Nmr relaxation studies and molecular modeling of 1-butyl-3-methylimidazolium PF6[BMIM][PF6]

The solution microdynamics of [BMIM][PF6] has been studied by C-13 NMR spectroscopy. The C-13 relaxation times and nuclear Overhauser enhancement (NOE) measurements are used to determine rotational correlation times. The continuous distribution of correlation times defined by Cole and Davidson that has been used to interpret the relaxation data of viscous liquids and glassy solids is used to model the [BMIM][PF6] system. The Cole Davidson model is combined with the model-free approach of Lipari and Szabo (LS). The combination of the Cole-Davidson and Lipari-Szabo models successfully represents the imidazolium ring carbons. The Bloembergen, Purcell and Pound (BPP) method is combined with the LS method to represent the flexible side chains in [BMIM][PF6]. The results of semi-empirical (AMI and PM3) and ab initio (Hartree-Fock and Density Functional Theory) calculations are compared for the [BMIM][PF6] ionic liquid. The ab initio calculations include fully optimized structures at the RHF/3-21G(*), RHF/6-31G*, RHF/6-31G**, MP2/6-31G*, B3LYP/6-31G* and B3LYP/6-31G** levels. In addition to the gas phase calculations of these ion pairs, semi-empirical modeling of the formation of multiple ion pair dimers also provides insight into their possible aggregation in the liquid state.