1999, Vol.72, No.5

1083-1091

The electron affinities (EAs) for a series of methyl benzoates, acetophenones, benzaldehydes, and benzophenones were determined by measuring the electron-transfer equilibria in the gas phase. The effect of substituents (X) in the benzene ring on the EAs of this series as well as the nitrobenzene and benzonitrile series were correlated in terms of the Yukawa–Tsuno equation,

It has been shown that the r value, representing the degree of π-delocalization of the negative charge into the substituent (X) at position 4, increases linearly with decreasing EA of the unsubstituted member (X = H) of the respective series, i.e., 0.65 for nitrobenzene vs. 1.05 for benzaldehyde and 1.30 for benzonitrile. This tendency of the r value leads to the conclusion that the negative charge of a less-stable radical anion is more significantly distributed in the benzene ring than in a stable radical anion, like that of nitrobenzene. Ab initio MO calculations show that when the EA of a Y-substituted benzene decreases, the total negative charge on the substituent (Y) in the radical anion also decreases, and that the bond lengths (C2C3/C5C6) shorten. These features are consistent with the conclusion derived from the experimentally obtained r value.