Instantaneous normal mode analysis of correlated cluster motions in hydrogen bonded liquids

Pressure response of the THz spectrum of bulk liquid water revealed by intermolecular instantaneous normal mode analysis

The radial distribution functions of liquid water are known to change significantly their shape upon hydrostatic compression from ambient conditions deep into the kbar pressure regime. It has been shown that despite their eye-catching changes, the fundamental locally tetrahedral fourfold H-bonding pattern that characterizes ambient water is preserved up to about 10 kbar (1 GPa), which is the stability limit of liquid water at 300 K. The observed increase in coordination number comes from pushing water molecules into the first coordination sphere without establishing an H-bond, resulting in roughly two such additional interstitial molecules at 10 kbar. THz spectroscopy has been firmly established as a powerful experimental technique to analyze H-bonding in aqueous solutions given that it directly probes the far-infrared lineshape and thus the prominent H-bond network mode around 180 cm^-1. We, therefore, set out to assess pressure effects on the THz response of liquid water at 10 kbar in comparison to the 1 bar (0.1 MPa) reference, both at 300 K, with the aim to trace back the related lineshape changes to the structural level. To this end, we employ the instantaneous normal mode approximation to rigorously separate the H-bonding peak from the large background arising from the pronounced librational tail. By exactly decomposing the total molecular dynamics into hindered translations, hindered rotations, and intramolecular vibrations, we find that the H-bonding peak arises from translation-translation and translation-rotation correlations, which are successively decomposed down to the level of distinct local H-bond environments. Our utmost detailed analysis based on molecular pair classifications unveils that H-bonded double-donor water pairs contribute most to the THz response around 180 cm^-1, whereas interstitial waters are negligible. Moreover, short double-donor H-bonds have their peak maximum significantly shifted toward higher frequencies with respect to such long H-bonds. In conjunction with an increasing relative population of these short H-bonds versus the long ones (while the population of other water pair classes is essentially pressure insensitive), this explains not only the blue-shift of the H-bonding peak by about 20-30 cm^-1 in total from 1 bar to 10 kbar but also the filling of the shallow local minimum of the THz lineshape located in between the network peak and the red-wing of the librational band at 1 bar. Based on the changing populations as a function of pressure, we are also able to roughly estimate the pressure-dependence of the H-bond network mode and find that its pressure response and thus the blue-shifting are most pronounced at low kbar pressures.

Reconsideration of the relaxational and vibrational line shapes of liquid water based on ultrabroadband dielectric spectroscopy

The Debye relaxation function is widely used to describe the large dielectric dispersion of ambient water around 20 GHz. However, from a theoretical point of view, this function is supposed to give incorrect predictions at high frequencies owing to the inappropriate assumption that inertial effects and intermolecular interactions do not affect the relaxation dynamics. Our ultrabroadband spectroscopy investigation of liquid water ranging from 500 MHz to 400 THz did demonstrate that the Debye function is inaccurate far above the microwave region. As an alternative, we tried a stochastic frequency modulation (SFM) model assuming instantaneous modification of the line shapes by the correlation with the surrounding system. The SFM relaxation model reproduced the experimental dielectric spectra up to 400 THz, showing that the hydrogen-bond dynamics are associated with the inertial effect that causes the non-exponential relaxation behaviour in a very short time (typically 25 fs). Within the framework of this relaxation model, the hindered translation modes are able to be approximated as fast modulation (homogeneous) line shapes because the interaction time with frequency modulation is too short. Compared with them, the libation mode is found to have a relatively slow modulation (inhomogeneous) origin, where disturbance of water hydrogen bonds induced by the hindered translations leads to fluctuations in the libration frequency.

The hydrogen-bond collective dynamics in liquid methanol

The relatively simple molecular structure of hydrogen-bonded (HB) systems is often belied by their exceptionally complex thermodynamic and microscopic behaviour. For this reason, after a thorough experimental, computational and theoretical scrutiny, the dynamics of molecules in HB systems still eludes a comprehensive understanding. Aiming at shedding some insight into this topic, we jointly used neutron Brillouin scattering and molecular dynamics simulations to probe the dynamics of a prototypical hydrogen-bonded alcohol, liquid methanol. The comparison with the most thoroughly investigated HB system, liquid water, pinpoints common behaviours of their THz microscopic dynamics, thereby providing additional information on the role of HB dynamics in these two systems. This study demonstrates that the dynamic behaviour of methanol is much richer than what so far known, and prompts us to establish striking analogies with the features of liquid and supercooled water. In particular, based on the strong differences between the structural properties of the two systems, our results suggest that the assignment of some dynamical properties to the tetrahedral character of water structure should be questioned. We finally highlight the similarities between the characteristic decay times of the time correlation function, as obtained from our data and the mean lifetime of hydrogen bond known in literature.

The hydrogen-bond network of water supports propagating optical phonon-like modes

The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. The assignment of spectral peaks depends on whether they are collective modes or single-molecule motions. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Using molecular dynamics simulations, here we find dispersive optical phonon-like modes in the librational and OH-stretching bands. We argue that on subpicosecond time scales these modes propagate through water's hydrogen-bond network over distances of up to 2 nm. In the long wavelength limit these optical modes exhibit longitudinal–transverse splitting, indicating the presence of coherent long-range dipole–dipole interactions, as in ice. Our results indicate the dynamics of liquid water have more similarities to ice than previously thought.

How the local structure of water varies as a function of temperature is a long-studied topic, which is still under debate. Here, the authors show that dielectric susceptibility measurements might be used to probe and identify propagating optical phonon-like modes in the hydrogen-bond network of water.

