A MO-theoretical calculation of the antiferromagnetism in the γ-phase of p-nitrophenyl nitronyl nitroxide

Semi-empirical and ab initio molecular orbital (MO) calculations have been carried out for clusters of p -nitrophenyl nitronyl nitroxide ( p -NPNN) extracted from the experimental crystal structure of its γ-phase. It is found that the intraplane exchange interactions ( J ab ) are ferromagnetic, whereas the interplane one is antiferromagnetic. The antiferromagnetic transition temperature ( T N ) is estimated by a generalized Langevin-Weiss-Neel (GLWN) model combined with the calculated J ab values by the MO methods. The Neel temperature ( T N ) by the mean-field GLWN model is of the order of the experimental value, although the former is much larger than the latter because of the low dimensionality of the γ-phase. A systematic theoretical explanation is presented for the magnetic long-range orderings in the β- and γ-phase crystals of p -NPNN.