Maria Grazia Giorgini

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.

The non-coincidence effect in highly diluted acetone-CCl I. Experimental results and theoretical predictions 4 binary mixtures

The concentration dependence of the first moments and peak frequencies of isotropic and anisotropic profiles of the υC=O mode in acetone–CCl4 binary mixtures is investigated. The emphasis is directed mainly towards the non-coincidence effect in the high dilution range, not accessed in previous experiments. Our results for the relative non-coincidence effect in non-isotopic binary mixtures appear largely consistent with the Logan theory. Moreover, the observation of the υC=O mode of the naturally present acetone–13C=O has been exploited to check the validity of the Logan theory for isotopic mixtures. It is verified that the observation of negative values for the non-coincidence effect, which is strictly related to that of a threshold concentration for the onset of this phenomenon, are artefacts due to the presence of spectral asymmetries in the υC=O mode.

Raman noncoincidence effect: A spectroscopic manifestation of the intermolecular vibrational coupling in dipolar molecular liquids

This lecture addresses the analysis of the noncoincidence effect (NCE), a spectroscopic manifestation of the intermolecular coupling in molecular liquids. The vibrational bandshapes of molecular groups like C=O (strongly active in the IR spectrum) in dipolar liquids exhibit this phenomenon at a rather large extent. It will be shown that the vibrational exciton approach, developed under the assumption of the transition dipole coupling (TDC) mechanism, predicts how the orientational structure of the molecular liquid determines the magnitude and sign of the NCE. Specifically, it predicts that in simple molecular liquids, solely structured by dipolar forces, the NCE is large and positive, whereas when liquid structures are dominated by non-dipolar forces (as those present in H-bonded liquids), this scenario dramatically changes and IR-active modes may give rise to negative NCEs. This lecture is intended to offer a general overview of NCEs observed in dipolar (simple and structured) liquids in different thermodynamic conditions and of the theoretical and simulation results that assisted in their interpretation.

Density, Refractive Index, and Related Properties for 2-Butanone + n-Hexane Binary Mixtures at Various Temperatures

Density refractive index n and the related properties molar volume V and molar refraction R have been investigated for 2-butanone + n-hexane liquid binary mixtures over the entire composition range and a wide range of temperatures. Some well-known relationships have been applied to study the temperature and composition dependence of the measured and derived quantities. Furthermore, the deviations of the respective excess properties VE, nE, and RE have been examined, with the aim of identifying particular intermolecular interaction patterns responsible for the macroscopic behavior of these binary mixtures. The results have been interpreted on the basis of structural and geometric effects between the components.

Concentration dependence of the band profile parameters for the ν3(12C=O) Raman band of acetone in acetone–CCl4 binary mixtures: Experimental and Monte Carlo simulation results and their interpretation

The concentration dependence of specific band profile parameters, i.e., bandwidth and asymmetry, of the isotropic and anisotropic components of the ν3(12C=O) Raman band of acetone is investigated in the acetone–CCl4 binary mixtures. Emphasis is mainly placed upon the comparison of the results obtained from Monte Carlo simulations with the experimental results obtained by Raman spectroscopy. The results on the concentration dependence of the bandwidth are interpreted within the lineshape theory of Bratos and Tarjus, while the results on the concentration dependence of the band asymmetry are examined with the help of the theoretical predictions given by Knapp and by Knapp and Fischer. We have found a good qualitative agreement between all the results.

The non-coincidence effect in highly diluted acetone-CCl4 binary mixtures II. Experimental, theoretical and Monte Carlo simulation results

The concentration dependence of the Raman non-coincidence effect is investigated for the υ3(12C=O) band of the acetone–CCl4 binary mixtures. Emphasis is placed mainly upon a comparison of the results obtained from Monte Carlo simulations with those from Raman measurements. There is very good quantitative agreement between the results of the simulations and the experiments for the concentration dependence of the first moments of the isotropic and anisotropic bands, and hence the non-coincidence effect. These results on the observed and simulated concentration dependence of the non-coincidence effect are compared with those derived from the theory of Logan. This work confirms the validity of the theoretical treatment of chemical binary mixtures proposed by Logan, at least for this particular case of a polar solute dissolved in a non-polar solvent. To our knowledge these Monte Carlo simulations of spectral observables have been performed for chemical mixtures for the first time.

