Paolo Mirone

Local order and vibrational coupling in solutions of polar molecules

The separation between the isotropic and the anisotropic component of some Raman bands of polar molecules is studied as a function of concentration in different solvents. It is found that it depends linearly on the ratio between the volume fraction and the static dielectric constant of the solution, and becomes zero at a finite concentration. It is suggested that this concentration threshold is related to the relaxation time of the vibrational energy. An explanation is proposed for the observed dependence on the dielectric constant.

Evidence for short-range orientation effects in dipolar aprotic liquids from vibrational spectroscopy. Part 1.—Ethylene and propylene carbonates

The CO stretching frequency of liquid ethylene and propylene carbonates is higher in the infrared than in the Raman spectrum. The difference amounts to 13 cm–1 for ethylene carbonate at 313 K. This effect is explained as the consequence of a coupling between the transition dipoles of neighbouring molecules, which is made possible by some degree of alignment of molecular dipoles due to the high dipole moment of these molecules (about 16 × 10–30 C m). A study of dilution and temperature effects confirms this interpretation.

Short-range orientation effects in dipolar aprotic liquids—III. Intermolecular coupling of vibrations in sulfoxides, sulfones, nitriles and other compounds

Abstract In the Raman spectrum of dipolar aprotic liquids the anisotropic component of bands arising from totally symmetric vibrations falls at a higher frequency than the isotropic component. The corresponding i.r. band maximum appears at about the same frequency as the Raman anisotropic component. These effects disappear with dilution in an inert solvent. This behaviour can be explained by assuming that these liquids are composed of clusters inside which the molecules are oriented in a partially ordered way.

Evidence for short-range orientation effects in dipolar aprotic liquids from vibrational spectroscopy. Part 2.—Carbonyl compounds

In the Raman spectrum of pure liquids, the anisotropic component of the CO stretching band falls at a higher frequency than the isotropic component. In the same liquids the infra-red band maximum appears at almost the same frequency as the Raman anisotropic component. In some cases the infra-red band may be resolved into two components, the stronger coinciding with the anisotropic, the weaker with the isotropic Raman band. These effects disappear with dilution and are reduced with increasing temperature.This behaviour is explained in terms of a model which assumes that dipolar aprotic liquids are composed of clusters of molecules oriented in an (at least partially) ordered way.

Infrared and Raman spectra and vibrational assignment of ethylene carbonate

Abstract The infrared spectra of gaseous, liquid and solid ethylene carbonate (the latter in polarized light) and the Raman spectra in the liquid and solid states have been recorded. A complete vibrational assignment is proposed on the basis of gas-phase band contours, depolarization ratios and dichroic ratios; it is found that, although the molecule has C2 symmetry in all three states of aggregation, the liquid and vapour-phase spectra are explainable in terms of C2v pseudo-symmetry. The complex pattern of Fermi resonances in the CO stretching region of the various spectra is interpreted. The correlation field splittings of the most intense totalsymmetric vibrations are quantitatively explained in terms of interactions between transition dipoles.

The effect of composition on the noncoincidence of the isotropic and anisotropic Raman frequencies and its intrepretation by means of a simple dielectric model

The difference between the anisotropic and the isotropic frequencies of the C=O stretching bands of acetone, acetophenone, methylbenzoate, and dimethylformamide was measured in several mixtures of known dielectric constant. Its change with concentration can be explained by making the assumption that the interaction energy of the dissolved dipoles is described by the dielectric model of Onsager–Frohlich over the whole composition range. The same model accounts for the change of the above difference in neat liquids with temperature.

Solvent effects on Raman band intensities

Abstract The intensities of polarized Raman bands of carbon tetrachloride, tetrachloroethylene, benzene, cyclohexane, carbon disulfide, chloroform, acetone, and acetonitrile have been measured in binary mixtures with a wide variety of solvents, at concentrations ranging from 20 to 80% in volume. The scattering coefficient increases, in most cases linearly, with the refractive index of the mixture; furthermore, its value extrapolated to zero concentration exhibits a close correlation with the refractive index of the pure solvent. The results are in fair agreement with a theoretical formula, derived from Onsagers theory of dielectric polarization.

Infrared and Raman spectra and vibrational assignment of cyclopentadiene

Abstract The infrared spectrum of cyclopentadiene has been recorded from 200 to 4000 cm −1 in the gaseous, liquid and crystalline phases. The Raman spectrum of liquid cyclopentadiene has also been determined with depolarization ratios. All but one of the fundamentals are assigned on the basis of depolarization ratios, gas-phase band contours, and solid-state selection rules. A very large number of overtones and combination tones have been observed and assigned. The interpretation of the combination tones confirms neatly the proposed assignments for the fundamentals, and allows a reasonable value to be suggested for the lacking fundamental.

Comment on the influence of the dielectric constant upon the noncoincidence of the isotropic and anisotropic Raman frequencies

The formula for the difference between the isotropic and anisotropic components of a Raman band for polar liquids given by McHale is discussed. Comments on the role of dielectric constant are presented. (AIP)

Vibrational spectra and normal-coordinate treatment of cyclopentadiene and its deuterated derivatives

Abstract The infrared spectra in polarized light and the Raman spectra of crystalline cyclopentadiene and hexadeuterocyclopentadiene and of their solid solutions have been obtained. Pentadeuterocyclopentadiene (1,2,3,4,5) has been prepared for the first time and its i.r. and Raman spectra have been recorded. On the basis of these and previous results, a vibrational assignment is proposed for cyclopentadiene and three of its deuterated derivatives. This assignment is supported by a normal coordinate analysis based on a valence force field.