Noncoincidence Effects of Dimethyl Carbonate in Binary Mixtures Probed by Raman Spectroscopy: Experimental and DFT Calculations

The components of isotropic Raman and anisotropic Raman for dimethyl carbonate (DMC) dispersed in cyclohexane and acetone at different volume fractions were recorded separately. The noncoincidence effects (NCE) of the nu(7)(C=O) stretching mode were calculated accordingly. The NCE values (Delta nu(NCE)) of the nu(7)(C= O) versus DMC volume fractions in the DMC/C6H12 mixtures exhibits a convex (upward) curvature pattern, while the Delta nu(NCE) vs concentration in the DMC/CH3COCH3 mixtures exhibits a concave (downward) curvature. These different NCE behaviors in the different binary mixtures may arise from the solvent-induced aggregation character. Thus, monomer and dimer structures of DMC were optimized and the vibration spectra were obtained using density functional theory (DFT) calculations. An aggregation model was suggested to expound the DMC's characteristic NCE behavior and concentration effect. We found that the theoretical spectra from DFT/polarizable continuum model calculation based on the aggregation model is in accordance with our experimental data. Solvent-dependent experiments show the Delta nu(NCE) values increase with the decrease of the solvent dielectric constant under the identical volume fractions.