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A new method of obtaining molecular reorientational dynamics from 13C spin-lattice relaxation data of aromatic carbons in viscous solutions is applied to 13C relaxation data of both the cation and anion in the ionic liquid, 1-ethyl-3-methylimidazolium butanesulfonate ([EMIM]BSO3). 13C pseudorotational correlation times are used to calculate corrected maximum NOE factors from a combined isotropic dipolar and nuclear Overhauser effect (NOE) equation. These corrected maximum NOE factors are then used to determine the dipolar relaxation rate part of the total relaxation rate for each aromatic 13C nucleus in the imidazolium ring. Rotational correlation times are compared with viscosity data and indicate several [EMIM]BSO3 phase changes over the temperature range from 278 to 328 K. Modifications of the Stokes-Einstein-Debye (SED) model are used to determine molecular radii for the 1-ethyl-3-methylimidazolium cation. The Hu-Zwanzig correction yields a cationic radius that compares favorably with a DFT gas-phase calculation, B3LYP/(6-311+G(2d,p)). Chemical shift anisotropy values, Deltasigma, are obtained for the ring and immediately adjacent methylene and methyl carbons in the imidazolium cation and for the three carbon atoms nearest to the sulfonate group in the anion.

The ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1-methyl-3-propylimidazolium tetrafluoroborate ([PMIM][BF4]) were studied by H,H-NOESY NMR using a cross-relaxation matrix analysis. Cross-peak intensities are seen to increase with increasing mixing time. Experimental and theoretical hydrogen-hydrogen distances are in agreement at short mixing times (50 ms). Mixing times longer than 50 ms result in an increasing contribution of spin diffusion that produces unrealistically short hydrogen-hydrogen distances. Gas-phase ab initio molecular structures are obtained using Hartree-Fock (HF) and density functional theory (B3LYP) methods at the 6311 + G(2d,p) basis set level. The hydrogen-hydrogen distances obtained from the theoretical structures are in reasonable agreement with those calculated from the cross-relaxation matrices.