High pressure Raman study of the CO stretching mode in liquid N,N-dimethylacetamide

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Abstract
The Raman ν4 symmetric C=O stretching mode of N,N‐dimethylacetamide (DMA) is reported as a function of pressure from 1 to 4000 bar within the temperature range 40 to 100 °C. As in acetone, the isotropic line width ω1/2 (ISO) decreases with increasing density, in contrast to the opposite trend usually found in other molecular liquids. The noncoincidence effect, δω, which is the frequency difference between the peak maxima of the isotropic and the anisotropic Raman bands, increases with increasing density. These trends indicate the presence of strong intermolecular dipolar interactions between DMA molecules. Two theories of the noncoincidence effect due to McHale and Logan are used to interpret the density dependence of δω for DMA. In addition, we also used these two theoretical models to calculate δω for acetone using the results obtained earlier in our laboratory. McHale’s theory can predict the correct density dependence for the noncoincidence effect when the dielectric screening factor is ignored. Logan’s theory can account for the density behavior of the noncoincidence effect in DMA and in acetone.

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