In this study the stability order and tautomeric equilibriums of the uracil conformers were investigated at the HF, MP2 and B3LYP levels of theory using different types of basis sets. Furthermore the compound method, CBS-4M,was employed to obtain accuracy in results. According to our calculations uracil has 4 tautomers and 18 conformers respectively. The calculations show that the DO-1 form is most stable than the DH, DO-2 and MH forms, that this result is in agreement with the previous studies. Consistent with results in all levels of theory, with the exceptions of HF method that its stability order is DO-1>DO- 2>DH>MH, we can suggest the following stability order for uracil tautomer; DO-1>DH>DO-2>MH On the other hand, DO-1 is a global minimumthat is followed by DH, DO-2 andMH tautomers respectively. Additionally all the possible Tautomeric equilibrium between uracil tautomers and their activation energies were calculated. Our results show that energy barriers for proton transfers are up to 155 kJ mol-1, therefore, proton transfer processes of uracil tautomers in gas phase are unfeasible. Finally the population analysis of some of the selected conformers at the NBO and AIM was under taken.

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