Structural study of various substituted biphenyls and their radical anions based on time-resolved resonance Raman spectroscopy combined with pulse radiolysis

Abstract

The structures of various para-substituted biphenyls (Bp-X; X = −OH, −OCH3, −CH3, −H, −CONH2, −COOH, and −CN) and their radical anions (Bp-X•−) were investigated by time-resolved resonance Raman spectroscopy combined with pulse radiolysis. The inter-ring C1−C1′ stretching modes (ν6) of Bp-X were observed at ∼1285 cm−1, whereas the ν6 modes of Bp-X•− with an electron-donating or -withdrawing substituent were significantly up-shifted. The difference (Δf) between the ν6 frequencies of Bp-X and Bp-X•− showed a significant dependence on the electron affinity of the substituent and exhibited a correlation with the Hammett substituent constants (σp). In contrast to Bp-H•− with a planar geometry, the theoretical and experimental results reveal that all Bp-X•− with an electron-donating or -withdrawing substituent have a slightly twisted structure. The twisted structure of Bp-X•− is due to the localization of the unpaired electron and negative charge density on one phenyl moiety in Bp-X•−.