The hyperfine splitting constants, A_N, of electron spin probe, 4-hydroxy-2,2,6,6-tetramethyl-1- piperidin-1-yloxy, free radical (4-OH-TEMPO) show positive linear correlation with model reaction parameters of solvent polarity, E_T(30) and solvent acceptor number (AN). Moreover, generally, A_N values are higher in halogenated than non halogenated solvents as dielectron constant or dipole moment is near. Besides the general solvent effect on A_N values, larger A_N values can be attributed to the specific halogen-bonding between N—O free radical and halogenated solvent molecule. The calculation shows the occurrence of C—Cl…O between chloroform or dichloromethane and free radical molecules.

Key words: halogen-bonding, electron spin resonance, specific solvent effect, halogenated solvent, solvent acceptor number