Substituent effects on the electronic spectra of aromatic hydrocarbons II. The quantitative determination of the inductive effect on π-electrons

The inductive effects of substituent groups on the π-electrons of aromatic hydrocarbons are estimated quantitatively from electronic spectroscopic data. Two methods are used to evaluate the parameters. The first is based on the first-order changes in the energy of the first transition of azulene on mono-substitution and the second is based on the second-order changes in the energy of the first transition of benzene on poly substitution. The agreement between the values obtained by the two methods is good except for acceptor substituents for which the values obtained by the first method are considerably smaller than those obtained by the second. Using these values of the inductive parameter we have calculated the splitting of the ground state of the benzene negative ion brought about by substituents; and the relationship of this splitting to the electron spin resonance spectra of these ions is discussed.