Modelling the crystal structure of the 2-hydronitronylnitroxide radical (HNN): observed and computer-generated polymorphs

The crystal structures of two polymorphs of 4,4,5,5-tetramethyl-4,5-dihydro -1H-imidazol-1-oxyl 3-oxide (the 2-hydronitronylnitroxide radical, HNN) are analyzed by packing energy criteria. Other unobserved polymorphic crystal structures are generated using a polymorph predictor package and three diff erent force fields, one of which is without explicit Coulomb-type terms. Th e relative importance of several structural motifs (hydrogen-bonded dimers, shape-interlocking dimers or extended hydrogen-bonded chains) is discussed . As usual, many crystal structures within a narrow energy range are genera ted by the polymorph predictor, confirming that ab initio crystal-structure prediction is still problematic. Comparisons of powder patterns generated from the atomic coordinates of the X-ray structure and from computational c rystal structures confirm that although the energy ranking depends on the f orce field used, the X-ray structure of the a, polymorph was found to be am ong the most stable ones produced by the polymorph predictor, even using th e chargeless force field.