Geometry and thermodynamic stabilities of rhodanine tautomers and rotamers: Quantum chemical study

Eleven tautomers and rotamers of rhodanine, R1-R11, were studied theoretically at MP2/6-311G+(d,p), CBS-QB3, B3LYP/6-311+G(d,p), B3LYP/6-31+G(d,p) and MPW1K/6-31+G(d,p). Optimized geometrical parameters of all the eleven forms were computed and compared with the available X-ray results. The geometry of MP2/6-311G+(d,p), and B3LYP/6-311+G(d,p) methods shows the best agreement with the experimental results. Thermodynamic parameters for the eleven forms were calculated at the above levels in the gas phase and water utilizing PCM method. Tautomer R1 is the most stable tautomer in all cases both in gas phase and in aqueous solution as also known from experimental data. Consideration of the solvent causes some reordering of the relative stability of rhodanine conformers. Natural charges and electric dipole moment were computed at different levels. The transition states for the rhodanine tautomerization and rotamerization processes were studied for selected systems at different levels. Energy barriers of rotamerization processes are only 10% of that for the proton transfer tautomerization processes.