Raman band shape analysis of cyanate-anion ionic liquids

- We compared the CN vibrational frequency and band shapes of cyanate-anions in ionic liquids with high-temperature molten salts.
- The comparisons unraveled the roles played by the strength of anion-cation interaction and the relatively low temperature of the system.
- ν(CN) band shapes analysis were made on the basis of vibrational dephasing and reorientational time correlation functions.
- The isotropic component of the Raman spectrum of the [SCN]− anion is determined by the distribution of vibrational frequency.
- The ν(CN) Raman bands of [N(CN)2]−, [C(CN)3]−, and [B(CN)4]− also depend on the correlation time of frequency fluctuation.

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 1.0 ps 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.