Exchange interactions in nitroxide biradicals

It has been shown by experimental and theoretical studies that exchange interactions in non planar pi systems are strongly dependent on the geometrical parameters characterizing the non planarity. A study of the exchange interaction in isolated nitroxide biradicals is presented for non planar paraphenylene bis nitroxide 3 [1,4-di-t-butyl-2,5-bis(t-butylaminoxyl) benzene (anti)], non planar metaphenylene bis nitroxides 7 and planar metaphenylene bis nitroxide 9. Determination of positive or weak negative exchange interactions in isolated molecules in dilute solution is difficult and can be inaccurate. 3 is a ground state singlet 2J/k = -35K. Metaphenylene bis nitroxide 7g [1-t-butyl,3,5-bis(t-butylaminoxyl) benzene] and 9 [1,2,9-tetrahydro 2,2,4,4,6,6,8,8-octamethyl-4H,6H-benzo[1,2d; 4,5d']bis(1,3 oxazine)-1,9 dioxyl] are probably ground state singlets, the triplet-singlet separation being smaller than 10K, contrary to theoretical predictions. X-Ray structure of 7a A [1,3,5-trimethyl-2,6-bis(t-butyl aminoxyl)benzene (syn)], 7g and 9 are reported. In pure crystalline solid, however, these three biradicals have a susceptibility characteristic of triplet radical-pairs in weak antiferromagnetic interaction with their neighbours. The problems (including the transferability of the exchange interaction) raised by these apparently contradictory results are discussed.