13C NMR relaxation and reorientation dynamics in imidazolium-based ionic liquids: revising interpretation

Abstract

The temperature dependencies of ¹³C NMR relaxation rates in [bmim]PF₆ ionic liquid have been measured and the characteristic times (τ_c) for the cation reorientation have been recalculated. We found the origin of the incorrect τ_c temperature dependencies that were earlier reported for ring carbons in a number of imidazolium-based ILs. After a correction of the approach ¹³C T₁, the relaxation data allowed us to obtain the characteristic times for an orientation mobility of each carbon, and a complicated experiment, such as NOE, was not required. Thus the applicability of ¹³C NMR relaxation rate measurements to the calculation of the characteristic times for reorientation of all the carbons of the [bmim]⁺ cation was confirmed and our findings have shown that a ¹³C NMR relaxation technique allowed its application to ionic liquids to be equally successful as for other liquid systems.