Ab initio study of the magnetic behavior of four dithiadiazolyl radical compounds.

We have studied, by means of ab initio calculations, the magnetic interaction mechanisms in four radical crystals, X-C6F4-CNSSN (X = O2N, alpha-NC, beta-NC, Br), which has allowed its to explain their different magnetic behaviour (ferromagnetism, antiferromagnetism, paramagnetism, spin frustration, etc.). First, we have identified the magnetic exchange pathways considering those with distances between two atoms of different dithiadiazolyl rings shorter than 7 angstrom and those with an intermolecular distance between an atom of the heterocyclic ring and an atom in a neighbouring radical shorter than 4 angstrom. Second, the calculations have been carried Out in the framework of the DFT Broken Symmetry. Following this procedure we have determined the magnitude and the sign of the relevant coupling constants for the X-C6F4-CNSSN (X = O2N, alpha-NC, beta-NC, Br) radicals. In the cases where the radicals order magnetically, ordering temperatures determined with our ab initio calculations agree very well with the experimental ones. Thus, in the case of the O2N derivative ferromagnetic ordering is observed below 1.3 K, in very good agreement with an ordering temperature around 1.6 K predicted from our calculated exchange constants and using a mean field approximation. (c) 2005 Elsevier Ltd. All rights reserved.