Abstract
The ion pair structures of various arylmethyl carbanions have been investigated with carbon-13 nuclear magnetic resonance spectroscopy. Arylmethyl carbanions exist as an equilibrium between contact and solvent separated ion pairs. Each of these two species has characteristic carbon chemical shifts. The ion pair equilibrium is governed by cation size, solvent structure, temperature, and anion. Contact ion pairs predominate at high temperatures in low polarity ethereal solvents with large cations. The alpha-carbon resonances are shielded and become constant with higher temperatures. These contact ion pairs shifts are linearly related to the reciprocal of the interionic distance. Solvent-separated ion pairing is favored by a strongly solvating ethers at low temperatures. Decreasing temperature results in a deshielding effect at the alpha-carbon. This shift becomes constant in value and reflects a loss of π-electron density at Ca, in comparison to the contact ion pair. Carbon chemical shifts ( average ¹³C shifts) of conjugated aromatic π-systems are linearly related to their average π-electron densities. The slope of the correlation is 156.3 ppm/electron. This empirical relationshi8p properly compares π-systems to each other, unlike previous approaches..
Russell, Charles Robert (1976). The ion pair structures of arylmethyl carbanions. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -474975.