Prediction of solvent effects on vibrational intensities and Raman activities in solution with the Polarizable Continuum Model: a study of push-pull molecules

We present a comparison between experimental and calculated vibrational infrared and Raman spectra (harmonic frequencies, absorption intensities, and scattering activities) for two push-pull molecules, [(2E,4E)-5-(dimethylamino)penta-2,4-dienylidene]malononitrile and 5-[(2E,4E)-5-(diethylamino)penta-2,4-dienylidene]-1,3-diethylpyrimidine-2,4,6(1H,3H,5H)-trione, widely studied for their nonlinear optical properties, in several solvents. The polarizable continuum model (PCM) has been used to describe the solvents, and the molecules have been treated at the density functional theory (DFT) level. Local field effects on IR intensities and Raman activities are included in the calculations. Solvent effects on absorption and scattering intensities are predicted fairly well. A number of reasons for discrepancies between calculated and experimental results are discussed. The variation of the bond length alternation (BLA) of the studied molecules as a function of the solvent is also discussed.