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 (AN) depends not only on the π spin density at the nitrogen site (ρNπ) but also on the π spin densities at the nearest-neighbor carbon atoms (ρC1π and ρC2π, respectively). If, to a first approximation, the relationship is written as AN=QNNNρNπ+QCCN(ρC1π+ρC2π),where C1 and C2 are the two nearest neighbors of N, the coupling factors QNNN and QCCN are found to be +19.1±1.7 and +9.1±1.7 G, respectively.