Charge-density analysis in polymorphs of urea-barbituric acid co-crystals.

High-resolution single-crystal X-ray diffraction measurements at 100 K were performed for the two polymorphs of urea-barbituric acid co-crystals: (I) P2(1)/c and (II) Cc. Experimental and theoretical charge density and its properties were analysed for (I) and (II) in order to confirm the previous observation that in the polymorphs studied the barbituric acid molecules adopt different mesomeric forms, leading to different hydrogen-bond systems. Koch and Popelier criteria were applied to distinguish between hydrogen bonds and van der Waals interactions in the structures presented.