On the lack of hydrogen bonds in the crystal structure of alloxan

The crystal structure of alloxan ((CO)3.NH.CO.NH) is unusual in that it contains no hydrogen bonds. We account for this by using a realistic model for the intermolecular forces which includes a distributed multipole electrostatic model, to find minimum energy dimer and crystal structures. Alloxan dimer structures with close (CO) … (CO) interactions are predicted to have similar energies to the hydrogen bonded dimers. An extensive systematic search for possible crystal structures for alloxan found the observed crystal packing to have the lowest lattice energy and hydrogen bonded polymorphs to be less stable by more than 5 kJ mol−1. This illustrates the limitations of the functional group approach to predicting molecular crystal structures.