Intermolecular Interactions and Vibrational Perturbations within Mixtures of 1-Ethyl-3-methylimidazolium Thiocyanate and Water

A Raman spectroscopic analysis of the room temperature ionic liquid 1-ethyl-3-methlimidazolium thiocyanate, [EMIM] [SCN], has revealed that certain stretching vibrations associated with both the anion and cation shift to higher energy (or blue-shift) as water is introduced to the system (by up to 15 cm(-1) for a CH stretching mode associated with EMIM+ and up to 12 cm(-1) for the CS stretch of SCN-). Density functional theory was employed to gain molecular level insight into the origins of these spectral perturbations by computing changes in the structures, energetics, and harmonic vibrational frequencies of the [EMIM] [SCN] ion pair as a single explicit water molecule was added to the system. The computed harmonic vibrational frequency shifts for the low-energy structures of the ion pair and the corresponding monohydrated complex reproduce the experimentally observed trends. These results indicate that the donation of a hydrogen bond from water to the N atom of SCN- produces the blue-shifts associated with the CN and CS stretching modes. In contrast, the vibrational frequency shifts associated with CH stretches of EMIM+ do not appear to require a direct interaction with the water molecule.