Charge-localizing effect in alkali-metal enolates and phenolates. Structure and aromaticity of the phenolate anion

Alkali-metal counterion interactions influence the enolate and phenolate anions considerably. Like the acetaldehyde enolate anion, the stabilization energies of the phenolate anion (vs OH-) are very much reduced in the corresponding alkali-metal ion pairs. The stabilization energies (relative to OH-) of 34.9 kcal/mol for the free enolate anion and 40.9 kcal/mol for the phenolate ion (computed with the Becke3LYP density functional on the MP2(full)/6-31+G* optimized structures) reproduce the experimental gas-phase data well. The metal gegenion induced negative charge localization on the enolate and phenolate oxygens varies only somewhat along the Li-Cs series but reduces the stabilization energies enormously: for the enolates to 8-14 kcal/mol and to 9-15 kcal/mol for the phenolates (vs MOH). These ion pair interactions influence the geometries, the charge distributions, the C-13 chemical shifts, and the reaction energies of alkali-metal enolates and phenolates but do so to; similar extents in both systems. Charge localization is still effective at long metal-oxygen distances, since electrostatic interaction energies decrease only with the inverse of the distance. The phenolate anion has considerable quinoid character, and judging from the geometry and the magnetic criteria (NICS and Lambda, the magnetic susceptibility exaltation) its aromaticity is reduced to about 60% of that of phenol and the alkali-metal ion-paired phenolates.