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.