Anhydrous barbituric acid is known as the triketo tautomer in several polymorphic forms. With three C=O H-bond acceptor groups and only two N-H H-bond donor groups it was anticipated in early work that enol tautomeric forms of the β-diketone would be observed. Circa 50 years later it has recently been shown that this is the case and that this monoenol form is the thermodynamically stable species under slurry conditions. The preparation of this crystalline material suitable for a single crystal diffraction study[1] was facilitated by use of a liquid density gradient that separates polymorphic forms.[2] There are only ca. 20 known examples of tautomeric forms of organic materials as single crystals in both forms. As part of a study of partially H/D exchanged material it was found that the slurry process that converts the triketo form to the enol does not proceed in solvents that have exchangeable deuterium. Our approach to the question of the relative stability of polymorphic forms and the effect of deuterium substitution on this relative stability involves atomic level lattice energy calculations including dispersion contributions with periodic methods followed by simulation of vibrational spectra, particularly inelastic neutron scattering (INS) vibrational spectra. Comparison of the observed INS spectra from VISION/SNS/ORNL with that computed serves to validate the computed set of energy levels permitting computation of the vibrational zero point and thermal vibrational excitation of the crystalline materials in H and hypothetical D forms. Presumably the difference in the dynamical contributions to the enthalpy reverses the order of stability of the triketo and enol crystals in the H and D isotopic forms.

Inelastic neutron scattering analysis of tautomeric polymorphic crystals: The strange case of barbituric acid

253rd National Meeting (2017)

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