Abstract: Free radicals produced by the electroreduction of substituted phenyldiazo-nium RC₆H₄N₂⁺BF₄^-(R=F, Cl,Br,NO₂,OCH₃, or CH₃) in acetonitrile were studied with the combination of ESR spectroscopy with spin trapping technique.The results show that:
1. Radical RC₆H₄^·is produced during the electroreduction of RC₆H₄N₂⁺BF₄^-, and it can be trapped by PEN to form stable spin adduct (RC₆H₄-PBN)^·.
2. From the hyper-fine splitting constant of the spin adduct, the dihedral angle θ_β can be obtained and the effects of the same substituent in different ring positions of θ_β are in the order of
θ_β (o-R) < θ_β(m-R) < θ_β (p-R)

Key words: arenediazonium, electroreduction, Spin trapping, ESR