Quantum chemical study of the molecular structure and vibrational spectra of the [CH$_3$-Fur-CH$_2$-Im-CH$_3$][N(CN)$_2$] ionic liquid

Using quantum chemical modeling methods at the $\omega$B97X-D/6-311++G(d,p) theory level, the equilibrium geometries, relative energies, and vibrational frequencies of isolated CH$_3$-Fur-CH$_2$-Im-CH$_3^+$ cations, N(CN)$_2^-$ anions, as well as seven binary cation-anion complexes for a promising hypergolic ionic liquid were calculated. Taking into account the conformational composition allowed us to synthesize the theoretical IR spectrum of the liquid. Characteristic frequencies for detecting structural fragments were determined. It is shown that the formation of ion pairs leads to significant shifts, band splitting, and redistribution of intensities in the IR spectra, especially in the regions of valence vibrations of C$\equiv$N bonds of the anion (2100-2250 cm$^{-1}$) and C–H bonds of the cation (2800–3200 cm$^{-1}$).