A Molecular Orbital Treatment of the Electronic Structure and Spectra of Tautomeric Rhodanines

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Abstract
An experimental as well as theoretical investigation of the electronic structure and tautomeric equilibria of 2,4-thiazolidinedione, 4-thiazolidinone-2-thione (rhodanine), 2-thiazolidinone-4-thione (isorhodanine) and 2,4-thiazolidinedithione (thiorhodanine) has been conducted. The origin of the electronic absorption transitions and the effect of replacing oxygen by sulfur atoms on coplanarity have been discussed. The solvent and pH dependence of the electronic spectra of the studied compounds indicate that there should be a statistical distribution of the ground and excited states between all possible tautomeric structures. This has been confirmed via molecular orbital computations. The calculated pK values have been interpreted with a mechanism based on the calculated deprotonation energies. Results of the present investigation concur with the chemistry of the studied compounds and reveal that these compounds are far from complete homocyclization.
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