We have investigated the effects of tert-butyl alcohol (TBA) on the structure and dynamics of water at the liquid–vapor interface. The structure of interfacial water has been studied by calculating the structural correlations and vibrational sum frequency generation (VSFG) spectrum from molecular dynamics simulations. It is found that the dangling peak of the VSFG spectrum of the air–water interface near ∼3700 cm–1 almost disappears in presence of TBA at the chosen concentration which means that the interfacial region is covered by the solute molecules. The hydrogen-bonded peak in the VSFG spectrum is found to be red-shifted by ∼100 cm–1 as compared to that of pure the air–water interface despite the fact that the strength of hydrogen bonds in the interfacial region is found to be similar to that of the bulk. This red shift in the VSFG spectrum is found to be a consequence of the cancellation of the nonlinear responses from “up-” and “down”-oriented O–H modes of water in that region. The local structure around the interfacial water is found to be similar to that of bulk water where the oxygen of TBA provides an environment similar to the oxygen of water in the bulk. However, the dynamical properties like the orientational relaxation and vibrational spectral diffusion are found to slow down when one moves from bulk toward the TBA layer at the surface. The effects of intra- and inter-molecular coupling and third-order susceptibility on the VSFG spectrum are also discussed.

The hydrogen-bonded network of water can be affected both structurally and dynamically by the presence of ions. In the present study, we have considered three aqueous solutions of metal nitrates to investigate the effects of divalent cations (Mg2+ and Ca2+), compared to that of monovalent Na+ ions, on hydrogen-bond fluctuations and vibrational spectral diffusion through calculations of linear and two-dimensional infrared spectra of these solutions at room temperature. We have employed the methods of molecular dynamics simulations using effective polarizable models of ions combined with quantum mechanical calculations of transition variables and statistical mechanical calculations of spectral response functions of vibrational spectroscopy. Divalent cations are found to have much stronger and longer-ranged effects on the structure and dynamics of the hydrogen-bonded network than that induced by the monovalent sodium ions. The blue shifts in the calculated linear spectra are found to follow the Hofmeister trend for the cations. The 2D-IR spectral lineshape and intensity corresponding to three-pulse echo peak shift (3PEPS) experiments are calculated. The timescales of these nonlinear spectral responses and also frequency–time correlations show significant slowing down of spectral diffusion for solutions containing divalent Mg2+ and Ca2+ ions compared to the corresponding dynamics of the solution containing Na+ ions. Unlike NaNO3 solution, the relaxation of frequency and dipole orientational fluctuations of anion-bound water in Mg­(NO3)2 and Ca­(NO3)2 solutions are found to be somewhat slower than bulk water, which can be attributed to the presence of divalent cations whose effects go beyond their first solvation shells. This is also seen in the dynamics of bulk water in these solutions which is found to be notably slower for the solutions containing divalent cations than that in the NaNO3 solution. Unlike Mg2+ and Ca2+ ions, no specific cationic effect is observed for the Na+ ions.

Non-Condon effects in vibrational spectroscopy refers to the dependence of a molecule's vibrational transition dipole and polarizability on the coordinates of the surrounding environment. Earlier studies have shown that such effects can be pronounced for hydrogen-bonded systems like liquid water. Here, we present a theoretical study of two-dimensional vibrational spectroscopy under the non-Condon and Condon approximations at varying temperatures. We have performed calculations of both two-dimensional infrared and two-dimensional vibrational Raman spectra to gain insights into the temperature dependence of non-Condon effects in nonlinear vibrational spectroscopy. The twodimensional spectra are calculated for the OH vibration of interest in the isotopic dilution limit where the coupling between the oscillators is ignored. Generally, both the infrared and Raman line shapes undergo red shifts with decrease in temperature due to strengthening of hydrogen bonds and decrease in the fraction of OH modes with weaker or no hydrogen bonds. The infrared line shape is further red-shifted under the non-Condon effects at a given temperature, while the Raman line shape does not show any such red shift due to non-Condon effects. The spectral dynamics becomes slower on decrease of temperature due to slower hydrogen bond relaxation and, for a given temperature, the spectral diffusion occurs at a faster rate upon inclusion of non-Condon effects. The time scales of spectral diffusion extracted from different metrics agree well with each other and also with experiments. The changes in the spectrum due to non-Condon effects are found to be more significant at lower temperatures.

The structure, dynamics and vibrational spectroscopy of water surface covered by a monolayer of stearyl alcohol (STA) are investigated by means of molecular dynamics simulations and vibrational sum frequency generation...

Natural gas hydrate creates a lot of problems during its transportation and production by plugging gas pipelines and process equipment. This work envisages the effect of electrolytes (NaCl and CaCl 2 ) at different concentrations on methane hydrate formation and dissociation. Extensive observations on equilibrium pressure and temperature during hydrate formation and dissociation have been made. The experiments were conducted in the temperature range of 261 to 270 K and pressure range of 2.6 to 3.0 MPa. Both sodium chloride and calcium chloride salts were found to have significant inhibiting effects on hydrate formation and dissociation with the latter having stronger effects. Gas consumption was found to vary with the progress of the hydrate formation as well as the concentration of salts in the hydrate cell. The dissociation enthalpies of methane hydrates in the presence of the above salts were also determined using the Clausius-Clapeyron equation based on the phase equilibrium data.