one, APO. ,-Unsaturated--ketoenamines, are capable to exist in three different tautomers at equilibrium, i.e., iminoketone, aminoketone, and iminoenol forms, see Fig. 1. The structure of all iminoketone, aminoketone, and iminoenol tautomers of 3-MeAPO, 3-EtAPO, and APO have been optimized at the B3LYP level, using 6-311++G** basis set. To confirm the relative stability of the iminoketone, aminoketone, and iminoenol forms of 3-MeAPO, 3-EtAPO, and APO, the obtained stable structures were also fully optimized at the B3LYP, using 6-31G*, 6-31G**, 6-311G**, 6-311++G**, D95** basis set, the second-order Moller-Plesset (MP2), using 6-31G* basis set, level in gas phase. Also, we optimized all iminoketone, aminoketone, and iminoenol tautomers of 3-MeAPO, 3-EtAPO, and APO at B3LYP/6-311++G** in CCl4, C2H5OH, and CH3CN solutions. According to the results of our calculations and previous works [1-2], aminoketone form of 3-MeAPO, 3-EtAPO, and APO is more stable than iminoketone and iminoenol forms. upon the substitution of methyl and ethyl group in the α position and because of the larger steric effect of this group with the hydrogen atoms of methyl groups in β positions, which pushes the methyl groups toward oxygen and nitrogen atoms, this effect, causes to reduce the N…O and O…H distances decrease, while the OHN bond angle and N-H bond length increase [4]. Therefore, the IHB strength of 3-MeAPO and 3-EtAPO increases compared to that of APO. AIM results confirm that the IHB in 3-MeAPO, 3-EtAPO are stronger than APO. This result is in agreement with the EHB for 3-MeAPO and 3-EtAPO molecules, which are obtained by the AIM method. The EHB values are 10.85, 11.00 and 7.67 kcal/mol, in 3-MeAPO, 3-EtAPO and APO, respectively.

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