Inhomogeneous broadening of Morse oscillators in liquids

Anomalous Infrared Absorbance of S═O: A Perturbation Study of α-C-H/D

Solvatochromic shifts of S═O vibrational probes describe the strength of the surrounding electric fields and the hydrogen bonding status. Herein, we demonstrated how the solvents alter the infrared (IR) spectra of the S═O vibrating mode. The experimental measurement of the involvement of α-H/D isotopic interactions with different solvents and their effects on the IR absorbance spectra of the vibrational probe provides detailed knowledge of the microsolvation environment despite the complexity of overlapping bands in the spectra. Herein, we discover how the solvents interact differently with DMSO and DMSO-d6, while being electronically and structurally the same. Interestingly, the IR spectrum of the S═O mode remains unaltered during α-isotopic replacement in the presence of aprotic solvents (acetone, acetonitrile, and dichloromethane), but in strongly coordinating polar solvents (D2O), it is altered remarkably. There is a lack of quantitative information about the influence of the α-H atom or α-isotopic substitution on the vibrational probe in the literature. Our experiments provide a detailed molecular understanding of the structure of DMSO in DMSO-solvent binary mixtures. As DMSO plays an important role in virtually all subdisciplines of chemistry and biology, we believe that our work will be of interest to a large diversity of studies in these fields.

Interpretation of the pressure-induced Raman frequency shift of the ν1 stretching bands of CH4 and N2 within CH4-CO2, N2-CO2 and CH4-N2 binary mixtures

The relationships between the frequency shift of the ν₁ stretching bands of CH₄ and N₂ with pressure (or density) and composition have been previously provided in the literature as reliable parameters for accurate empirical barometers and densimeters for the direct determination of the pressure or density of gas mixtures. However, the latter results still remain a pure description of the experimental data without any interpretation of the physical mechanisms hidden behind the variation trend of the observed peak position. The present paper is devoted to interpreting the origin of the pressure-induced vibrational frequency shifts of the ν₁ stretching bands of CH₄ and N₂ within CH₄-CO₂, N₂-CO₂ and CH₄-N₂ binary mixtures at the molecular level. Two different theoretical models (i.e., the Lennard-Jones 6-12 potential approximation - LJ, and the generalized perturbed hard-sphere fluid - PHF) are used to intuitively and qualitatively assess the variation trend as well as the magnitude of the frequency shift of the CH₄ and N₂ν₁ bands for an in-depth understanding. Thereby, the contribution of the attractive and repulsive solvation-mean forces to the variation of the Raman frequency shift as a function of pressure and composition is assessed. A predictive model of the variation trend of the frequency shift of the CH₄ν₁ band as a function of pressure (up to 3000 bars), density and composition within CH₄-N₂ and CH₄-CO₂ binary mixtures is then provided.

Dynamical characterization of rotationally hindered species in liquids

The rotational dynamics of HCl in liquid Ar has been studied by means of molecular-dynamics simulations. We calculate the lifetimes of weakly bound HCl-Ar dimers induced by the anisotropic pair interaction. It is shown that, although lifetimes are small with respect to the reorientational decorrelation, the time interval between the breaking down and formation of the next dimer is negligibly small. Thus, with respect to the rotational dynamics of the probe, the effect is similar to that and eventually would cause a time-stable complex. This provokes a peculiar hindered rotation of the diatomic in the liquid which is macroscopically embodied in the infrared spectrum of the solution as a Q-branch nonexistent otherwise.

Infrared spectral profiles in liquids and atom-diatom interactions

Molecular dynamics simulations of the infrared spectrum of a generic simple polar diatomic in a liquid nonpolar solvent allow to reproduce the different prototypical experimental line shapes of this kind of systems. This is feasible by using different solute-solvent anisotropic potentials at fixed thermodynamic conditions. In the limit cases, the rotation of the diatomic is explained in terms of a quasifree motion or a rotational diffusion evolution and the spectra show a doublet structure formed by P and R branches or a unique collapsed branch, respectively. When the profile contains three branches, including an intense Q branch in the vicinity of the center of the band, rotational evolution presents a particular hindering that can be understood by studying the influence on rotational spectral densities of the different time scales involved in rotational relaxation. Cancellation/enhancement effects among spectral density terms arising from intermediate and long times (0.4-1 ps) are essential to understand rotational hindering.