Dissecting the THz spectrum of liquid water from first principles via correlations in time and space

Abstract

Solvation of molecules in water is at the heart of a myriad of molecular phenomena and of crucial importance to understanding such diverse issues as chemical reactivity or biomolecular function. Complementing well-established approaches, it has been shown that laser spectroscopy in the THz frequency domain offers new insights into hydration from small solutes to proteins. Upon introducing spatially-resolved analyses of the absorption cross section by simulations, the sensitivity of THz spectroscopy is traced back to characteristic distance-dependent modulations of absorption intensities for bulk water. The prominent peak at ≈200 cm -1 is dominated by first-shell dynamics, whereas a concerted motion involving the second solvation shell contributes most significantly to the absorption at about 80 cm -1 ≈2.4 THz. The latter can be understood in terms of an umbrella-like motion of two hydrogen-bonded tetrahedra along the connecting hydrogen bond axis. Thus, a modification of the hydrogen bond network, e.g., due to the presence of a solute, is expected to affect vibrational motion and THz absorption intensity at least on a length scale that corresponds to two layers of solvating water molecules. This result provides a molecular mechanism explaining the experimentally determined sensitivity of absorption changes in the THz domain in terms of distinct, solute-induced dynamical properties in solvation shells of (bio)molecules—even in the absence of well-defined resonances.

Three-Dimensional Infrared Spectroscopy of Isotope-Substituted Liquid Water Reveals Heterogeneous Dynamics

“…In the other case (Figure 4c), the hydrogen is fully tetra- THz spectra 59,60 as well as ab initio molecular dynamics simulations. 61 This is consistent with our basic picture that the motions involve primarily reorganization within one hydrogen bonding motif (the clusters in the network analysis). Our results show that 3D-IR can at least partially decompose this highly overlapped spectral region, and detect motions which are due to different hydrogen bonding motifs.…”

Vibrational features of water at the low-density/high-density liquid structural transformations

“…This indicates on the correlation of the low-frequency molecular vibrational properties with the structural transformations at the pressure. While high-frequency mode corresponds to the inherent vibrational processes of the molecules, the time scale τ s ∼ 1/ω s is related with the vibrational dynamics of the molecular conglomerates [40,41]. The immediate correlation of the low-frequency vibrations with the structural transformations at the local spatial scales in the range of the second coordination shell can be directly seen from inset (b) of Fig.…”

2D-Raman-THz spectroscopy: A sensitive test of polarizable water models

“…That band is better represented in more sophisticated polarizable models of water, [43][44][45] presumably since those are polarizable also at the hydrogen sites and as such feel more closely the formation of a hydrogen bond, or since those are flexible models. It is however very well established that this band originates from charge flows between water molecules upon hydrogen bonding, 5,[36][37][38][39] and it is quite questionable whether polarizability can describe that effect. The electric field at a polarizable site scales decays quadratically with the distance to an hydrogen bond partner, whereas charge flow effects decrease much steeper with an exponential dependence.…”

“…32 It is well established that simple point charge models of water, such as TIP4P/2005 34 or SPC/E, 35 cannot account for the intensity of that band, since that band originates from charge flows within and between water molecules upon hydrogen bonding. 5,[36][37][38][39] Adding polarizability to a water model, either in an ad hoc manner to a trajectory that has been precalculated with the help of a pointcharge model, [39][40][41][42] or explicitly as part of the force field, [43][44][45] reveals the band in the THz absorption spectrum, albeit, often, with severely underestimated intensity.…”