Effects of Ion Pair Association on the Barrier to Hindered Internal Rotation in Nitrobenzophenone and Dinitrobenzophenone Radical Anions

Radical anions formed by benzophenone (BF), 4,4′-dinitrobenzophenone (DNBF), and 4-nitrobenzophenone (NBF) and ion pairs formed by these compounds with alkali metals show hindered rotation of the two rings on the ESR time scale in the temperature range explored. Activation parameters for the rotation have been obtained. For NBF, ion pair formation lowers the enthalpy barriers to internal rotation, in agreement with the results from INDO UHF calculations. Furthermore, the barrier height depends significantly on the strength of ionic association. In every case the ring rotational barrier is larger than the activation energy of the viscosity of the system studied, whose value is generally close to that of the pure solvent, as expected for very dilute solutions in the absence of strong solute-solvent interactions.