Local density effects on the hyperfine splitting constant and line width of TEMPO radical in gaseous and supercritical carbon dioxide

Abstract

The density dependences of the nitrogen hyperfine coupling constant ( a N) and line width (Δ H pp) of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) free radical have been investigated at near-infinite dilution in gaseous and supercritical carbon dioxide using steady-state EPR spectroscopy. The local density augmentation around the radical is quantified based on the density dependences of the a N. The viscosity dependences of the Δ H pp are reproduced by the modified Stokes–Einstein equation, suggesting the local solute-fluid density augmentation.