  In this work we present crystal structure determination of two elusive
polymorphs of furazidin, an antibacterial agent, employing a combination of
crystal structure prediction (CSP) calculations and NMR crystallography
approach. Two previously uncharacterized neat crystal forms, one of which has
two symmetry independent molecules (form I), whereas the other one has only one
molecule (form II), crystallize in P21/c and P-1 space groups, respectively,
and are both built by different conformers, displaying different intermolecular
interactions. We demonstrate that the usage of either crystal structure
prediction or NMR crystallography alone is insufficient to successfully
elucidate the mentioned crystal structures, especially in the case with two
independent molecules in the polymorph structure. In addition, cases of
serendipitous agreement in terms of 1H or 13C NMR data obtained for the crystal
structure prediction generated crystal structures different from the ones
observed in laboratory (false-positive matches) are analyzed and described.
Although for majority of analyzed crystal structures the obtained agreement
with the NMR experiment is indicative of some structural features in common
with the experimental structure, the mentioned serendipity observed in
exceptional cases points to the necessity of caution when using NMR
crystallography approach in crystal structure determination.
