IMINE-REVERSIBLE-ARROW-ENAMINE TAUTOMERISM

Acetaldimine half arrow right over half arrow left vinylamine tautomer ism and their conformational preferences are studied using MP2-(full)/ 6-31G geometries and Gaussian-2 energies. Geometrical parameters, har monic vibrational frequencies, relative stabilities, conformational st abilities, the amine inversion barrier, ionization energies, atomizati on energies, and equilibrium constants are compared with experimental gas-phase data when available. Their less than satisfactory agreement results mainly from a lack of definitive experimental data. The imine half arrow right over half arrow left enamine tautomerism is also comp ared with the keto half arrow right over half arrow left enol and nitr o half arrow right over half arrow left enamine tautomeric processes. Solvent effects are estimated by Onsager's reaction field model (SCRF) at HF/6-31G for structural parameters and vibrational frequencies, b oth of which are minimal, and at QCISD for energies. The anti-acetaldi mine tautomer is energetically favored over vinylamine by 3.9 kcal/mol at G2 and by 4.3 kcal/ mol after extrapolation of the acetonitrile so lvent effect. The theoretical (gas-phase) proton affinity of the enami ne anion amounts to 377 kcal/mol, and the atomization energy of acetal dimine is 697 kcal/mol. The anti conformation of acetaldimine is energ etically preferred over the syn form by <1 kcal/mol, which becomes eve n less after inclusion of solvent effects. Similarly, the energetic pr eference of the syn conformer over the anti form of the enamine anion becomes negligible after inclusion of solvent effects. The effect of c harges in Coulombic versus resonance delocalization in the allylic ena mine anion is discussed in detail.