13C NMR Relaxation Rates: Separation of Dipolar and Chemical Shift Anisotropy Effects

Abstract

The process of obtaining molecular reorientational dynamics from 13C spin−lattice relaxation data is simplified for aromatic carbons in viscous solutions. Spin−lattice relaxation times (13C) are used to determine pseudorotational correlation times for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). The pseudorotational correlation times are used to calculate corrected maximum nuclear Overhauser effect (NOE) factors from a combined isotropic dipolar and NOE equation. These corrected maximum NOE factors are then used to determine the dipolar relaxation rate part of the total relaxation rate for each 13C nucleus in the imidazolium ring. A consequence of this analysis is that a plot of the maximum NOE factors and the total spin−lattice relaxation times have minima at similar temperatures. Chemical shift anisotropy values, Δσ, for the aromatic carbons in the imidazolium cation are temperature dependent with maximum Δσ values at ca. the same temperature as observed for the spin−lattice relaxation times. The average Δσ values for the imidazolium ring carbons are similar to those of pyrimidine in liquid crystal solutions.