AB-INITIO STUDY ON THE IMINE-ENAMINE TAUTOMERISM OF THE ALPHA-SUBSTITUTED IMINES (XH(2)CCH=NH, X=H, BH2, CH3, NH2, OH, F, CL, CN, NO)

Ab initio molecular orbital calculations have been carried out on the alpha-substituted acetaldimines XH(2)CCH=NH (X = H, BH2, CH3, NH2, OH, F, Cl, CN, NO) to investigate the substituent effects on the imine re versible arrow enamine tautomerisms. All stationary points (imines, en amines, and transition states) were optimized at HF/6-31G() and MP2(f ull)/6-31G() levels of theory and subsequently characterized by the f requency calculations. Single-point calculations at the MP4(FC)/6-31G( )//MP2(full)/6-31G(*) and MP4(FC)/6-311 ++ G(**)//MP2(full)/6-31G(*) levels were performed for all stationary points. The intrinsic reactio n coordinates (IRC) were traced in order to connect the transition str uctures and the corresponding substituted tautomers. MP2(full)/6-31G( ) results favor the imines over the enamines by 7.89, 6.56, 5.95, 5.61 , 5.57, and 5.55 kcal/mol, respectively, for X = H, F, OH, Cl, NH2, an d CH3. While, for X = BH2, CN, and NO, the enamines are lower in energ ies than the imines by 5.67, 1.20, and 3.77 kcal/mol, respectively. An alysis of NBO calculations shows the empty 2p atomic orbital in boron and the pi() orbitals in CN and NO interact with the adjacent C=C dou ble bond.