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

The density dependences of the nitrogen hyperfine coupling constant (aN) and line width (ΔHpp) 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 aN. The viscosity dependences of the ΔHpp are reproduced by the modified Stokes–Einstein equation, suggesting the local solute-fluid density augmentation.