Infrared spectroscopy of the water OH stretch provides a sensitive probe of the local hydrogen-bonding structure and dynamics of water molecules. Previously, we have utilized a mixed quantum/classical model to calculate vibrational spectroscopic observables for bulk water, ice, the liquid/vapor interface, and small water clusters, as well as water interacting with ions and biological molecules. These studies rely on spectroscopic maps that relate the OH stretching frequency and transition dipole to the local environment around a water molecule. Our spectroscopic maps were parametrized based on water clusters taken from bulk water simulations; in this article, we test the robustness of these maps for water in nonbulk-liquid environments. We find that the frequency, transition dipole, and coupling maps work as well for the water surface, ice Ih, and the water hexamer as they do for liquid water. This suggests that these maps may be generally applied to study the vibrational spectroscopy of water in diverse, potentially heterogeneous environments.

Vibrational spectroscopy of the water bending mode has been investigated experimentally to study the structure of water in condensed phases. In the present work, we calculate the theoretical infrared (IR) and sum-frequency generation (SFG) spectra of the HOH bend in liquid water and at the water liquid/vapor interface using a mixed quantum/classical approach. Classical molecular dynamics simulation is performed by using a recently developed water model that explicitly includes three-body interactions and yields a better description of the water surface. Ab-initio-based transition frequency, dipole, polarizability, and intermolecular coupling maps are developed for the spectral calculations. The calculated IR and SFG spectra show good agreement with the experimental measurements. In the theoretical imaginary part of the SFG susceptibility for the water liquid/vapor interface, we find two features: a negative band centered at 1615 cm(-1) and a positive band centered at 1670 cm(-1). We analyze this spectrum in terms of the contributions from molecules in different hydrogen-bond classes to the SFG spectral density and also compare to SFG results for the OH stretch. SFG of the water bending mode provides a complementary picture of the heterogeneous hydrogen-bond configurations at the water surface.

We present a unified picture of how OH-stretch spectroscopy in water can be understood in terms of hydrogen bonding for the four systems listed in the title. To understand the strength, and hence OH-stretch frequency, of a hydrogen bond, it is crucial to consider the number of additional acceptor hydrogen bonds made by both the donor and acceptor molecules. This necessity for focusing on the hydrogen-bond environment of both donor and acceptor molecules follows from quantum chemical considerations and is related to the three-body interactions in water. Armed with this understanding we can make a detailed interpretation of the OH-stretch IR absorption spectrum of the cage conformer for HOD(D2O)5 and the imaginary part of the ssp OH-stretch sum-frequency spectrum of the surface of liquid D2O with dilute HOD.

Using our newly developed explicit three-body (E3B) water model, we simulate the surface of liquid water. We find that the timescale for hydrogen-bond switching dynamics at the surface is about three times slower than that in the bulk. In contrast, with this model rotational dynamics are slightly faster at the surface than in the bulk. We consider vibrational two-dimensional (2D) sum-frequency generation (2DSFG) spectroscopy as a technique for observing hydrogen-bond rearrangement dynamics at the water surface. We calculate the nonlinear susceptibility for this spectroscopy for two different polarization conditions, and in each case we see the appearance of cross-peaks on the timescale of a few picoseconds, signaling hydrogen-bond rearrangement on this timescale. We thus conclude that this 2D spectroscopy will be an excellent experimental technique for observing slow hydrogen-bond switching dynamics at the water surface. Interfaces play important roles in many disciplines of science. The water liquid/vapor interface, for example, is of great interest in chemistry, biology, and earth science and is an important model system for water in a heterogeneous environment. Of particular interest is understanding the extent to which the structure and dynamics, and ultimately reactivity, of water at the interface differ from those in the bulk. For example, how does the distribution of hydrogen bonds differ between interfacial and bulk water? How anisotropic is the orientation of the water molecules at the interface? In terms of dynamics, how do the diffusion constant, rotational relaxation time, and hydrogen-bond rearrangement time vary as the interface is approached? One can also consider vibrational dynamics processes such as energy relaxation and transfer.One important technique for addressing these questions is computer simulation. Models used in these calculations for the water surface range from rigid, fixed-point-charge two-body models (1-3), to fluctuating charge or polarizable models (4, 5), to ab initio molecular dynamics calculations (6-10). Regarding static properties, for example, some effort has been expended toward understanding what fraction of H atoms in the surface layer are hydrogen bonded, and what fraction of molecules do not donate any hydrogen bonds (nondonors or "acceptor-only" molecules) (6, 9). In terms of dynamics, it is generally found that diffusion is faster at the interface than in the bulk (1, 4, 10), and rotational relaxation is also faster (3,6,7,10). On the other hand, two studies with fixed-charge two-body models show that hydrogenbond rearrangement is slower at the interface (2, 3), whereas one study with a fluctuating-charge model shows that hydrogen-bond rearrangement is faster (5). In this latter study the authors conclude that this is generally true for polarizable models.Because of its surface sensitivity, vibrational sum-frequency generation (SFG) spectroscopy (11, 12) has become one of the most powerful experimental techniques for the study of interfaces, including the one separa...

