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 different force fields, one of which is without explicit Coulomb-type terms. The 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 generated 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 crystal structures confirm that although the energy ranking depends on the force field used, the X-ray structure of the alpha polymorph was found to be among the most stable ones produced by the polymorph predictor, even using the chargeless force field.
Centro CNR per lo Studio delle Relazioni tra Struttura e Reattività Chimica, Università di Milano, Milano, Italy. fili@rs6.csrsrc.mi.cnr.it
G
Filippini
A
Gavezzotti
JJ
Novoa