Single‐Crystal X‐ray Diffraction Structure of the Stable Enol Tautomer Polymorph of Barbituric Acid at 224 and 95 K

Abstract

The thermodynamically stable enol crystal form of barbituric acid, previously prepared as powder by grinding or slurry methods, has been obtained as single crystals by slow cooling from methanol solution. The selection of the enol crystal was facilitated by a density‐gradient method. The structure at 224 and 95 K confirms the enol inferred on the basis of powder data. The enol has bond lengths that are consistent with the expected bond order and with DFT calculations that include treatment of hydrogen bonding. In isolation, the enol is higher in energy than the tri‐keto form by 50 kJ mol−1 which must be more than compensated by enhanced hydrogen bonding. Both crystal forms have four normal H‐bonds; the enol has two additional H‐bonds with O–O distances of 2.49 Å. Conversion into the enol form occurs spontaneously in the solid state upon prolonged storage of the commercial tri‐keto material. Slurry conversion of tri‐one to enol in ethanol is reversed in direction in ethanol‐D1.

Invariant and Variable Supramolecular Self-Assembly in 6-Substituted Uracil Derivatives: Insights from X-ray Structures and Quantum Chemical Study

“…The former molecular dimer M1 stabilizes with the N1–H1···O2 hydrogen bond, which is relatively stronger than the latter dimer M2. It should be noted that the S3 type molecular tape is one of the recurrent synthon topologies observed in some of the 6-substituted uracil compounds (CSD refcodes: XOJFAE, UPAWIT, IYAQOP02, GEDYAR, HIMMUM, IYAQOP03, OROTAC01, and CEWVOP03). − The 6- n -propyl-2-uracil (csd refocde: PELJIA) produces the S3 type synthon but utilizes a different combination of donors and acceptors of the uracil moiety . In 6-substituted uracil-cocrystal complexes, the orotic-melamine·H 2 O and 6-substituted uracil-tetrafluoroterephthalic acid complexes exhibit the S3 type synthon, and co-formers do not affect one of the recurring S3 synthons.…”

Supramolecular Assemblies of Melamine-2-Thiobarbiturate and Melamine-Barbiturate-2-Thiobarbiturate: Experimental and Theoretical Studies

“…Three days later, the resulting precipitate was filtered. Another important property of TBA is the presence of many tautomeric forms [12,20,21]. Five tautomeric forms were experimentally observed out of ten theoretically possible [12].…”

“…The presence of an acidic proton in the C5 position in its structure makes it a convenient substrate for various modifications, including amination [7] and makes it possible to form complexes with metals [15,16], and the presence of carboxyl and amine groups allows the formation of intermolecular hydrogen bonds, which could be utilized to create multimodal supramolecular polymers with desired properties [17,18] and control crystallization [19]. Another important property of TBA is the presence of many tautomeric forms [12,20,21]. Five tautomeric forms were experimentally observed out of ten theoretically possible [12].…”