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 (H₂O, D₂O, and H¹⁸₂O). 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 D₂O is 6.5±1% slower than that of H₂O, 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 H¹⁸₂O and H₂O are the same within error. D₂O and H¹⁸₂O have identical masses, yet H¹⁸₂O is  much  closer  to  H₂O  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 H₂O and D₂O at different temperatures. Full 2D signals for H₂O at (A) 293 K and (B) 276 K as well as for D₂O 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 H₂O at 293 K (red line) and 276 K (blue line), and F compares those for D₂O 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