ENTHALPIES OF CLEAVAGE FOR A SERIES OF RADICAL CATIONS AND RADICAL-ANIONS - COMPARISON WITH HEATS OF HOMOLYSIS AND HETEROLYSIS OF THEIR NEUTRAL PRECURSORS

Enthalpies of cleavage (DELTA-H(cleav)) of the derived radical ions were determined at 25-degrees-C in 95% sulfolane/5% 3-methylsulfolane by carrying out electrochemical oxidation and reduction of neutral molecules prepared by the reactions of a series of 9-substituted fluorenyl anion salts with four resonance-stabilized carbenium ion salts. Correlations are attempted between these results and other previously determined thermodynamic properties. Excellent correlations were obtained between DELTA-H(cleav) for both types of radical ions and the heats of homolysis of their neutral precursors. Only scatter diagrams resulted from plots of the DELTA-H (cleav)s versus the corresponding heats of heterolysis for the same neutral precursors. This behavior follows a previously observed pattern that, for these compounds (and perhaps more generally), properties which involve the gain or loss of charge correlate with each other but not with properties involving no change in charge. DELTA-H(cleav)s for radical cations are strongly exothermic, implying a low barrier for rupture of the sessile bond. In every case, the slope for DELTA-H(cleav) of the radical cations versus DELTA-H(homo) of their precursors was positive and less than unity while those of corresponding plots for the radical anions were negative and close to one. The results presented here provide an unprecedented comparison of bond cleavage energies for radical cations, radical anions, heats of homolysis and heterolysis of directly comparable neutral structures, and also redox properties of the component ions for which pK(a) and pK(R)+ values are well established. The acidities of the radical cations derived from fluorenes were also determined from their pK(a)s in DMSO, their oxidation potentials, and those of their conjugate bases, viz. fluorenide ions. Three different electrochemical methods, cyclic voltammetry (CV), second harmonic alternating current voltammetry (SHACV), and Osteryoung square wave voltammetry (OSWV), were used to determine the redox potentials of the radical ionic species. Despite the time scales and apparent degrees of reversibility of these techniques, there was excellent agreement among the redox potentials obtained by these different methods.