Two Tautomers of Thiobarbituric Acid in One Crystal: The Experimental Charge Density Perspective

High-quality crystals of a certain polymorphic form of thiobarbituric acid containing both keto and enol tautomers in the asymmetric unit were obtained. High-resolution X-ray diffraction data up to sinθ/λ = 1.0 Å^-1 were collected and subsequently successfully used for the refining of the multipolar model of electron density distribution. The use of a crystal containing both ketone and enol forms allowed a direct comparison of the topological analysis results and a closer look at the differences between these two forms. The similarities and differences between the deformation densities, electrostatic potentials, Laplacian maps and bond characteristics of the tautomers were analysed. Additionally, the spectrum of the intermolecular interactions was identified and studied from classical, relatively strong N-H···O and O-H···O hydrogen bonds through weaker N-H···S hydrogen bonds to weak interactions (for instance, C-H···O, C-H···S and N···O). The results of these studies point toward the importance of including both the geometrical features and the details of the electron density distribution in the analysis of such weak interactions.