Ab initio study on the substituent effect in the transition state of keto-enol tautomerism of acetyl derivatives

Ab initio molecular orbital calculations have been performed on the intramolecular tautomerism of some acetyl derivatives, CH(3)COX (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 sigma* 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.