Towards modeling of ionic liqudis with density functional theory methods

Resumen

Ionic liquids (ILs) are salts with low melting points, in most cases close to room temperature. In the last decade, this kind of compounds have so far aroused interest from the academic point of view, as well as in their applications. Their notable properties, among which it is remarkable the extremely low vapor pressure, make them an important alternative to the common organic solvents. Besides, there are lots of organic cations and anions that can form an IL leading to a huge amount of possible combinations that present different properties. In consequence, it is possible to design an IL for a determined practical application. This is the reason why this systems are labeled as design solvents. However, the experimentation is expensive and presents some difficulties that make the selection of the most favorable ions a hard task. Because of this, theoretical investigations play an important role opening up the possibility of predicting the properties of different ILs before the experimentation. Theoretical research of ILs is conducted at different levels. From fundamental quantum mechanic studies, to process simulation of their implementation in chemical industry, through the study of the macroscopic properties of the liquid using mechanical molecular methods and other methods specially developed for the prediction of liquid phase systems as COnductor–like Screening MOdel (COSMO) methodology, in particular COnductor–like Screening MOdel for Realistic Solvents (COSMO–RS). In most cases, the starting point are calculations at quantum mechanics level of the structure and properties of the molecules that form the ILs. In this sense, Density Functional Theory (DFT) methods play an important role due to the big size of the molecules that make almost unapproachable the calculations at a higher level of theory. In consequence, DFT methods are nowadays a fundamental tool in the theoretical approach to the study of the structure of the ions that form the ILs, and calculations with these methods are, as it was previously mentioned, the starting point for the estimation of properties using specific methodologies. Despite the generalized use of DFT methods, the factors that concern to the quality of the calculations and how they affect to the predicted properties have not been sufficiently studied. Bearing this in mind, one of the main goals of this thesis is the evaluation of the ability of DFT methods to correctly describe the structure using different families of functionals. With this purpose, the errors that usually affect DFT calculations have been analyzed and the most appropriate functional to describe the structure and the properties has been selected. Once the most suitable method has been chosen, families of ILs with anions containing iron have been haracterized because of their interest as potential magnetic ionic liquids. The macroscopic properties have been estimated using COSMO–RS method. Finally, as an alternative to COSMO methodology, the results obtained using molecular dynamic methods have been analyzed by characterizing the family NMImCl, which, within the ILs based on imidazolium, is the most simple one.