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 bands, We account for this by using a realist ic model for the intermolecular forces which includes a distributed mu ltipole electrostatic model, to find minimum energy dimer and crystal structures. Alloxan dimer structures with close (C=O)...(C=O) interact ions are predicted to have similar energies to the hydrogen bonded dim ers. An extensive systematic search for possible crystal structures fo r alloxan found the observed crystal packing to have the lowest lattic e energy and hydrogen bonded polymorphs to be less stable by more than 5 kT mol(-1). This illustrates the limitations of the functional grou p approach to predicting molecular crystal structures.