Comparative theoretical study of transition structures, barrier heights, and reaction energies for the intramolecular tautomerization in acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems

The transition structures associated with the possible intramolecular tautomerization for acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems as models of keto/enol and imine/enamine interconversion processes, respectively, were characterized. The relative stabilities of the tautomers and the associated barrier heights were calculated. Ab initio analytical gradients and second derivatives at the HF level of theory and 3‐21G, 6‐31G, 6‐31G**, 6‐31++G**, and 6‐311++G** basis‐set, DFT (BP86/6‐311++G** and BLYP/6‐311++G**), and semiempirical (AM1 and PM3) procedures were used to identify the stationary points. Correlation effects were estimated using the perturbational approach at MP2/6‐31G**, MP2/6‐311++G**, and MP2/6‐311++G (3df,2p) levels. The geometry, electronic structure, harmonic vibrational frequencies, and transition vector associated with the transition structures as well as the relative stabilities of different isomers and barrier heights were analyzed. The dependence of these properties upon theoretical methods is analyzed and discussed. The transition structures are four‐membered rings and the corresponding transition vectors are associated to collective fluctuations. The 1,3 intramolecular hydrogen migration is much more advanced than are the hybridization changes on donor and acceptor centers at the transition structure. The corresponding barrier heights can be related to the change of bond orders and acid/base properties of these centers. A comparison of the results obtained with different methods renders that the nature of the transition structure seems to be a rather robust entity. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 9–24, 1998