SOLVATION OF ALKALI-METAL CATIONS OF ARYLMETHYL ANIONS BY ETHEREAL SOLVENTS

The ion pair equilibria for triphenylmethyl, fluorenyl, diphenylmethyl, a-trimethylsilylbenzyl, and benzyl alkali metal anions dissolved in 2-methyltetrahydrofuran, tetrahydrofuran, and dimethoxyethane have been investigated with variable temperature carbon-13 magnetic resonance. The α-carbon chemical shifts for contact ions are very sensitive to cation and are proportional to the reciprocal of the interionic distance. Large a-carbon chemical shift changes are observed for the lithium and sodium salts when the change from contact ions at high temperatures to solvent-separated ions at low temperatures is within the experimentally accessible temperature range. The α-carbon chemical shifts for lithium and sodium solvent-separated ions are cation independent and comparison of these shifts to the contact shifts of the larger alkali metal carbanions gives an estimate of the interionic distance in the solvent-separated state. Little or no temperature-dependent chemical shift change is observed for the potassium, rubidium, or cesium salts either because they are contact ions at all temperatures or because carbon-13 is insensitive to solvation if it occurs. Entropies and enthalpies of solvation were determined for those salts that undergo approximately 40% of the change from contact to solvent-separated ions within the accessible temperature range. © 1979, American Chemical Society. All rights reserved.