Ab initio molecular orbital calculations have been carried out on the a-substituted acetaldimines XH2CCH=NH (X = H, BH2, CH3, NH2, OH, F, Cl, CN, NO) to investigate the substituent effects on the imineenamine tautomerisms. All stationary points (imines, enamines, and transition states) were optimized at HF/6-31G* and MP2(full)/6-31G* levels of theory and subsequently characterized by the frequency calculations. Singlepoint calculations at the MP4(FC)/6-31G*//MP2(full)/6-31G* and MP4(FC)/6-31 l++G**//MP2(full)/6-31G* levels were performed for all stationary points. The intrinsic reaction coordinates (IRC) were traced in order to connect the transition structures 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, and CH3. While, for X = BH2, CN, and NO, the enamines are lower in energies than the imines by 5.67, 1.20, and 3.77 kcal/mol, respectively. Analysis of NBO calculations shows the empty 2p atomic orbital in boron and the jz* orbitals in CN and NO interact with the adjacent C=C double bond. Computational MethodsAb initio molecular orbital calculations were carried out using the Gaussian 9017 and Gaussian 9218 packages. The geometrical structures of the substituted imines, enamines, and transition states for the tautomeric interconversion were optimized at the MP2(full)19 and HF levels with the 6-31G*20 basis set. The calculated equilibrium structures (local minima and saddle

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The potential energy surface for C4H5 + as calculated by ab initio molecular orbital theory is reported at two levels of theory, HF/6-31G(d,p) and MP2(full)/6-311G(d,p). Fourteen minima have been located at HF/6-31G(d,p), but inclusion of electron correlation reduced this number to nine. The methylcyclopropenyl cation, 1, is the global minimum, and the 2-cyclobutenyl (2), α-vinylvinyl (3), γ-methylpropargyl (4), α-methylpropargyl (5), and 1-cyclobutenyl (6) cations are 9.1, 19.9, 25.3, 27.5, and 26.9 kcal/mol, respectively, above 1. Eleven transition structures, permitting interconversion between the nine minima, are reported. Enthalpies of formation (in kcal/mol) calculated at MP4SDTQ/6-311++G(2df,p) are 231.4 for 1, 241.7 for 2, 246.9 for 3, 255.7 for 5, 259.9 for 6, and 264.3 for 7.

Ab initio molecular orbital calculations have been performed on the R-substituted acetaldehydes XH 2 CCHdO (X)H, BH 2 , CH 3 , NH 2 , OH, F, CN, NC, and Cl) to investigate the substituent effects on the keto-enol tautomerisms. Structures for all stationary point (ketones, enols, and transition states) were optimized and characterized at the MP2(full)/6-31G* and MP2(full)/6-31G** levels of theory. Intrinsic reaction coordinates (IRC) calculations were performed in order to connect transition structures with the appropriate tautomeric pairs. Results from various levels of calculations all show that the keto forms are thermodynamically more stable than the enol forms. At the G2 level, the former tautomers are energetically favored over the latter forms by 2.

Ab initio molecular orbital calculations have been performed on the intramolecular tautomerism of some acetyl derivatives, CH3COX (X = H, BH2, CH3, NH2, OH, F, Cl, CN, NC). All stationary points and the keto, enol, and transition structures were optimized at the HF/6-31G* and MP2(full)/6-31G* levels of theory and confirmed by frequency calculations. Single-point calculations at MP4(FC)/6-311++G**//MP2(Full)/6-31G* were also carried out for all stationary points. The intrinsic reaction coordinate (IRC) for the tautomeric processes were traced to connect the transition structures and the corresponding substituted tautomeric pairs. The natural bond orbital (NBO) analyses on the transition states show that the interactions of the lone pair electrons on the oxygen atom and the σ* C−H bonds have a significant effect on their stabilities, which consequently affects the activation energies of the tautomeric processes. The energy barriers calculated at various levels of theory are reported for each tautomeric interconversion.