OH-Stretch Vibrational Relaxation of HOD in Liquid to Supercritical D2O

Intramolecular O-H⋯S hydrogen bonding in threefold symmetry: Line broadening dynamics from ultrafast 2DIR-spectroscopy and ab initio calculations

The dynamics of intramolecular hydrogen-bonding involving sulfur atoms as acceptors is studied using two-dimensional infrared (2DIR) spectroscopy. The molecular system is a tertiary alcohol whose donating hydroxy group is embedded in a hydrogen-bond potential with torsional C₃-symmetry about the carbon-oxygen bond. The linear and 2DIR-spectra recorded in the OH-stretching region of the alcohol can be simulated very well using Kubo's line shape theory based on the cumulant expansion for evaluating the linear and nonlinear optical response functions. The correlation function for OH-stretching frequency fluctuations reveals an ultrafast component decaying with a time constant of 700 fs, which is in line with the apparent decay of the center line slopes averaged over absorption and bleach/emission signals. In addition, a quasi-static inhomogeneity is detected, which prevents the 2DIR line shape to fully homogenize within the observation window of 4 ps. The experimental data were then analyzed in more detail using a full ab initio approach that merges time-dependent structural information from classical molecular dynamics (MD) simulations with an OH-stretching frequency map derived from density functional theory (DFT). The latter method was also used to obtain a complementary transition dipole map to account for non-Condon effects. The 2DIR-spectra obtained from the MD/DFT method are in good agreement with the experimental data at early waiting delays, thereby corroborating an assignment of the fast decay of the correlation function to the dynamics of hydrogen-bond breakage and formation.

Two-dimensional infrared spectroscopy from the gas to liquid phase: density dependent J-scrambling, vibrational relaxation, and the onset of liquid character

Ultrafast 2DIR spectra and pump-probe responses of the N₂O ν₃ asymmetric stretch in SF₆ as a function of density from the gas to supercritical phase and liquid are reported. 2DIR spectra unequivocally reveal free rotor character at all densities studied in the gas and supercritical region. Analysis of the 2DIR spectra determines that J-scrambling or rotational relaxation in N₂O is highly efficient, occurring in ∼1.5 to ∼2 collisions with SF₆ at all non-liquid densities. In contrast, N₂O ν₃ vibrational energy relaxation requires ∼15 collisions, and complete vibrational equilibrium occurs on the ∼ns scale at all densities. An independent binary collision model is sufficient to describe these supercritical state point dynamics. The N₂O ν₃ in liquid SF₆ 2DIR spectrum shows no evidence of free rotor character or spectral diffusion. Using these 2DIR results, hindered rotor or liquid-like character is found in gas and all supercritical solutions for SF₆ densities ≥ρ* = 0.3, and increases with SF₆ density. 2DIR spectral analysis offers direct time domain evidence of critical slowing for SF₆ solutions closest to the critical point density. Applications of 2DIR to other high density and supercritical solution dynamics and descriptions are discussed.

Water under Supercritical Conditions: Hydrogen Bonds, Polarity, and Vibrational Frequency Fluctuations from Ab Initio Simulations with a Dispersion Corrected Density Functional

We have studied the effects of dispersion interactions on the dynamics of vibrational frequency fluctuations, hydrogen bonds, and free OD modes in supercritical heavy water at three different densities by means of ab initio molecular dynamics simulations. The vibrational spectral diffusion, as described by the frequency fluctuations, is studied through calculations of frequency time correlation of stretch modes of deuterated water, and its relations to the dynamics of hydrogen bonds and free OD modes are described. In addition, some of the other dynamical, structural, and electronic properties such as diffusion, rotational relaxation, radial distribution functions, hydrogen bond and coordination numbers, and dipole moments are also investigated from the perspectives of their variation with inclusion of dispersion interactions at varying density of the solvent. Although some changes in the structural properties are found on inclusion of dispersion corrections, no significant difference in the fluctuation dynamics of OD stretching frequencies and also in other dynamical quantities of supercritical water are found because of dispersion effects. The dynamics of water molecules under supercritical conditions is very fast compared to the corresponding dynamics under ambient conditions. The large thermal effects at such a high temperature seem to take over any relatively minor changes that might be introduced by weak dispersion interaction.

Vibrational Energy Relaxation of Thiocyanate Ions in Liquid-to-Supercritical Light and Heavy Water. A Fermi's Golden Rule Analysis

The vibrational relaxation dynamics following an ultrafast nitrile stretching (ν3) excitation of thiocyanate anions dissolved in light and heavy water have been studied over a wide temperature and density range corresponding to the aqueous liquid up to the supercritical phase. In both solvents, the relaxation of the ν3 = 1 state of the anion leads to a direct recovery of the vibrational ground state and involves the resonant transfer of the excess vibrational energy onto the solvent. In light water, the energy-accepting states are provided by the bending-librational combination band (νb + νL), while in heavy water, the relaxation is thermally assisted by virtual acceptor states derived from the stretching-librational/restricted translational hot band (νS - νL,T). The relaxation rate is found to strictly obey Fermi's Golden Rule when the density of resonant solvent states is estimated from the linear infrared spectra of the solute and the pure solvents.

Fluctuations and Relaxation Dynamics of Liquid Water Revealed by Linear and Nonlinear Spectroscopy

Many efforts have been devoted to elucidating the intra- and intermolecular dynamics of liquid water because of their important roles in many fields of science and engineering. Nonlinear spectroscopy is a powerful tool to investigate the dynamics. Because nonlinear response functions are described by more than one time variable, it is possible to analyze static and dynamic mode couplings. Here we review the intra- and intermolecular dynamics of liquid water revealed by recent linear and nonlinear spectroscopic experiments and computer simulations. In particular, we discuss the population relaxation, anisotropy decay, and spectral diffusion of the intra- and intermolecular motions of water and their temperature dependence, which play important roles in ultrafast dynamics and relaxations in water.

