2D IR spectroscopy of high-pressure phases of ice

Two-dimensional infrared-Raman spectroscopy as a probe of water's tetrahedrality

Two-dimensional spectroscopic techniques combining terahertz (THz), infrared (IR), and visible pulses offer a wealth of information about coupling among vibrational modes in molecular liquids, thus providing a promising probe of their local structure. However, the capabilities of these spectroscopies are still largely unexplored due to experimental limitations and inherently weak nonlinear signals. Here, through a combination of equilibrium-nonequilibrium molecular dynamics (MD) and a tailored spectrum decomposition scheme, we identify a relationship between the tetrahedral order of liquid water and its two-dimensional IR-IR-Raman (IIR) spectrum. The structure-spectrum relationship can explain the temperature dependence of the spectral features corresponding to the anharmonic coupling between low-frequency intermolecular and high-frequency intramolecular vibrational modes of water. In light of these results, we propose new experiments and discuss the implications for the study of tetrahedrality of liquid water.

Detailed crystallographic analysis of the ice V to ice XIII hydrogen-ordering phase transition

Ice V is a structurally highly complex material with 28 water molecules in its monoclinic unit cell. It is classified as a hydrogen-disordered phase of ice. Yet, some of its hydrogen-bonded water molecules display significant orientational order. Upon cooling pure ice V, additional orientational ordering cannot be achieved on the experimental time scale. Doping with hydrochloric acid has been shown to be most effective in enabling the phase transition of ice V to its hydrogen-ordered counterpart ice XIII. Here, we present a detailed crystallographic study of this phase transition investigating the effects of hydrochloric and hydrofluoric acid as well as lithium and potassium hydroxide doping. The magnitudes of the stepwise changes in the lattice constants during the phase transition are found to be more sensitive indicators for the extent of hydrogen order in ice XIII than the appearance of new Bragg peaks. Hydrofluoric acid and lithium hydroxide doping enable similar ordering processes as hydrochloric acid but with slower kinetics. The various possible space groups and ordered configurations of ice XIII are examined systematically, and the previously determined P2₁/a structure is confirmed. Interestingly, the partial hydrogen order already present in ice V is found to perpetuate into ice XIII, and these ordering processes are found to be independent of pressure. Overall, the hydrogen ordering goes along with a small increase in volume, which appears to be the origin of the slower hydrogen-ordering kinetics under pressure. Heating pressure-quenched samples at ambient pressure revealed low-temperature "transient ordering" features in both diffraction and calorimetry.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction

Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an all-encompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semiempirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository Web site (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-the-art multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future.

Vibrational echo spectroscopy of aqueous sodium bromide solutions from first principles simulations

A theoretical study of the time-dependent vibrational echo spectroscopy of sodium bromide solutions in deuterated water at two different concentrations of 0.5 and 5.0 M and at temperatures of 300 and 350 K is presented using the method of ab initio molecular dynamics simulations. The instantaneous fluctuations in frequencies of local OD stretch modes are calculated using time-series analysis of the simulated trajectories. The third-order polarization and intensities of three pulse photon-echo are calculated from ab initio simulations. The timescales of vibrational spectral diffusion are determined from the frequency time correlation functions (FTCF) and short-time slope of three pulse photon echo (S3PE) calculated within the second-order cumulant and Condon approximations. It is found that under ambient conditions, the rate of vibrational spectral diffusion becomes slower with increase in ionic concentration. Decay of S3PE calculated for different systems give timescales, which are in close agreement with those of FTCF and also with the results of experimental time-dependent vibrational spectroscopic experiments. © 2019 Wiley Periodicals, Inc.

Advances in the experimental exploration of water's phase diagram

Water's phase diagram displays enormous complexity with currently 17 experimentally confirmed polymorphs of ice and several more predicted computationally. For almost 120 years, it has been a stomping ground for scientific discovery, and ice research has often been a trailblazer for investigations into a wide range of materials-related phenomena. Here, the experimental progress of the last couple of years is reviewed, and open questions as well as future challenges are discussed. The specific topics include (i) the polytypism and stacking disorder of ice I, (ii) the mechanism of the pressure amorphization of ice I, (iii) the emptying of gas-filled clathrate hydrates to give new low-density ice polymorphs, (iv) the effects of acid/base doping on hydrogen-ordering phase transitions as well as (v) the formation of solid solutions between salts and the ice polymorphs, and the effect this has on the appearance of the phase diagram. In addition to continuing efforts to push the boundaries in terms of the extremes of pressure and temperature, the exploration of the "chemical" dimensions of ice research appears to now be a newly emerging trend. It is without question that ice research has entered a very exciting era.

Temperature dependence of the ultrafast vibrational echo spectroscopy of OD modes in liquid water from first principles simulations

The temperature dependence of the vibrational spectral diffusion of OD modes in liquid water is investigated through calculations of vibrational echo spectral observables from first principles molecular dynamics.

Simple Quantum Dynamics with Thermalization

In this paper, we introduce two simple quantum dynamics methods. One is based on the popular surface-hopping method, and the other is based on rescaling of the propagation on the bath ground-state potential surface. The first method is special, as it avoids specific feedback from the simulated quantum system to the bath and can be applied for precalculated classical trajectories. It is based on the equipartition theorem to determine if hops between different potential energy surfaces are allowed. By comparing with the formally exact Hierarchical Equations Of Motion approach for four model systems we find that the method generally approximates the quantum dynamics toward thermal equilibrium very well. The second method is based on rescaling of the nonadiabatic coupling and also neglect the effect of the state of the quantum system on the bath. By the nature of the approximations, they cannot reproduce the effect of bath relaxation following excitation. However, the methods are both computationally more tractable than the conventional fewest switches surface hopping, and we foresee that the methods will be powerful for simulations of quantum dynamics in systems with complex bath dynamics, where the system-bath coupling is not too strong compared to the thermal energy.