Abstract
The amino–imino tautomerism of mono- and diaminothiazoles in the gas, water and CCl
4 phases was investigated using the B3LYP and MP2(full) levels with the 6-311++G(d,p) basis set. The MP2 method predicted accurately the geometrical parameters of the compounds studied and showed that the amino forms predominate in solution as well as the gas phases. The tautomeric process is endothermic with a small entropy contribution to Gibbs free energy. Single point calculations at the MP4/6-311++G**, CISD/6-31+G**or QCISD/6-31+G**levels were also performed to improve the thermochemical values. 2-Aminothiazole-4(5
H)-imino was found to be the most stable structure in gas and solution phases in good agreement with the experimental data. At the QCISD/6-31G*and CISD/6-31G*levels the diimino form was more stable than the corresponding diamino form
24ATH whereas the MP2 level resulted in stabilization of the diamino form
24ATH over the imino form
2A5ITH. For the 2,5-diamino isomer
25ATH the calculations at all levels showed that the diamino form is the stable configuration in the gas or solution phases.