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.
Filippini
G
Centro CNR per lo Studio delle Relazioni tra Struttura e Reattività Chimica, Università di Milano, Milano, Italy. fili@rs6.csrsrc.mi.cnr.it
Gavezzotti
A
Novoa
JJ