Use of Electron Paramagnetic Resonance Spectroscopy to Study Dielectric Properties of Liquids

Abstract EN

The signal response of an EPR active species is attenuated by the medium it is in.

Keeping all other parameters the same, the higher the dielectric constant of the medium, the lower the EPR signal response.

This behavior is problematic in studying EPR active species in high dielectric media but can be capitalized upon to monitor changes in the dielectric constant or estimate the dielectric constant of the medium.

Using a coaxial EPR cell design, the EPR signal of a stable nitroxyl radical compound (2,2,6,6-tetramethyl-piperidin-1-oxyl radical) in a low dielectric constant solvent in an inner tube is attenuated by the solvent present between the inner and outer tubes (jacket medium).

The attenuation increases monotonically with an increase in the dielectric constant of the jacket medium.

Calibration curves can be constructed using jacket media of known dielectric constants ranging from 2 to 80 and the dielectric constant of a sample used as the jacket medium can be estimated by interpolation.

This technique is applied to estimate the dielectric constants and/or composition of mixed solvents and to monitor the rate of a reaction.