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

Femtosecond mid-infrared pump-probe spectroscopy was carried out to obtain information about the dynamics of vibrational energy relaxation in liquid ethanolamine at room temperature and ambient pressure. Through partial deuteration it was possible to disentangle the dynamics resulting from the OH and the NH stretching modes that proceed independently and simultaneously in the hydrogen-bonded liquid following an ultrafast vibrational excitation by a resonant mid-infrared pulse. The OH-stretching vibrational lifetime was determined to be 450 fs while the NH-stretching lifetime was found to be 1.2 ps. This large difference in lifetimes highlights the importance of the hydrogen-donating and the hydrogen-accepting character of the vibrating groups that are engaged in hydrogen-bonding.

Vibrationally excited ultrafast thermodynamic phase transitions at the water/air interface

The extraordinary ability of the hydrogen-bond network of water in the condensed phase to thermalize vibrational excitations within several picoseconds, even under supercritical conditions, offers the possibility of creating highly excited thermodynamic states at water surfaces on ultrafast time scales using vibrationally resonant short infrared laser pulses. We experimentally and numerically studied such states created by depositing ~100 ps long pulses tuned to the 3400 cm(-1) O–H stretch vibration at the water/air interface using time-resolved dark-field imaging and time-resolved optical reflectivity. The results are reasonably well described by using a hydrodynamic ablation model under the assumption of impulsive heat deposition. The large thermoelastic stress amplitudes on the order of 1 GPa created within 100 ps by depositing laser pulses with ~1 J cm(-2) fluence were inferred from the numerical simulations. Stresses of this magnitude drive the excited water layer into a very fast expansion resulting in rapid adiabatic cooling and thorough vaporization within a few nanoseconds. The spatial and temporal lengths scales of the ablation plume are nearly ideal for ejecting molecules into the gas phase with minimum perturbation for applications ranging from mass spectrometry and laser surgery to the development of extremely high pressure molecular beams.

On the cluster composition of supercritical water combining molecular modeling and vibrational spectroscopic data

The present study is aimed at a detailed analysis of supercritical water structure based on the combination of experimental vibrational spectra as well as molecular modeling calculations of isolated water clusters. We propose an equilibrium cluster composition model where supercritical water is considered as an ideal mixture of small water clusters (n=1-3) at the chemical equilibrium and the vibrational spectra are expected to result from the superposition of the spectra of the individual clusters, Thus, it was possible to extract from the decomposition of the midinfrared spectra the evolution of the partition of clusters in supercritical water as a function of density. The cluster composition predicted by this model was found to be quantitatively consistent with the near infrared and Raman spectra of supercritical water analyzed using the same procedure. We emphasize that such methodology could be applied to determine the portion of cluster in water in a wider thermodynamic range as well as in more complex aqueous supercritical solutions.

Effects of nonadditive interactions on ion solvation at the water/vapor interface: a molecular dynamics study

The solvation of halide ions at the water/vapor interface is investigated by using molecular dynamics simulations with nonpolarizable molecular mechanical (MM), polarizable MM, and quantum mechanical (QM)/MM methods. The free energy profile of the ion solvation is decomposed into the energy and the entropic contributions along the ion displacement from inside to the surface of water. It is found that the surface affinity of the ion, relative to the bulk value, is determined by a subtle balance between the energetic destabilization and the entropic stabilization with the ion displacement. The amount of energetic destabilization is found to be reduced when nonadditive interactions are included, as in the polarizable MM and QM/MM models. The structure of water around the ion at the interface is also largely modified when the higher order effects are considered. For example, the induced dipole effect enhances the solvation structure around the ion at the interface significantly and thus reduces the amount of entropic stabilization at the interface, relative to in the bulk. It is found that this induced dipole effect causes the slowing in the ion-water hydrogen bond dynamics at the interface. On the other hand, the higher order induced multipole effects in the QM/MM method suppress both the excessive enhancement of the solvation structure and the slowing of the ion-water hydrogen bond dynamics at the interface. The present study demonstrates that not only the induced dipole moment but also the higher order induced multipole moments, which are neglected in standard empirical models, are essential for the correct description of the ion solvation at the water/vapor interface.

Thermal lensing in a supercritical water medium

A pulsed, two-beam, thermal lensing experiment was performed to determine the concentration of aqueous solutes above the critical point of water. Despite a very significant mirage effect due to thermal gradients in the cell and absorption by water itself, the thermal lensing signal strength for aqueous benzoic acid in supercritical water was found to be linear with concentration in the sub-millimolar range. Although thermal lensing experiments in aqueous media are notoriously insensitive, the sharp density gradient near the critical point considerably improves the signal intensity. In this study a short-pulse pump 266 nm YAG laser and continuous low-power probe Ar ion beam were both focused into a supercritical water cell, giving a lensing signal whose strength could be maximized by changing the overlap of the two beams.

Mode-specific energy absorption by solvent molecules during CO2 vibrational cooling

Non-equilibrium molecular dynamics (NEMD) simulations of energy transfer from vibrationally excited CO(2) to CCl(4) and CH(2)Cl(2) solvent molecules are performed to identify the efficiency of different energy pathways into the solvent bath. Studying in detail the work performed by the vibrationally excited solute on the different solvent degrees of freedom, it is shown that vibration-to-vibration (V-V) processes are strongly dominant and controlled by those accepting modes which are close in frequency to the CO(2) bend and symmetric stretch vibration.