| Abstract
| - The relative stabilities of the tautomers of 2-aminothiazolidine-4-one and 4-aminothiazolidine-2-one werecalculated at the MP2/6-31+G(d,p) level by considering their mono- and trihydrated complexes. Single-point calculations at the MP4/6-31+G(d,p)//MP2/6-31+G(d,p) level of theory were performed to obtain moreaccurate energies. The values of proton transfer barriers in the isolated, mono- and trihydrated tautomers of2-aminothiazolidine-4-one (2AT) and 4-aminothiazolidine-2-one (4AT) were calculated for two differentmechanisms of tautomerisation. In the absence of water, the process of proton transfer should not occur.Addition of water molecules decreases the barrier making the process faster, as the participation of two watermolecules in a proton transfer reaction is more favorable than the participation of only one water molecule.To estimate the effect of the medium (water) on the relative stabilities of the tautomers of the studied compoundswe applied the polarizable continuum model (PCM). 13C NMR chemical shieldings were calculated using theGIAO approach at MP2/6-31+G(d,p) optimized geometries. HF and the DFT B3LYP functional with 6-31+G(d,p) basis set were employed. The quantum chemical results for the chemical shifts in gas phase and in polarsolvents (water and DMSO) were compared with experimental data. TD DFT B3LYP/aug-cc-pVTZ calculationswere performed to predict the absorption maxima of tautomers A and B of 2AT and 4AT.
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