Prediction of Infrared Spectra of Ionic Liquids with ab initio Molecular Dynamics

The combination of acids and amino acids anions with the choline cation gives origin to a new and potentially important class of organic ionic liquids (4th generation) that represents a valid bio- compatible alternative to the traditional ones, (i. e. imidazolium/pyrrolium based ionic liquids). This new class of ILs, in fact, has been found to have low toxicity to humans and to the environment, and is prepared from renewable feedstocks. For these two aspects, they can really be considered a “green” class of materials, and have already found a wide range of important technological applications, the most important being the possibility of selectively dissolving cellulose and lignin from biomasses, without the need of “heavy” chemical processes [1-4]. In this study, we have characterized two prototypes in both series of compounds (Choline-Alanine and Choline-Salicilate), employing a combination of ab initio and classical molecular dynamics (AIMD, MD), DFT studies of several ion pairs (in vacuo) and experimental measurements with the aim of interpreting the geometric structure and the dynamics of the corresponding liquid systems. Our models were able to reproduce successfully both X-Ray diffraction patterns and vibrational spectra, and show the existence of very strong hydrogen bond interactions in the bulk liquid, that are responsible for the very large red-shift observed in infrared spectra. The assignment of the vibrational absorption was carried out applying the localization in frequency of the Vibrational Density of States (VDOS), using effective normal modes along the AIMD trajectory as proposed by Gaigeot et al. [5]. This method offers a very valid alternative to more traditional quantum chemistry methods based on Hessian Matrix diagonalization, and takes into account finite temperature, solvation and anharmonicity effects directly.