Massively parallel molecular dynamics simulation of formation of ice-crystallite precursors in supercooled water: incipient-nucleation behavior and role of system size

Ice-crystallite precursor formation in supercooled water was studied via molecular dynamics for systems ranging from ∼10^{6} to 8.6×10^{6} molecules, using a tetrahedrally biased single-site "mW" model. This has established system-size effects in the early onset of nucleation, so as to study often-transient precursors' beguiling propensity to "flicker" into instantaneous locally ordered molecular arrangements redolent of ice. In addition, the adoption of solidlike and liquidlike bimodal local configurational-energy distributions was observed, characteristic of early nucleation. Larger systems favored a higher probability of precursor formation, although such ones were not usually longer lived relative to those in smaller systems (which themselves are rather transient). It was concluded tentatively that subtle effects of differences in systemwide density fluctuations and accessible lower-frequency modes tend to favor precursor formation in larger systems, although not necessarily the precursor's kinetic stability.

Thermal Conductivity of Supercooled Water: An Equilibrium Molecular Dynamics Exploration

The thermal conductivity of both supercooled and ambient-temperature water at atmospheric pressure has been computed over the 140-270 K temperature range for three popular water models via equilibrium molecular dynamics in the Green-Kubo setting. No strong temperature dependence of thermal conductivity was observed. The underlying phonon modes contributing to thermal conduction processes have been examined in the present work, and it has been established that (translational) acoustic modes dominate in supercooled water.

Single-particle and collective dynamics of methanol confined in carbon nanotubes: a computer simulation study

We present the results of computer simulations of methanol confined in carbon nanotubes. Different levels of confinement were identified as a function of the nanotube radius and characterized using a pair-distribution function adapted to the cylindrical geometry of these systems. Dynamical properties of methanol were also analysed as a function of the nanotube size, both at the level of single-particle and collective properties. We found that confinement in narrow carbon nanotubes strongly affects the dynamical properties of methanol with respect to the bulk phase, due to the strong interaction with the carbon nanotube. In the other cases, confined methanol shows properties quite similar to those of the bulk phase. These phenomena are related to the peculiar hydrogen bonded network of methanol and are compared to the behaviour of water confined in similar conditions. The effect of nanotube flexibility on the dynamical properties of confined methanol is also discussed.

Dynamical properties of supercooled water close to the liquid-liquid coexistence lines, and their relation to those at ambient conditions

We report results of molecular dynamics simulations of supercooled bulk water at 180 K, close to the liquid/liquid coexistence lines recently discovered in the supercooled regime, both experimentally and by computer simulations. Despite the considerable differences in the densities of the three states considered, the obtained velocity autocorrelation functions display very similar behaviour in every case. On the other hand, the corresponding spectra show the presence of three well-defined modes. The two modes at higher frequencies are assigned to the symmetric and asymmetric stretching motions, respectively, whereas the lowest frequency mode to the bending and torsional motions in clusters of five hydrogen-bonded water molecules. A careful fitting procedure demonstrates the presence of such clusters also in the normal liquid phase of water at ambient conditions.

Collective properties of water confined in carbon nanotubes: A computer simulation study

The collective properties of water confined in the (10,10), (8,8) and (6,6) carbon nanotubes are studied by analysing the longitudinal-current autocorrelation function, calculated from computer-simulated trajectories. The corresponding spectra clearly show the presence of two excitations, but their behaviour is quite different from that observed in the case of bulk water. Instead of the strong positive dispersion of the hydrodynamic sound mode characteristic of bulk water (the fast-sound phenomenon), the sound dispersion relation of confined water is observed to flatten into a non-propagating mode, while a second excitation appears at a higher frequency. This behaviour is analysed in terms of the localized oscillation modes of the hydrogen-bond network.

Instantaneous normal mode analysis of orientational motions in liquid water: local structural effects

We have investigated the effects of local structures on the orientational motions in liquid water in terms of the instantaneous normal mode (INM) analysis. The local structures of a molecule in liquid water are characterized by two different kinds of index: the asphericity parameter of its Voronoi polyhedron and the numbers of the H bonds donated and accepted by the molecule. According to the two kinds of index, the molecules in the simulated water are classified into subensembles, for which the rotational contributions to the INM spectrum are calculated. Our results indicate that by increasing the asphericity, the rotational contribution has a shift toward the high-frequency end in the real spectrum and a decrease in the fraction of the imaginary modes. Furthermore, we find that this shift essentially relies on the number of the donated H bonds of a molecule, but has almost nothing to do with that of the accepted H bonds. The local structural effects resulting from the geometry of water molecule are also discussed.

The influence of chain dynamics on the far-infrared spectrum of liquid methanol-water mixtures

Far-infrared-absorption spectroscopy has been used to study the low-frequency (<or=100 cm-1) intermolecular modes of methanol in mixtures with water. With the aid of a first-principles molecular-dynamics simulation on an equivalent system, a detailed understanding about the origin of the low-frequency IR modes has been established. The total dipole spectrum from the simulation suggests that the bands appearing in the experimental spectra at approximately 55 and 70 cm-1 in methanol and methanol-rich mixtures arise from both fluctuations and torsional motions occurring within the methanol hydrogen-bonded chains. The influence of these modes on both the solvation dynamics and the relaxation mechanisms in the liquid is discussed within the context of recent experimental and theoretical results that have emerged from studies focusing on the short-time dynamics in the methanol hydrogen bond network.

The influence of chain dynamics on the far-infrared spectrum of liquid methanol

Far-infrared-absorption spectroscopy is used to investigate the low-frequency (<or=100 cm-1) intermolecular interactions in liquid methanol. Using an intense source of far-infrared radiation, modes are elucidated at approximately 30 and 70 cm-1 in the absorption spectrum. These modes are believed to arise from intermolecular bending and librational motions, respectively, and are successfully reproduced in an ab initio molecular-dynamics simulation of methanol.