The non-coincidence effect in N,N-dimethylformamide-CCl4 highly diluted binary mixtures. Experimental and theoretical results

We report results on the concentration dependence of the non-coincidence effect in the v(C=O) mode in N,N-dimethylformamide-CCl4 mixtures. This study has been mainly addressed to the investigation of the experimentally observed sign inversion for this quantity in the low concentration range of chemical mixtures, which is also reported in the literature. We argue that the sign inversion of the non-coincidence effect in N,N-dimethylformamide-CCl4 mixtures is an artefact originated by the presence of an intrinsic asymmetry which with dilution moves from the blue to the red side of the band. The Logan theory for the thermodynamic state dependence of the non-coincidence effect in chemical mixtures offers a valuable tool for the interpretation of the results in the whole concentration range. Different models are compared and discussed.

Molecular dynamics and vibrational relaxations in liquid nitromethane. II. Raman, coherent anti‐Stokes Raman spectroscopy, and transient optical Kerr effects in the totally symmetric ν4 mode in CH3NO2

The vibrational relaxation of the ν4 mode of CH3NO2 liquid and its molecular dynamics are investigated. Temperature and solvent effects on vibrational dephasing and correlation times are discussed in terms of the adiabatic impact theory to infer that, in the molecular collision, the attractive part of intermolecular potential is explored. The statistical uncorrelation of rotational and vibrational relaxations is experimentally demonstrated.

Vibrational analysis of tellurophene and its deuterated derivatives

Abstract The i.r. and Raman spectra of tellurophene, tellurophene-2- d 1 , tellurophene-2-5- d 2 , tellurophene-3-4- d 2 , tellurophene-2-3-4- d 3 and tellurophene-2-3-4-5- d 4 have been studied. A vibrational assignment of the fundamental frequencies of all these molecules has been carried out on the basis of i.r. vapor band contours and depolarization states of Raman lines. Such an assignment conveniently fits the isotopic “sum” and “product” rules. A comparison of the vibrational behaviour of tellurophene and that of the other congeners is made and a rationalization of some frequency shifts is attempted.

Isotopic and chemical dilution effects on the vibrational relaxation rate of some totally symmetric motions of liquid acetonitrile

Abstract The vibrational dephasing of the ν 1 (C–H, C–D stretching) and ν 3 (C–H, C–D bending) symmetric motions of liquid acetonitrile in its light and fully deuterated forms has been studied in the frame of the vibrational time correlation functions obtained as Fourier transforms of the isotropic Raman spectral distributions and interpreted within the Kubo theory. In addition, the experimental isotropic profiles have been analysed within the bandshape approach formulated by analytical Fourier transformation of the Kubo vibrational time correlation functions in order to derive the relaxation parameters in the frequency domain. The effects of the isotopic (CH 3 CN/CD 3 CN and vice versa) and chemical (CCl 4 ) dilution on the bandshapes and on the vibrational relaxation parameters have been studied. It was observed that the decay rate of ν 1 mode is insensitive to the isotopic dilution but varies appreciably with chemical (CCl 4 ) dilution. The vibrational dephasing of ν 3 mode is qualitatively, but not quantitatively, affected in the same way by chemical dilution and shows a slower modulation regime than that exhibited by the stretching mode. Unlikely from the latter, the ν 3 mode results are slightly affected by the isotopic dilution. Phase relaxation mechanisms of these two motions of acetonitrile in the liquid state are proposed on the basis of these data, and a comparison is made with the results earlier published.