian, non-perturbative, and nonlinear description of the interactions between intramolecular modes and their baths. To this end, we adopt a hierarchical equations of motion (HEOM) framework to compute the 2D spectra. By comparing the resulting spectra of H2O and D2O, we explore the underlying mechanisms governing their complex energy and phase relaxation dynamics.

Figures (10)

• Figure 1: Overall workflow used in this study: MD trajectory generation, ML-based parameterization of the MAB model (sbml4md), assembly of the S-B Hamiltonian, CHFPE/HEOM propagation, and construction/analysis of 1D and 2D correlation IR spectra.
• Figure 2: Linear absorption spectra of (a) H$_2$O and (b) D$_2$O calculated using the three-mode MAB models with the parameter sets listed in Tables \ref{['tab:ferguson_drude_3bo_bath']} and \ref{['tab::LL_bbdb_mode']}. For comparison, each panel also includes results from MD simulations (blue lines) and experimental data (black dashed curves). Each spectrum is normalized to its maximum peak intensity. The H$_2$O experimental spectrum is reproduced with permission from Y. Maréchal, J. Mol. Struct. 1004, 146 (2011). Copyright (2011) Elsevier.IRexp2011 The D$_2$O experimental spectrum is reproduced from J. Chem. Phys. 131, 184505 (2009), with the permission of AIP Publishing.D2OIR
• Figure 3: The quantum-mechanically computed 2D correlation IR spectra of H$_2$O are shown for the stretching modes (1) and (1$'$) in the upper panel, and for the coupled stretching (1, 1$'$)–bending (2) motions in the lower panels. These spectra were obtained at several $t_2$ periods using the parameter sets listed in Tables \ref{['tab:ferguson_drude_3bo_bath']} and \ref{['tab::LL_bbdb_mode']}. The orientation of the red dashed nodal lines in the upper panel indicates the degree of correlation between the vibrational coherences during the $t_1$ and $t_3$ periods. For clarity, the contour interval in the lower panels has been increased by a factor of five.
• Figure 4: The quantum‑mechanically computed 2D correlation IR spectra of H$_2$O for the (2) bending motions were obtained using the parameter sets listed in Tables \ref{['tab:ferguson_drude_3bo_bath']} and \ref{['tab::LL_bbdb_mode']}. The spectral intensities were normalized to the maximum stretching amplitude. For emphasis, the peak intensity of the bending mode was multiplied by a factor of five relative to the intensity shown in Fig. \ref{['fgr:qst-bnH2Ombpol']}.
• Figure 5: The quantum‑mechanically computed 2D correlation IR spectra of D$_2$O for the stretching modes (1) and (1') [upper panel] and the stretching (1, 1')--bending (2) motions [lower panels] at different $t_2$ periods. The orientation of the red dashed nodal lines in the upper panel indicates the degree of correlation between the vibrational coherences during the $t_1$ and $t_3$ periods. For emphasis, the contour interval in the lower panels was increased by a factor of five.