A Bloembergen-Purcell-Pound 13C NMR relaxation study of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate

The molecular structure and rotational motion of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) were studied over a wide temperature range using the Bloembergen-Purcell-Pound C-13 NMR spin-lattice relaxation method and NOE factors. Examination of the spin-lattice relaxation times (T-1) and the rates (R-1 = 1/T-1) of the 1-butyl-3-methylimidazolium cation reveals the relative motions of each carbon in the imidazolium cation. The rotational characteristics of the [BMIM] cation are supported by ab-initio molecular structures of [BMIM][PF6] using density functional theory (DFT) and Hartree-Fock (HF) methods. The ab-initio gas phase structures of [BMIM] [PF6] indicate that the 1-butyl-3-methylimidazolium C2 hydrogen, the ring methyl group, and the butyl side-chain hydrogen atoms form hydrogen bonds with the hexafluorophosphate anion.