Abstract
We have performed ab initio molecular orbital calculations on 1‐ and 2‐substituted propenes, acetaldimines, and aldehydes — H~3~CC(X)=Y and XCH~2~CH=Y (Y = CH~2~, NH, O; X = H, CH~3~, NH~2~, OH, F) — to investigate the effects that substituents and functional groups have on their isoelectronic (except X = H) tautomerism. Structures for all stationary points (keto forms, enol forms, and transitions states) were optimized at the HF/6−31G**, MP2(full)/6−31G*, and B3LYP/6−31G** levels of theory and were characterized by frequency calculations. We performed intrinsic reaction coordinate (IRC) calculations at the HF/6−31G** level of theory to connect the transition structures with their local minima along the reaction path. In the transition structures, the migrating H4 atom is slightly out of plane, with a dihedral angle H4−C2−C1−Y3 (d4) of < 12°. In the keto−enol and imine−enamine series, the keto forms are thermodynamically more stable than their counterparts by ca. 30 kcal·mol^−1^ regardless of the site of substitution. In the 2‐substituted propene (Y = CH~2~) series, the enol forms are lower in energy than the keto forms by ca. 3−10 kcal·mol^−1^. There is a very good linear correlation between the relative energies and the electronegativity (Pauling scale) of the non‐hydrogen atom in Y of the functional group C=Y; the coefficient of the linear regression ranges from 0.98624 to 0.99963 for the six tautomeric series at the G2 level of theory. We explain the effects that the substituents have on the relative energies in terms of the changes in bond dissociation energies (by means of isodesmic reactions) and the dispersion of the charge through resonance effects (NBO analysis). These “enolization” processes all have rather high activation energies of 56−78 kcal·mol^−1^ at the G2 levels of theory. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)