Theoretical study of methylidene rhodanine tautomerism

Rhodanines is non-aromatic heterocyclic compounds, which are basis for the synthesis of some sensitizing dyes. In papers reported that rhodanine cycle can exist in three tautomeric forms - ketone, enol and thioenol. This is a consequence of large polar N-H bond in rhodanine cycle. Enol and thioenol forms can exist in two equivalent conformations, different in the direction of S-H and O-H bonds (Fig. 1). This conclusion was confirming by the data.