X-ray Absorption Spectral Signature of Quantum Nuclei in Liquid Water

Abstract

Based on electron-hole excitation theory, we investigate the X-ray absorption spectral signature of nuclear quantum effect in liquid water, whose molecular structure is simulated by path-integral molecular dynamics using the MB-pol model. Compared to spectra generated from classically modeled water structure, quantum nuclei has important effect on spectra in terms of both the spectral energies and their line shapes. At the short-range ordering of H-bond network, the delocalized protons increase the fluctuations on the intramolecular covalency and broaden the pre-edge of the spectra. For intermediate-range and long-range orderings, the observed red and blue shifts of the main-edge and post-edge are attributed to the so-called competing quantum effects, under which both the weak and well-formed H-bonds are promoted. The theoretical spectra are in nearly quantitative agreement with the available experimental data.The nature of H-bond network in water continues to be at the center of scientific interests [1][2][3][4][5][6][7][8][9][10][11][12]. Recently, high-resolution X-ray absorption spectroscopy (XAS), has emerged to be a powerful experimental technique to probe the water structure at molecular scale [13][14][15][16][17][18][19]. Allowed by Franck-Condon principle [20], the electrons leaving the oxygen core can be described as excited electronic states from an instantaneously frozen snapshot of water at equilibrium. Therefore, XAS spectrum inherits a unique local signature of its molecular environment, complimentary to the scattering experiments [21,22].Ab initio theory is desired for an unambiguous interpretation of the underlying molecular structure. However, theoretical prediction has posed a major challenge by itself. The computed XAS spectrum is sensitive to the accuracy of methods in both the adopted electronic and molecular theories. Rigorously, the electron-hole excitation theories, such as the Bethe-Salpeter equation (BSE) [13,[23][24][25], should be applied beyond the unoccupied electronic structure from density functional theory [26][27][28][29]. However, the BSE is computational formidable and has not yet widely applied in water. The above computational burden has been greatly alleviated by the recently introduced approximate BSE solution utilizing localized basis [30]. On the other hand, the modeling of water at molecular level is no less difficult. The difficulty lies at the complex nature of the H-bond structure balanced by many physical interactions. The directional H-bond, as the building block of the near-tetrahedral structure in water, is much weaker than the ordinary (covalent or ionic) chemical bond. Furthermore, the H-bond network is largely modified by the van der Waals interaction, whose energy is even weaker than H-bond by one order of magnitude [31][32][33][34][35]. Worse still, nuclear quantum effects (NQEs) should be properly considered due to the low mass of proton [36][37][38]. Under the influence of NQEs, the more delocalized protons are found to introduce an unexpected competing e...