# Impact of nuclear quantum effects on the structural inhomogeneity of liquid water

January 28, 2019**The degree to which water is structured is an extremely intriguing problem and a matter of ongoing debate.**

The 2D Raman–terahertz (THz) response of liquid water is studied in dependence of temperature and isotope substitution (H_{2}O, D_{2}O, and H^{18}_{2}O). In either case, a very short-lived (i.e., between 75 and 95 fs) echo is observed that reports on the inhomogeneity of the low-frequency intermolecular modes and hence, on the heterogeneity of the hydrogen bond networks of water. The echo lifetime slows down by about 20% when cooling the liquid from room temperature to the freezing point. Furthermore, the echo lifetime of D_{2}O is 6.5±1% slower than that of H_{2}O, and both can be mapped on each other by introducing an effective temperature shift of ∆T=4.5±1 K. In contrast, the temperature-dependent echo lifetimes of H^{18}_{2}O and H_{2}O are the same within error. D_{2}O and H^{18}_{2}O have identical masses, yet H^{18}_{2}O is much closer to H_{2}O in terms of nuclear quantum effects. It is, therefore, concluded that the echo is a measure of the structural inhomogeneity of liquid water induced by nuclear quantum effects.

**Figure**: *2D Raman–THz–THz responses of neat H _{2}O and D_{2}O at different temperatures. Full 2D signals for H_{2}O at (A) 293 K and (B) 276 K as well as for D_{2}O at (D) 293 K and (E) 280 K. The upper right quadrants, which correspond to the Raman–THz–THz pulse sequence, and the main diagonals t1=t2 (dotted lines) are indicated. C compares 1D cuts along the t1=t2 diagonal for H_{2}O at 293 K (red line) and 276 K (blue line), and F compares those for D_{2}O at 293 K (red line) and 280 K (blue line), in either case together with single exponential fits (solid lines). The 1D and 2D data are normalized to the maximum signal, and the 1D cuts start at 50 fs, after which time the effects of the pump-probe pulse overlap can be neglected.*

See also:

**Reference:**Berger, A., G. Ciardi, D. Sidler,

**P. Hamm**and A. Shalit (2019). Impact of nuclear quantum effects on the structural inhomogeneity of liquid water. Proc. Natl. Acad. Sci. U.S.A.: 201818182 (10.1073/pnas.1818182116) Berger-20191

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