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.