Predicting the hydrogen bond strength from water reorientation dynamics at short timescales

TL;DR

The paper investigates how hydrogen-bond (H-bond) strength and its asymmetry at the water/air interface relate to short-time water reorientation and SFG spectra. Using path-integral MD with the q-TIP4P/F model and ALMO EDA, it resolves layer-specific H-bond energetics and delocalization, and connects these to observable reorientation dynamics and SFG signatures across instantaneous interface layers. A key finding is a strong, layer-dependent link between the short-time librational dynamics and the local strongest H-bond donor interaction, expressed as a linear relation $E_{D_1}=137.78\,P_2(\tau_{L2})-92.35$ with high predictive accuracy, while long-time dynamics correlate less strongly with H-bond strength. The results show that H-bond strength and asymmetry increase away from the interface, consistent with SFG red-shifts and a notable strong H-bond feature in layer 2, and they provide a practical route to estimate interfacial H-bond strength from short-time experimental observables in hydrophobic environments.

Abstract

Path-integral molecular dynamics simulations and electronic structure-based energy decomposition analysis (EDA) are employed to connect hydrogen bond (H-bond) strength, its asymmetry, and the total delocalization energy at the water/air interface to experimentally measurable observables, such as the reorientation dynamics and the sum-frequency generation (SFG) spectrum. Using SFG spectra for distinct layers at the water/air interface, we validate the accuracy of our simulations and report a red-shift from the interface to bulk and a strongly bonded water peak at around 3250 cm$^{-1}$ in the layer closest to bulk. The reorientation dynamics of water molecules slow down from the interface to bulk, which correlates with the SFG results. From our EDA based on absolutely localized molecular orbitals, we observe a strong decline in total delocalization energy from bulk to the interface, as well as a decline in the strength of the strongest donor and acceptor interactions. The asymmetry between the two strongest interactions similarly rises towards the interface, while the importance of interactions from the outer solvation shells is greatly diminished and is lower than previously reported. Finally, we find that the strength of the strongest H-bond donor/acceptor is best correlated with the local minimum of the autocorrelation function resembling the L2 band librational motions. Following that, we propose a simple yet quantitative relationship between H-bond strength and the short-time reorientation dynamics at the water/air interface that could potentially be extended to predict H-bond strength in other hydrophobic systems from experimentally obtainable observables.

Figures (3)

• Figure 1: SFG response for water layer 0 (orange), layer 1 (blue), layer 2 (red) and bulk water (green), respectively. The imaginary response is sensitive to the O-H group orientation. Negative values are indicates O-H stretches pointing towards the water bulk, whereas positive contributions are due to O-H stretches pointing towards the water vapor phase.
• Figure 2: Reorientation dynamics of water hydroxy groups, as measured by the second order auto-correlation function $P_{2}(\tau)$ and a fitted bi-exponential function for layer 1 (blue), layer 2 (red) and bulk (green) at the water/air interface.
• Figure 3: Short-time ($\le$ 0.1 ps) reorientation dynamics $P_{2}(\tau)$ for layer 1 (blue) and layer 2 (red) and bulk (green) at the water/air interface. Dashed lines mark the local minima labeled as $P_{2}(\tau_{L2})$.