Raman band shape analysis of cyanate-anion ionic liquids

Abstract

Raman band shape analysis of the C≡N stretching mode was carried out for ionic liquids based on [SCN]−, [N(CN)2]−, [C(CN)3]−, and [B(CN)4]−, with a common cation, 1-ethyl-3-methylimidazolium, [C2C1im]+. Vibrational and reorientational time correlation functions were obtained by Fourier transforming isotropic and anisotropic Raman spectra. Comparison is provided between results for [C2C1im][SCN] and literature data available for molten alkali thiocyanates. Vibrational dephasing of the ωCN mode in [C2C1im][SCN] belongs to the regime of inhomogeneous broadening as it is determined by the distribution of vibrational frequencies, <Δω2>. Vibrational dephasing in [N(CN)2]−, [C(CN)3]−, and [B(CN)4]−, depends on both the <Δω2> and the correlation time of frequency fluctuation. Decay rates of reorientational time correlation functions within the range of 1.0ps do not correlate with the viscosity of these ionic liquids. Molecular dynamics simulations of these ionic liquids provide support to the experimental finding that the reorientational time correlation function of [SCN]− decays faster than the other anions. The Raman band shape analysis suggests that the short-time dynamics of these ionic liquids is structure-limited because of steric hindrance.