Antiferromagnetic interactions arising from a close contact between nitroxyl oxygen and β-methyl carbon atoms carrying an α-spin in the solid state

Antiferromagnetic interactions (|J|/k_B > 4.0), which were observed by the magnetization measurement of the powder samples of the racemic nitroxyl radicals trans-2,5-bis(4-methylphenyl)-2,5-dimethylpyrrolidine-1-oxy [(±)-1a] and trans-2,5-bis(4-bromophenyl)-2,5-dimethylpyrrolidine-1-oxy [(±)-2a] at low temperatures, were rationalised in terms of the intermolecular spin polarization exchange coupling or the SOMO–SOMO overlapping owing to a short contact between the nitroxyl oxygen atom and the β-carbon atom of a neighbouring molecule by X-ray crystallographic analysis and DFT calculations.