Investigating unused hydrogen bond acceptors using known and hypothetical crystal polymorphism

The crystal structures found in a manual search for polymorphs are discussed in conjunction with low energy crystal structures found in a computational search for minima in the lattice energy, for barbituric acid, cyanuric acid, alloxan, parabanic acid, and urazole. Since all these molecules, with the exception of urazole, have crystal structures in which there are carbonyl groups not used in conventional hydrogen bonding, these results and the electrostatic properties of the molecules are used to interpret this unusual behavior. It appears that there is no great difference between the strengths of the various N-H donors and C=O acceptors within these molecules, and the observed crystal structures result from the compromise between the intermolecular interactions of the molecules.