14N Hyperfine Structure in ESR Spectra of Heterocyclic Anions

ESR spectra of the anions of several aromatic heterocyclics, containing nitrogen, have been recorded with the object of correlating the observed nitrogen hyperfine splittings with the π‐electron spin‐density distributions. As a result of this analysis it is found that the ¹⁴N hfs splitting constant (A^N) depends not only on the π spin density at the nitrogen site (ρ_N^π) but also on the π spin densities at the nearest‐neighbor carbon atoms (ρC₁^π and ρC₂^π, respectively). If, to a first approximation, the relationship is written as $$A^{\mathrm{N}}{\mathrm{=Q}}_{\mathrm{NN}}^{\mathrm{N}}{\rho }_{\mathrm{N}}^{\pi }{\mathrm{+Q}}_{\mathrm{CC}}^{\mathrm{N}}{\mathrm{(\rho }}_{{\mathrm{C}}_1}^{\pi }{\mathrm{+\rho }}_{{\mathrm{C}}_2}^{\pi }\mathrm{),}$$ where C₁ and C₂ are the two nearest neighbors of N, the coupling factors Q_NN^N and Q_CC^N are found to be +19.1±1.7 and +9.1±1.7 G, respectively.