CARBANION MECHANISMS .9. SPECTROPHOTOMETRIC STUDY OF TRIPHENYLMETHYL ALKALI-METAL SALTS - CONTACT AND SOLVENT-SEPARATED ION-PAIRS IN ETHEREAL SOLVENTS

Results of a spectrophotometric study of (triphenylmethyl)lithium (TPM-Li+) and (triphenylmethyl)potassium (TPM-Li+) in dimethoxyethane (DME), tetrahydrofuran (THF), and diethyl ether are reported. The UV-vis spectral absorptions show characteristic dependence on the nature of the cation and the solvent, which can be interpreted on the basis of formation of contact and solvent-separated ion pair species. Addition of 18-crown-6 polyether causes a shift in the equilibria, from contact to crown ether complexed ion pair species having spectral characteristics closely similar to the solvent-separated ion pair. The temperature variation of the spectra for the (TPM-Li+)/ Et2O, (TPM-Li+)/THF, (TPM-Li+)/THF, and (TPM-Li+)/DME systems has allowed evaluation of the thermodynamic parameters for the contact ⇄ solvent-separated ion pair equilibria in these systems. The overall results compare satisfactorily with data obtained by means of a 1H NMR study of triphenylmethyl alkali metal salts in ethereal solvents. © 1979, American Chemical Society. All rights reserved.