Evidence for a liquid-liquid critical point in supercooled water within the E3B3 model and a possible interpretation of the kink in the homogeneous nucleation line

Supercooled water exhibits many thermodynamic anomalies, and several scenarios have been proposed to interpret them, among which the liquid-liquid critical point (LLCP) hypothesis is the most commonly discussed. We investigated Widom lines and the LLCP of deeply supercooled water, by using molecular dynamics simulation with a newly reparameterized water model that explicitly includes three-body interactions. Seven isobars are studied from ambient pressure to 2.5 kbar, and Widom lines are identified by calculating maxima in the coefficient of thermal expansion and the isothermal compressibility (both with respect to temperature). From these data we estimate that the LLCP of the new water model is at 180 K and 2.1 kbar. The oxygen radial distribution function is calculated along the 2 kbar isobar. It shows a steep change in the height of its second peak between 180 and 185 K, which indicates a transition between the high-density liquid and low-density liquid phases and which is consistent with the ascribed location of the critical point. The good agreement of the height of the second peak of the radial distribution function between simulation and experiment at 1 bar, as a function of temperature, supports the validity of the model. The location of the LLCP within the model is close to the kink in the experimental homogeneous nucleation line. We use existing experimental data to argue that the experimental LLCP is at 168 K and 1.95 kbar and speculate how this LLCP and its Widom line might be responsible for the kink in the homogeneous nucleation line.

Two intrinsic difficulties in modeling condensed-phase water with conventional rigid non-polarizable water models are: reproducing the static dielectric constants for liquid water and ice Ih, and generating the peak at about 200 cm(-1) in the low-frequency infrared spectrum for liquid water. The primary physical reason for these failures is believed to be the missing polarization effect in these models, and consequently various sophisticated polarizable water models have been developed. However, in this work we pursue a different strategy and propose a simple empirical scheme to include the polarization effect only on the dipole surface (without modifying a model's intermolecular interaction potential). We implement this strategy for our explicit three-body (E3B) model. Our calculated static dielectric constants and low-frequency infrared spectra are in good agreement with experiment for both liquid water and ice Ih over wide temperature ranges, albeit with one fitting parameter for each phase. The success of our modeling also suggests that thermal fluctuations about local minima and the energy differences between different proton-disordered configurations play minor roles in the static dielectric constant of ice Ih. Our analysis shows that the polarization effect is important in resolving the two difficulties mentioned above and sheds some light on the origin of several features in the low-frequency infrared spectra for liquid water and ice Ih.

No man's land is the region in the metastable phase diagram of water where it is very difficult to do experiments on liquid water because of homogeneous nucleation to the crystal. There are a number of estimates of the location in no man's land of the liquid-liquid critical point, if it exists. We suggest that published IR absorption experiments on water droplets in no man's land can provide information about the correct location. To this end, we calculate theoretical IR spectra for liquid water over a wide range of temperatures and pressures, using our E3B3 model, and use the results to argue that the temperature dependence of the experimental spectra is inconsistent with several of the estimated critical point locations, but consistent with others.