Computer Simulation of Crystal Structure for Three Furazan Derivatives

The crystal structure simulations for three azoxy furazans, 4,4‘-dimethylazoxyfurazan (1), 4,4‘-dichloroazoxyfurazan (2), and 4,4‘-di(morpholin-1-yl)azoxyfurazan (3), were carried out to test the reliability of
standard force fields for furazan derivatives. All calculations were performed using the Polymorph Predictor module
from the Cerius2 package with three force fields implemented in this package:  Dreiding, Universal, and Scheraga's.
The predicted crystal structures were compared with experimental ones, obtained by X-ray diffraction analysis. To
reveal the effect of initial molecular geometry on crystal packing prediction, the crystal structure simulations were
carried out for molecular geometry from X-ray data, and from quantum calculations as well. To evaluate the
importance of Coulomb interactions in these crystals, we carried out two types of calculations, taking into account,
and disregarding the Coulomb term. Our study showed that crystal structures 2 and 3, which are mainly governed
by Coulomb and steric factors, respectively, are readily predicted, while structure 1 has many almost equal energy
minima and is difficult to predict. It was shown that a set of predicted structures allows for estimation of the density
of the calculated compound.