Hyperfine and quadrupole interactions at nitrogen in hemin

Abstract

The hyperfine and quadrupole coupling constants at nitrogen in iron (III) porphyrin chloride have been calculated. The wave functions were obtained by means of a self-consistent charge extended Huckel method [2]. The spin Hamiltonian for the system is , where the double-primed axis refers to the principal axis system of the field-gradient tensor. In the calculation of the nitrogen hyperfine constant, three contributions were considered: 1) the Fermi contact term; 2) the exchange polarization contribution; 3) the many body contribution. The last contribution was included because of its large relative importance in atomic nitrogen. The Fermi contact contribution dominates the nitrogen hyperfine constant, and the contact term has a 15% contribution from non-local effects, indicating significant unpaired electron density between iron and nitrogen. The exchange polarization term is negative and smaller than the Fermi contact term. Correlation effects are quite small, in contrast to the atomic nitrogen case. The dipolar hyperfine terms and the principal axis system of the hyperfine interaction were also evaluated. The quadrupole coupling constant for nitrogen was calculated, as well as the corresponding principal axis system. The contribution of unpaired electrons to the field gradient at nitrogen is substantial. The results of the calculations suggest considerable delocalization of unpaired electrons, in contrast to what one assumes using a crystal field model.