Effects of dipole interaction and solvation on the CO stretching band of N,N-dimethylacetamide in nonpolar solutions: Infrared, isotropic and anisotropic Raman measurements

The concentration dependence of the CO stretching (νCO) band of N,N-dimethylacetamide (NdMA) in cyclohexane, n-hexane, and CCl4 has been investigated by infrared (IR) and polarized Raman spectroscopy. For the neat liquid of NdMA, the noncoincidence of the aniso- and isotropic Raman wavenumbers is evident. In the 0.47 M cyclohexane solution of NdMA, the noncoincidence effect almost disappears and the νCO envelopes in both the Raman and IR spectra are asymmetric to the low-wavenumber side. When the concentration of NdMA decreases from 0.33 to 0.023 M, the peak of these bands slightly shifts to a higher wavenumber and the band shape becomes symmetric. The shape of the νCO envelope does not show any significant change below 0.023 M. These results suggest that the asymmetric shape of the νCO band observed for the 0.33 M cyclohexane solution is associated with the intermolecular interaction among NdMA molecules, which vanishes at around 0.02 M. Spectral changes for the CCl4 solution of NdMA show a similar tendency. However, the shape and peak wavenumber of the νCO band observed in a highly diluted CCl4 solution (≤0.023 M) indicate that the solvation effect of CCl4 is more complicated than those of cyclohexane and n-hexane. The analyses of the νCO band, which is sensitive to the intermolecular interaction between solutes and between solute and solvent for NdMA dissolved in nonpolar solvents, would serve to clarify the electronic property of the molecule in a solution.