A. Morresi

Light Scattering Spectra of Water in Trehalose Aqueous Solutions: Evidence for Two Different Solvent Relaxation Processes

Light scattering spectra on aqueous solutions of trehalose were recorded in a wide frequency range combining the use of a double monochromator and a multipass Fabry-Perot interferometer. Experimental results indicate the presence of a slow relaxation mode related to the solute dynamics, which is clearly separated from the solvent one. The spectral analysis reveals the existence of two separate solvent relaxation processes assigned to hydrating and bulk water molecules. The picosecond dynamics of water molecules directly interacting with the solute (proximal water) is consistently delayed with the corresponding relaxation time increase is about 5-6 times compared to the bulk. The slowing down induced by the sugar on the water dynamics mainly involves a restricted hydration layer constituted of 16-18 water molecules. These results improve our knowledge about the influence of carbohydrates on the fast rearrangement dynamics of water and may serve as a model to gain important insight on basic solvation properties of other biorelevant systems in aqueous media.

Hydration and Aggregation in Mono- and Disaccharide Aqueous Solutions by Gigahertz-to-Terahertz Light Scattering and Molecular Dynamics Simulations

The relaxation properties of hydration water around fructose, glucose, sucrose, and trehalose molecules have been studied by means of extended frequency range depolarized light scattering and molecular dynamics simulations. Evidence is given of hydration dynamics retarded by a factor ξ = 5-6 for all the analyzed solutes. A dynamical hydration shell is defined based on the solute-induced slowing down of water mobility at picosecond time scales. The number of dynamically perturbed water molecules N(h) and its concentration dependence have been determined in glucose and trehalose aqueous solutions up to high solute weight fractions (ca. 45%). For highly dilute solutions, about 3.3 water molecules per sugar hydroxyl group are found to be part of the hydration shell of mono- and disaccharide. For increasing concentrations, a noticeable solute-dependent reduction of hydration number occurs, which has been attributed, in addition to simple statistical shells overlapping, to aggregation of solute molecules. A scaling law based on the number of hydroxyl groups collapses the N(h) concentration dependence of glucose and trehalose into a single master plot, suggesting hydration and aggregation properties independent of the size of the sugar. As a whole, the present results point to the concentration of hydroxyl groups as the parameter guiding both sugar-water and sugar-sugar interactions, without appreciable difference between mono- and disaccharides.

The negative Raman non-coincidence effect of ring vibrations

The totally symmetric ring modes of benzene, benzene-d 6, pentafluorobenzene, hexafluorobenzene, cyclopentane, cycloheptane, cyclopentanone, cyclohexanone were investigated with customary Raman devices. For the first time it is shown that a negative or anomalous noncoincidence effect seems to be a general property of aromatic and aliphatic ring vibrations. In agreement with the results of isotopic dilution experiments carried out with benzene and benzene-d 6 this new effect is explained in terms of the pair excitation model as a result of a dominant repulsive resonant intermolecular vibrational coupling.

Rotational dynamics of trehalose in aqueous solutions studied by depolarized light scattering

High resolution depolarized light scattering spectra, extended from 0.5 to 2x10(4) GHz by the combined used of a dispersive and an interferometric setup, give evidence of separated solute and solvent dynamics in diluted trehalose aqueous solutions. The slow relaxation process, located in the gigahertz frequency region, is analyzed as a function of temperature and concentration and assigned to the rotational diffusion of the sugar molecule. The results are discussed in comparison with the data obtained on glucose solutions and they are used to clarify the molecular origin of some among the several relaxation processes reported in literature for oligosaccharides solutions. The concentration dependence of relaxation time and of shear viscosity are also discussed, suggesting that the main effect of carbohydrate molecules on the structural relaxation of diluted aqueous solutions is the perturbation induced on the dynamics of the first hydration shell of each solute molecule.

Application of the Kubo–Anderson band shape equation to vibrational relaxation studies in the frequency domain and to an improved determination of spectral second moments from experimental data

The well‐known stochastic equation of Kubo–Anderson, widely adopted in the studies of time vibrational correlation functions, is put in a single‐parametric form (defining a parameter α that describes the modulation regime of the dephasing) and Fourier transformed. It is shown how the corresponding analytical expression in the frequency domain can be compared with any experimental band shape, allowing the computation of the dynamical parameters. In particular, the more extensively addressed problems, in literature, in the field of vibrational relaxation studies, have been afforded and a contribution to their solution has been given; these are: the baseline positioning, the uncertainties of the dynamical variables involved in the stochastic model, and the frequency second moment computation. The algebraic development has been implemented in the KUBOFREQ© computer program; it has been applied in the fitting of two sets of experimental data: ν4 and ν5 mode of liquid CH3NO2 at various temperatures. The compari...

