Ultrafast dynamics of nitrile liquids and their mixtures with ionic liquids studied by optical heterodyne detected Raman-induced Kerr effect spectroscopy

This work is aimed at understanding the low-frequency (0-450 cm-1) dynamics of mixtures of n-alkyl nitrile liquids with the ionic liquid 3-methyl-1-pentylimidazolium bis(trifluoromethylsulfonyl)imide ([C5mim][NTf2]) using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Three studies are reported in the dissertation. The first study is a study of the low-frequency dynamics of n-alkyl nitriles. In this study, density functional theory (DFT) calculations were used to assign the intramolecular vibrational modes in the reduced spectral densities of these n-alkyl nitriles. This study showed that the peak of the intermolecular band shifts to a higher frequency with decreasing alkyl chain length. Based on the multicomponent line shape analysis model, the intermolecular spectrum was primarily determined by the translational motions. The second study is a comparative study of models for the mixtures of alkyl nitriles and the IL [C5mim][NTf2]. From this study, the mole-fraction weighted mixture model gave a more reasonable interpretation of the experimental spectrum than the volume-fraction weighted mixture model. It is found that ideal mixture model well describes the spectrum of the mixture containing the longer alkyl chain nitrile. The spectrum of nitrile in the mixture, which is broader than the spectrum of neat nitrile, is enhanced at low/high frequencies. In the third study the intermolecular dynamics of mixtures of pentyl nitrile and the IL [C5mim][NTf2] were studied as a function of composition. In the IL [C5mim][NTf2], the pentyl nitrile spectrum was higher in frequency and broader than the spectrum in neat pentyl nitrile, which suggests that it sees a stiffer intermolecular potential. We propose that the appearance of enhanced shoulders in the pentyl nitrile spectrum in mixtures at low/high frequencies is a signature of two interaction sites or different local environments for the interactions between pentyl nitrile and cation of IL as shown previously in combined OHD-RIKES studies and MD simulations on acetonitrile/IL mixtures.

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