New ε-Polymorphic Modification of 4-Nitrophenyl-Substituted Nitronylnitroxyl

A new ε-polymorph of 4,4,5,5-tetramethyl-2-(4-nitrophenyl)-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl is prepared and characterized by single-crystal XRD. The arrays of XRD data have been accumulated and the structures of known α–γ-polymorphs of this paramagnetic are solved under the same conditions. The Hirshfeld surfaces of the crystal structures of the studied polymorphs are analyzed qualitatively and quantitatively. It is shown that the known α–δ-polymorphs contain a supramolecular associate in the form of a centrosymmetric dimer whose molecules are connected by C–H⋯O hydrogen bonds. At the same time, only the ε-polymorph contains π-bonded infinite stacks of nitrophenyl substituents, which in turn determine the formation of short distances between oxygen atoms of nitroxide groups (3.508(2) Å and 3.627(2) Å).