Crystal Engineering and Charge Density Study of Pharmaceutical Nonlinear Optical Material: Melamine-Barbital Co-Crystal

This
paper presents topological analysis of charge density from multipolar
refinement and ab initio calculations for both melamine-barbital co-crystal
and isolated molecules. The comparison of electron density properties
for the isolated molecules and for the co-crystal allowed one to unravel
the factors contributing to the formation of specific hydrogen bond
patterns in the co-crystal structure. The inspection of charge density
properties revealed that N1A of barbital and N3B of melamine compete
for H1A atom in hydrogen bond formation. The hydrogen bond shows an
intermediate character between closed and shared shell interaction.
The tendency of barbital to adopt tautomeric forms in solution which
can be correlated with the shift of electron density (mesomeric effect)
was emphasized. Charge density studies confirmed the existence of
the mesomeric form B of barbital in the studied co-crystal. The form
B was also reported in polar co-crystals comprised of barbital and
barbituric acid.