Sub-picosecond hydrogen-bond anticorrelations of water*

Liquid water is traditionally viewed to form a distorted tetrahedral hydrogen-bond (H-bond) network. Yet, this rather symmetric structure has been challenged by x-ray [1] and computer simulation [2] studies. These studies have revealed a highly asymmetric H-bond geometry resulting in chain- or ring-like configurations, relevant to understanding the various anomalies of water [2]. Here, we characterize the inherent H-bond symmetry of water by studying water diluted in dimethylformamide with two-dimensional infrared (2D-IR) spectroscopy. We characterize the distribution of local H-bonds via coupling and broadening of the OD stretching vibrations of HOD and D₂O. Experiments on the coupled OD stretch oscillators of D₂O – the asymmetric and symmetric stretching modes – reveal a markedly narrower inhomogeneous linewidth for the coupled modes as compared to the uncoupled vibration of HOD. Frequency maps obtained from density functional theory calculations show that these differing linewidths can be explained by anti-correlated H-bond strengths of water. This anti-correlation is confirmed by the cross-peaks in the 2D-IR spectra. Yet, our results indicate that this anti-correlation is rather short-lived (few 100s of fs) and a mere results of the H-bond potential energy landscape. In fact, experiments on urea dissolved in dimethylsulfoxide provide evidence for a similar asymmetry of urea’s N-H H-bonds. As such, our results suggest that asymmetric H-bonding is an inherent feature of XH₂ groups (X=N,O) and the structural consequences are rapidly smeared out at ambient conditions.

References

[1] P. Wernet, D. Nordlund, U. Bergmann, M. Cavalleri, M. Odelius, H. Ogasawara, L. Å. Näslund, T. K. Hirsch, L. Ojamäe, P. Glatzel, L. G. M. Pettersson and A. Nilsson, Science (2004), 304, 995.

[2] T.D. Kühne and R. Z. Khaliullin, J. Am. Chem. Soc. (2014) 136, 3395.