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MNDO calculations have been employed to study the effect of some substituents of the acetaldemine-vinylamine tautomeric system at the imine-carbon position of CH 3 CXNH, where X = H, F, CN, NH 2 , NO 2 , BH 2 , CH 3 and CF 3 . It is found that the substitutents F, NH 2 and NO 2 encourage the formation of the enamine tautomer. The substitutents CN, CH 3 , CF 3 and BH 2 encourage the formation of the imine tautomer. Isodesmic reactions, free energy change (∆G), charge distribution and energy gap (E g ) between HOMO and LUMO were used to prove these findings. Resonance stabilization was a major factor in the determination of the most stable tautomer. These results were compared with previous work on the substitution at the σ -carbon position.

The Acetaldimine-Vinylamine tautomeric system has been studied by employing the MNDO semiempirical method. The imine structure was found to be energetically favorable, as indicated by the calculated heats of formation, Gibbs free energy, LUMO and HOMO, and charges. The substitution of F, Cl, CN, CH