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

Abstract

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.water | multidimensional spectroscopy | THz spectroscopy | nuclear quantum effects

Velocity echoes in water

“…In our experiments, the echoes became more pronounced when adding salts to the solution to artificially increase the inhomogeneity of the hydrogen-bond networks, 6 when cooling neat water that makes those networks more persistent, 7 or when deuterating water that makes hydrogen bonds slightly stronger. 7 We do consider these echoes, and their dependence on external parameters, the most important outcome from 2D-Raman-THz spectroscopy so far. Recently, we fitted the 2D-Raman-THz response to a model consisting of an ensemble of anharmonic oscillators, augmented with electrical anharmonicity, suggesting that the low frequency bands are indeed in the "quasi-inhomogenous" limit.…”

“…2,3 It is the directionality of the hydrogen-bonding between water molecules, forming networks of connected water molecules, that is responsible for the peculiar properties of water. With the introduction of hybrid 2D-Raman-THz techniques, [4][5][6][7] it became possible recently to measure the 2D response of water in a very low frequency regime, where these intermolecular hydrogen-bond modes are found. 2D spectroscopic methods can enhance the information that can be extracted from the very blurred spectra in this spectral range, and indeed, echoes have been observed.…”

Anomalies in Supercooled Water at ∼230 K Arise from a 1D Polymer to 2D Network Topological Transformation

“…polydispersity) of the HB in liquid water has been confirmed in experiments using 2D Raman-terahertz (THz) technique, 25 which has the ability to distinguish between the homogeneous and inhomogeneous broadening. Indeed, the 2D Raman-THz of Berger et al 25 found the HB lifetime of H2O to be in the range of 75 to 95 fs for the temperature range of 298 to 276 K which agrees well with the range of SHB lifetimes predicted independently by RexPoN FF, 90 to 100 fs, confirming the heterogeneity of the SHB network predicted by RexPoN. 24 .…”

Feynman diagram description of 2D-Raman-THz spectroscopy applied to water

“…[20][21][22][23] Due to technical limitations, the latter is as of now the only 2D spectroscopy in the THz range that has been successfully applied to water and aqueous salt solutions. [22][23][24] Of particular interest in these experiments is the observation of a very short-lived echo, whose decay time has been related to the relevant time-scales in water. However, a detailed understanding of the 2D-Raman-THz response is still lacking.…”

“…[27][28][29][43][44][45][46][47][48] Since typical THz experiments work in a frequency range equivalent to k B T , the response can be derived in the classical limit from molecular dynamics (MD) simulation. 38 This approach appears to be the method of choice for complicated systems like water, since basically all effects, apart from possible quantum effects, 24 are captured implicitly by a MD force field, including anharmonicities, mode coupling, chemical exchange, and orientational averaging. It has been shown that the MD approach reveals responses, which strongly depend on the force field used, especially on the description of polarizability, albeit in a rather nonintuitive and indirect way.…”