We have investigated the pairs of rotational isomers for six 3-(o-aryl)-5-methyl-rhodanines (Z = H, F, Cl, Br, OH, and CH3) using NMR spectroscopy and density functional theory (DFT) calculations. Electron density topological and NBO analysis has demonstrated the importance of non-covalent interactions, characterised by (3, -1) bond critical points (BCPs), between the oxygen and sulfur atoms on the thiazolidine ring with the aryl substitutents in stabilizing the transition states. The energetic activation barriers to rotation have also been determined using computational results; rotational barriers for 3-(o-chlorophenyl)-5-methyl-rhodanine (3S) and 3-(o-tolyl)-5-methyl-rhodanine (6S) were determined experimentally based on NMR separation of the diastereoisomeric pairs, and the first-order rate constants used to derive the value of the rotational barrier from the Eyring equation.

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The enolisation tendencies of 1-(o-substituted phenyl)barbituric and -2-thiobarbituric acid derivatives have been studied by observing the behaviour of the compounds in different solvents by 1 H and 13 C NMR. It has been found that the enolisation tendencies of the thiobarbituric acid derivatives observed in polar solvents are greater than those of the barbituric acid derivatives. The ratio of keto -enol tautomers of thiobarbituric acid derivatives in DMSO and in DMF has been calculated.

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