(j.7r Interaction and Proton Hyperfine Interaction

Proton hyperfine interactions in aromatic radicals and aromatic ions are calculated by the MO (molecular orbital) method, and the basis of the semi-empirical equaticon, aN=Qp, is clarified; here aN is the splitting constant referred to proton N, p is the unpaired electron density at the carbon atom adjacent to proton N, and Q is the semi-empirical constant assumed to be the same for all CH bonds. Two essential approximations in this treatment are: a) the singlet-triplet excitation energies in cr orbitals are replaced by an averaged value JEAV, b) all cr orbitals containing CH bonds are transformed into 0 = orbitals which are strongly localized to each CH bond, and are approximated to be CH bond orbitals between the 1 s hydrogen atomic orbitals and the carbon sh hybrid orbitals. Then it is shown that the calculation of proton hyperfine splittings in the hypothetical CH fragment leads to the same results as in the case of the entire set of the aromatic system. The above two appro-ximations are shown to be reasonable for aromatic systems. The calculated value of Q in the present paper is-22.2 gauss, which is quite close to the semi-empirical value-22.!5 gauss