Nonpolar π electron delocalization effect on the stabilities of C=C, C=N, and N=N double bonds

Para substituent effects on the stabilities of C=C, C=N, and N=N bonds are studied with B3LYP/6-311++-g(2d,2p)//B3LYP/6-31g(d) method, which reveals a particular substituent effect that completely differs from the conventional Hammett type of induction/field or resonance effect. Analogous substituent effect has only been observed with carbon radicals, where it is called the spin-delocalization effect. However, as no spin is involved in the present systems, nonpolar π electron delocalization instead of any radical effect is proposed to explain of the observed behaviors. Further analyses show that the nonpolar π electron delocalization effect can only be clearly exhibited by the conjugation systems where the polar resonance effect is not significant, which implies the difficulty to observe such an effect. Nevertheless, dual-parameter regressions can clearly show the general existence of the nonpolar π electron delocalization effect.