The (C=O) Raman band frequencies of acetone have been analyzed to separate the contributions of the environmental effect and the vibrational coupling to the gas-to-liquid frequency shifts of this band and to elucidate the changes in these two contributions upon dilution in DMSO. We have measured the frequencies of the (12C=O) band in acetone/DMSO binary mixtures, the (13C=O) band of the acetone-13C=O present as natural abundance isotopic impurity in these mixtures, and both the (12C=O) and (13C=O) bands in the acetone-12C=O/acetone-13C=O isotopic mixtures at infinite dilution. These frequencies are compared with those of the (12C=O) band in the acetone/CCl4 binary mixtures measured previously. We have found the following three points: (i) The negative environmental contribution for the (12C=O) oscillator of acetone completely surrounded by DMSO is reduced in magnitude by +5.5 cm-1 and +7.8 cm-1 upon the complete substitution of DMSO with acetone and CCl4 molecules, respectively, indicating the progressive reduction of the attractive forces exerted by the environment on the (12C=O) mode of acetone. (ii) In DMSO and other solvents, the contribution of the vibrational coupling to the frequency of the isotropic Raman (12C=O) band of acetone becomes progressively more negative with increasing acetone concentration up to a value of –5.5 cm-1, while the contribution to the frequency of the anisotropic Raman band remains approximately unchanged. The only difference resides in the curvatures of the concentration dependencies of these contributions which depend on the relative solute/solvent polarity. (iii) The non-coincidence effect (separation between the anisotropic and isotropic Raman band frequencies) of the (C=O) mode in the acetone/DMSO mixtures exhibits a downward (concave) curvature, in contrast to that in the acetone/CCl4 mixtures which shows an upward (convex) curvature. This result is supported by MD simulations and by theoretical predictions, and is interpreted as arising from the reduction and the enhancement of the short-range orientational order of acetone in the acetone/DMSO and acetone/CCl4 mixtures, respectively.

Concentration dependent frequency shifts and Raman spectroscopic noncoincidence effect of the C=O stretching mode in dipolar mixtures Acetone/Dimethylsulfoxide. Experimental, theoretical and simulation results.

GIORGINI, MARIA GRAZIA;
2005

Abstract

The (C=O) Raman band frequencies of acetone have been analyzed to separate the contributions of the environmental effect and the vibrational coupling to the gas-to-liquid frequency shifts of this band and to elucidate the changes in these two contributions upon dilution in DMSO. We have measured the frequencies of the (12C=O) band in acetone/DMSO binary mixtures, the (13C=O) band of the acetone-13C=O present as natural abundance isotopic impurity in these mixtures, and both the (12C=O) and (13C=O) bands in the acetone-12C=O/acetone-13C=O isotopic mixtures at infinite dilution. These frequencies are compared with those of the (12C=O) band in the acetone/CCl4 binary mixtures measured previously. We have found the following three points: (i) The negative environmental contribution for the (12C=O) oscillator of acetone completely surrounded by DMSO is reduced in magnitude by +5.5 cm-1 and +7.8 cm-1 upon the complete substitution of DMSO with acetone and CCl4 molecules, respectively, indicating the progressive reduction of the attractive forces exerted by the environment on the (12C=O) mode of acetone. (ii) In DMSO and other solvents, the contribution of the vibrational coupling to the frequency of the isotropic Raman (12C=O) band of acetone becomes progressively more negative with increasing acetone concentration up to a value of –5.5 cm-1, while the contribution to the frequency of the anisotropic Raman band remains approximately unchanged. The only difference resides in the curvatures of the concentration dependencies of these contributions which depend on the relative solute/solvent polarity. (iii) The non-coincidence effect (separation between the anisotropic and isotropic Raman band frequencies) of the (C=O) mode in the acetone/DMSO mixtures exhibits a downward (concave) curvature, in contrast to that in the acetone/CCl4 mixtures which shows an upward (convex) curvature. This result is supported by MD simulations and by theoretical predictions, and is interpreted as arising from the reduction and the enhancement of the short-range orientational order of acetone in the acetone/DMSO and acetone/CCl4 mixtures, respectively.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11585/5498
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