Structural changes in concentrated solutions of alkali metal monosubstituted benzophenone radical anions as deduced from ¹H NMR measurements

Abstract

The radical anions of monosubstituted benzophenones (including 2-, 3-and 4-methyl-, 4-tert-butyl-and 4-phenylbenzophenone) with lithium, sodium and potassium as countercations in tetrahydrofuran (THF) were studied by paramagnetic solvent 1 H NMR shift measurements in the concentration range ca 0.1-1.0 M. In addition to the cation effects, marked positional isomer and substituent effects were found for (1) molar paramagnetism, (2) magnetic state, (3) solubility, (4) structural stability with respect to concentration or addition of cosolvents and (5) rapidity of attainment of their equilibrium structure. The major conclusions which can be drawn are (a) the solvation behavior of the countercation is governed by the anion, (b) an alkyl group in the 4-position makes the structure of the lithium and sodium monosubstituted benzophenone radical anions concentration dependent and (c) in most cases, alkali metal aromatic ketone radical anions generated by electron transfer from the corresponding naphthalene radical anion attain their equilibrium structure rapidly. The data are interpreted on the basis of the involvement of spin-triplet interactions within 'dimers' or higher clusters.