The structures of various para-substituted biphenyls (Bp-X; X = -OH, -OCH3, -CH3, -H, -CONH2, -COOH, and -CN) and their radical anions (Bp-X center dot-) were investigated by time-resolved resonance Raman spectroscopy combined with pulse radiolysis. The inter-ring C1-C1' stretching modes (nu(6)) of Bp-X were observed at similar to 1285 cm(-1), whereas the nu(6) modes of Bp-X center dot- with an electron-donating or -withdrawing substituent were significantly up-shifted. The difference (Delta f) between the nu(6) frequencies of Bp-X and Bp-X center dot- showed a significant dependence on the electron affinity of the substituent and exhibited a correlation with the Hammett substituent constants (sigma(p)). In contrast to Bp-H center dot- with a planar geometry, the theoretical and experimental results reveal that all Bp-X center dot- with an electron-donating or -withdrawing substituent have a slightly twisted structure. The twisted structure of Bp-X center dot- is due to the localization of the unpaired electron and negative charge density on one phenyl moiety in Bp-X center dot-.