13C NMR relaxation rates in the ionic liquid 1-ethyl-3-methylimidazolium butanesulfonate

A new method of obtaining molecular reorientational dynamics from C-13 spin-lattice relaxation data of aromatic carbons in viscous solutions is applied to C-13 relaxation data of both the cation and anion in the ionic liquid, 1-ethyl-3-methylimidazolium butanesulfonate ([EMIM]BSO3). C-13 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 C-13 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, Delta sigma, 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.