Solvent effect on the vibrational dephasing of the ν2 (CN) and ν4 (CC) stretching modes in liquid acetonitrile and acetonitrile-d3

Abstract The vibrational dynamics of CH 3 CN and CD 3 CN have been examined in pure substances and in mixtures with CCl 4 at constant, room temperature, analysing the band shapes and the frequency shifts of ν 2 (CN) and ν 4 (CC) stretching modes. The effective temperature of the scattering volume, instead of the thermostatic value, was used to calculate and eliminate some secondary structures present in all the examined profiles. By doing this, it has been possible to correctly extract the dynamic parameters from the Raman isotropic profiles and from the relevant time correlation functions. The nonideality of the CH 3 CN/CCl 4 mixture clearly emerges from the application of the Kubo theory in the frequency domain, following a calculation procedure previously developed in our laboratory. The concentration dependence of the bandwidths and the frequency shifts in ν 2 mode have been interpreted within the theory of the local concentration fluctuation.

Non-coincidence effect of aromatic ring vibrations

The non-coincidence of the maxima of the isotropic and the anisotropic profiles in Raman spectra has always been considered an efficacious probe of the structure and dynamics of polar liquids. An anomalous, negative non-coincidence effect has been observed in ring breathing modes of aromatic molecules, without dipole moment (p -xylene, mesitylene) or with a weak permanent dipole (toluene, o -xylene, m -xylene). Temperature effects and isotopic dilution have also been performed in toluene and o -xylene, confirming the presence of a resonant coupling mechanism. Previous analogous results of other aromatic non-polar liquids (benzene, hexafluorobenzene) have been confirmed. It has been hypothesized that quadrupolar intermolecular forces and steric repulsive interactions balance each other giving final negative values of non-coincidence effect.

Hydrophobic hydration of tert-butyl alcohol studied by Brillouin light and inelastic ultraviolet scattering

The longitudinal viscosity of diluted water-tert-butyl alcohol solutions in the 10 GHz frequency region has been measured by means of Brillouin light scattering and inelastic ultraviolet scattering. The main advantage of our hypersonic investigation compared to more traditional ultrasonic measurements is that in the gigahertz frequency range slow relaxation processes involving the alcohol dynamics are completely unrelaxed, so that the measured viscosity mainly originates from the hydrogen bond restructuring of water. In contrast with previous determinations, we estimate an activation energy which is independent from the alcohol mole fraction up to X = 0.1, and comparable to that of bulk water. A simple two-component model is used to describe the steep increase of viscosity with increasing alcohol mole fraction, and a retardation factor 1.7 ± 0.2 is found between the relaxation times of hydration and bulk water. These findings endorse a dynamic scenario where the slowing down of hydration water is mainly due to a reduction of configurational entropy and does not involve an arrested, icelike, dynamics.

ORIENTATIONAL PROCESSES IN LIQUID NITROMETHANE STUDIED BY DEPOLARIZED LIGHT SCATTERING AND TRANSIENT OPTICAL KERR EFFECT

The dynamics of nitromethane in the liquid phase is investigated by steady‐state (Raman and Rayleigh) and time‐resolved (optical Kerr effect) spectroscopic experiments performed at variable temperature. Both experiments show that the entire relaxation process is completed in a few picoseconds and that the dynamics can be described by assuming a biexponential function for the molecular response. The prominent dynamical feature at longer times appears to be that of a diffusionally reorienting symmetric top. The time dependence for orientational correlation at the shorter times observed in the coherent optical processes (Rayleigh and OKE) suggests that perturbative phenomena in the subpicosecond time scale (collisional and cage effects) are operative prior to the onset of the diffusional regime. Reorientational times from steady‐state coherent and incoherent light scattering experiments enable us to exclude that pair‐particle orientational correlation is effective. The same orientational activation energy wa...

Solvent Sharing Models for Non-Interacting Solute Molecules: The Case of Glucose and Trehalose Water Solutions

The concentration dependence of hydration numbers of molecules modelled as nearly spherical particles is studied by simple analytic and numerical approaches, in the ideal limit of absence of intermolecular interactions. It is shown that the random close-to-contact condition achieved by solute molecules, noticeably affects the average hydration numbers. Comparison with experimental results obtained by light scattering in glucose and trehalose water solutions shows a reduction of the hydration number that is twice faster than that calculated in absence of interactions, suggesting important aggregation phenomena to occur in both systems, even at relatively low solute concentration. The effect of concentration on shear viscosity is also reported, suggesting that the leading contribution to the increase of viscosity arises from hydration water.