A SINGLE-CRYSTAL X-RAY AND ELECTRON-SPIN-RESONANCE STUDY OF THE 5-NITRO-1,1,3,3-TETRAMETHYLISOINDOLIN-2-YLOXYL FREE-RADICAL

X-ray structural analyses have been carried out on the 5-nitro-1,1,3,3 -tetramethylisoindolin-2-yloxyl free radical (NTMIO) and its precursor 5-nitro-1,1,3,3-tetramethylisoindoline (NTMI). A mixed single crystal of about 2% NTMIO in NTMI has been prepared and subjected to a detail ed ESR examination. The X-ray analyses of NTMIO and NTMI showed that t he two have different crystal habits but the dimensions of the two mol ecules are almost identical hence a small amount of NTMIO in NTMI woul d not be expected to perturb the lattice of the latter. The unit cell of NTMIO shows that the packing of the structure is normal and that th ere are no intermolecular distances below 3.41 A showing that hydrogen bonding is not present. The bond distances and angles of the heterocy clic ring of NTMIO are generally more closely related to saturated rat her than to unsaturated five-membered ring nitroxyl radicals. ESR para meters are compared for TMIO in solution, in frozen solution and in a single crystal of NTMI.