Structure-magnetism relationships in alpha-nitronyl nitroxide radicals

The crystal packing of alpha-nitronyl nitroxide radicals that have dominant ferromagnetic or antiferromagnetic interactions is analyzed in order to te st if there are characteristic orientations of their functional groups that can be associated with these magnetic interactions. From a large crystalli ne structural database of compounds containing alpha-nitronyl nitroxide rad ical units (143 structures), 23 representative cases with dominant intermol ecular ferromagnetic interactions, and 24 cases exhibiting dominant antifer romagnetic interactions were selected. The spatial distribution of the N-O ... O-N, C(sp(3))-H ... ON, and C(sp(2))-H ... ON contacts whose distance i s smaller than 10 Angstrom was analyzed, with special emphasis on the 0-5 A ngstrom region for the N-O ... O-N contacts and 0-3.8 Angstrom for the C-H O-N contacts. No correspondence is found between the presence of intermolec ular ferro- or antiferromagnetic interactions and the geometry of any of th e previous isolated contacts. Therefore, there is a need to change the way in which some structure-magnetism correlations are obtained in a-nitronyl n itroxide crystals. These results also show that the intermolecular magnetic interaction is related to the relative orientation of the nearby molecules as a whole, that is, with the collection of intermolecular contacts made b y them.