Dynamic Anti-correlations Revealed in Water Hydrogen Bonds

Water is a fascinating substance with unique properties attributed to its strong intermolecular hydrogen bonds. In this study, we use two-dimensional infrared (2D-IR) spectroscopy and density functional theory (DFT) calculations to explore the hydrogen bonding in water. Our findings reveal important insights into the structure and dynamics of water's hydrogen bonds.

Unraveling the Mysteries of Water's Hydrogen Bonding

Section 1: Introduction to Water's Hydrogen Bonding

The peculiar properties of water arise from its intermolecular interactions, particularly strong hydrogen bonds. Traditional views of liquid water's symmetric coordination structure have been challenged by recent studies. Transient deviations from symmetric coordination are connected to local H-bond strength correlations and asymmetric coordination geometries, which may explain water's anomalous properties. However, experimental evidence on the exact details of H-bond symmetry is lacking.Water's O-H stretching vibrations reflect the length and symmetry of its hydrogen bonds. The frequencies and linewidths of these vibrations are affected by intermolecular coupling and local mode interactions. To eliminate delocalization, we dilute water in dimethylformamide (DMF).

Section 2: Measuring O-D Stretching Vibration in DMF

We measure the O-D stretching vibration of 5 mol% water in DMF, where water-water H-bonds are negligible. By characterizing the distribution of O-D stretching frequencies using isotopically diluted water, we observe differences in the linewidths of D2O and HOD. 2D-IR spectroscopy helps disentangle homogeneous and inhomogeneous broadening contributions. The inhomogeneous broadening of the O-D stretching frequencies is revealed, and we find that coupling of the two O-D stretching modes in D2O results in a narrower distribution of frequencies compared to HOD.

Section 3: DFT Calculations and H-Bond Distance Correlations

Using DFT calculations, we investigate the effect of H-bond distance correlations on vibrational frequencies. By calculating normal mode frequencies for different H-bond geometries, we show that anti-correlated H-bonds lead to a narrower distribution of symmetric and asymmetric O-D stretching frequencies. The correlation between these frequencies can be directly interrogated with 2D-IR spectroscopy, providing evidence for anti-correlated H-bond distances.

Section 4: Dynamics of Correlations and Inhomogeneous Linewidths

The frequency-frequency anti-correlations in water are short-lived and rapidly randomized by thermal fluctuations. Molecular dynamics simulations suggest that these oscillations originate from the inherent dynamics of water. The decay of the center-line slope (CLS) of the off-diagonal peak in 2D-IR spectra demonstrates the modulation of H-bond anti-correlations by low-frequency motions.

Section 5: Origin of H-Bond Anti-correlation

We estimate the distribution of H-bond conformations by simultaneously using information from all vibrational modes. The anti-correlated H-bond distances are found to be intrinsic to the H-bonding potential of XH2 groups. Experiments on urea's N(H/D)2 groups in dimethylsulfoxide (DMSO) confirm the presence of anti-correlated H-bonds, suggesting that this phenomenon is common to H-bonding XD2/XH2 moieties.

Section 6: Conclusions and Implications

We find that the H-bond distances of D2O and urea's ND2 groups are anti-correlated. These anti-correlations are encoded in the inhomogeneous linewidths and frequency-frequency correlations. Similar H-bond conformations are likely present in liquid water, and the dynamics of H-bond asymmetry are key to understanding water's phase behavior. Our findings also have implications for water as a solvent and the ability of water to hydrate solutes.