Dissecting the Molecular Structure of the Air/Water Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum

Abstract

The molecular characterization of the air/water interface is a key step in understanding fundamental multiphase phenomena ranging from heterogeneous chemical processes in the atmosphere to the hydration of biomolecules. The apparent simplicity of the air/water interface, however, masks an underlying complexity associated with the dynamic nature of the water hydrogen-bond network that has so far hindered an unambiguous characterization of its microscopic properties. Here, we demonstrate that the application of quantum many-body molecular dynamics, which enables spectroscopically accurate simulations of water from the gas to the condensed phase, leads to a definitive molecular-level picture of the interface region. For the first time, excellent agreement is obtained between the simulated vibrational sum-frequency generation spectrum and the most recent state-of-the-art measurements, without requiring any empirical frequency shift or ad hoc scaling of the spectral intensity. A systematic dissection of the spectral features demonstrates that a rigorous representation of nuclear quantum effects as well as of many-body energy and electrostatic contributions is necessary for a quantitative reproduction of the experimental data. The unprecedented accuracy of the simulations presented here indicates that quantum many-body molecular dynamics can enable predictive studies of aqueous interfaces, which by complementing analogous experimental measurements will provide unique molecular insights into multiphase and heterogeneous processes of relevance in chemistry, biology, materials science, and environmental research.

Communication: Vibrational sum-frequency spectrum of the air-water interface, revisited

“…17 In 2015, two new heterodyne-detected experiments with a more careful consideration of the reference used to determine the phase were performed by Yamaguchi, and Tahara and co-workers, which showed, in fact, that for neat H 2 O the low-frequency positive feature was absent 19,20 21 which does essentially confirm the result that there is no positive feature at low frequencies. We note that Nagata and co-workers have recently calculated sum-frequency spectra with ab initio MD simulations, 22 and Medders and Paesani have done the same 23 using their MB-pol model, 24 and the results of both groups are in good agreement with these new experiments.…”

Modeling Molecular Interactions in Water: From Pairwise to Many-Body Potential Energy Functions

“…61,62 Although it was shown that many-body effects can be decomposed, at least to some extent, into effective pairwise contributions using force-matching, 88 more recent studies have demonstrated that the explicit account of 3B effects is essential for a physically correct description of heterogeneous environments such as the air/water interface. 89,90 …”

“…Although the E3B models represent an improvement over common pairwise additive force fields, a recent analysis of the heterodyne-detected vibrational sum-frequency generation (HD-vSFG) spectrum of the air/water interface in terms of many-body contributions suggests that, due to the empirical parametrization, 3B effects are possibly overemphasized in the E3B models. 90 …”

“…In addition, as shown in Table 5, molecular dynamics simulations of liquid water at ambient conditions carried out with MB-pol at both classical and quantum mechanical levels predict thermodynamic and dynamical properties in excellent agreement with the corresponding experimental values. 90,249 …”

Molecular dynamics study of structure and vibrational spectra at zwitterionoic lipid/aqueous KCl, NaCl, and CaCl2 solution interfaces

“…23,[31][32][33][34] While the conventional SFG measurement detects the square of the second-order nonlinear susceptibility | χ (2) | 2 (hereafter χ (2) is denoted as χ), phase-sensitive 35 or heterodyne-detected 36 vibrational SFG (HD-VSFG) measurement can detect the phase of χ (or the imaginary part of χ, Im[ χ]) besides its amplitude, which enables us to detect the orientation of water molecules at the interface, proton-up or proton-down (H-up or H-down). 37 Several HD-VSFG studies clarified detailed molecular orientation at air/water [38][39][40][41][42][43][44][45][46] and lipid (or surfactant)/water [47][48][49][50][51] interfaces.…”