Dynamic NMR study of rotational isomerism in the carbanion of <TOGGLE>p</TOGGLE>-trifluoromethanesulfonylbenzyltriflone in aprotic solvents

Rotational isomerism in the conjugate carbanion of p-trifluoromethanesulfonylbenzyltriflone was studied by dynamic NMR spectroscopy (¹H, ¹³C). Treatment of the experimental spectra affords the activation parameters of the processes. The activation enthalpies (~48 kJ mol⁻¹) for the rotational barrier around the C<INF>Ar</INF>—C<INF>α</INF> carbon– carbon bond are indicative of a relatively weak π-order of this bond and are nearly identical in the three dipolar aprotic solvents studied, namely acetonitrile, dimethyl sulfoxide and acetone. Changes in the rates of the conformation exchange around the C<INF>Ar</INF>—C<INF>α</INF> bond are mainly a reflection of the solvent dependence of the entropy change (from almost 0 in acetonitrile, to −8 in DMSO and −13 J K⁻¹ mol⁻¹ in acetone). These data are discussed in relation to information on the electronic density distribution, as obtained from an analysis of the ¹³C chemical shift of the anion of the benzyltriflone. Copyright © 2000 John Wiley & Sons, Ltd.