Polarographic Half-wave Reduction Research Articles

New description of the substituent effect on electronic spectra by means of substituent constants. II. n-.PI.* and .PI.-.PI.* Transitions.

In order to confirm and to extend the scope of the equation 1, 3EUVho→lu=aF+bR+c, presented by us in the foregoing paper, the polarographic half-wave reduction (Ered1/2) and oxidation (Eoxd1/2) potentials in nonaqueous media and the electronic spectra of 4-substituted azobenzenes have been analyzed. Although the equation was originally derived for π-π* type HOMO→LUMO transitions with the same character in a series of substances, in this paper we have theoretically verified that the equation is also applicable to the n-π* bands of conjugated systems. The singlet n-π* and π-π* bands of 4-substituted azobenzenes and the triplet n-π* bands of 4, 4'-disubstituted benzophenones were employed to examine the above equation. The results were reasonable, and are discussed from the viewpoint of molecular orbital theory.

Mutual correlation between nonaqueous oxidation-reduction potentials and electronic spectra, and its extension to describe the substituent effect on electronic spectra by means of substituent constants.

In order to confirm and to extend the scope of the equation (Eoxd1/2-Ered1/2)=k11, 3 EUVho-lu+k2, presented previously by us, the polarographic half-wave reduction (Ered1/2) and oxidation (Eoxd1/2) potentials in nonaqueous media and the electronic spectra of 4, 4'-disubstituted stilbenes have been measured and analyzed by means of the equation with reasonable results. The substituent effect on Eoxd1/2 and Ered1/2 values has been well described by using the Hammett-type substituent constants. This result has been combined with the above equation, and thus a general equation describing the substituent effect on electronic spectra in terms of substituent constants has been derived as EUVho-lu=aF+bR+c, F and R being Swain and Lupton's substituent constants. This equation has been experimentally examined using the strong longest-wavelength π-π* bands of 4, 4'-disubstituted stibenes and azoxybenzenes. The results were quite reasonable. Finally, the description of the substituent effect on electronic spectra in terms of substituent constants is discussed on the basis of molecular orbital theory.