Low-frequency (0-450 cm^-1) dynamics of n-alkyl cyanide liquids studied by optical Kerr effect spectroscopy and...

This article presents a study of the dynamics of n-alkyl cyanides (CH₂CN with n = 1-5) in the 0-450 cm^-1 spectral region as a function of alkyl chain length. The spectra were measured using femtosecond optical Kerr effect (OKE) spectroscopy. The OKE spectra are characterized by a broad band in the 0-150 cm^-1 region arising mainly from intermolecular modes and by narrow bands in the 150-450 cm^-1 region arising from intramolecular modes. The intramolecular bands in the OKE spectra are in good agreement with the bands in simulated spectra obtained from density functional theory calculations. For cyanide molecules with long alkyl chains (n = 3-5), a low frequency intramolecular band associated with a torsional vibrational mode overlaps with the intermolecular band. Applying multicomponent line-shape analysis to the broad low-frequency band allowed us to obtain the intermolecular component of the band. We find the frequency and amplitude of the intermolecular band decrease with increasing chain length. Based on previous theoretical studies of the OKE spectrum of methyl cyanide and OKE reorientational studies of n-alkyl cyanides, we show that the dependence of the frequency of the intermolecular band on alkyl chain length is consistent with the band being primarily due to the hindered rotational motion of the molecules in the extended configuration about an axis perpendicular to the long axis of the molecule. The dependence of the amplitude of the intermolecular band on alkyl chain length is a mass effect associated with suppression of the collision-induced contribution to the intermolecular band.