ELECTRODE-POTENTIALS AND THE THERMODYNAMICS OF ISODESMIC REACTIONS

The free energies of isodesmic reactions can be calculated from appropriate electrode potential differences without the necessity of evaluating reference electrode potentials or the free energies of any chemical or physical processes. Class I reactions involve different oxidation states of the same species while reactions involving different species at the same oxidation state level are grouped in Class II. The free energy change of H. exchange (i) above the double line is equal to that for electron transfer (ii) below the double line. The first aryl group appearing on both sides of the equations is designated with an asterisk [GRAPHICS] for clarity, i.e. the compositions of Ar* and Ar are identical. Similar equations relate differences in acid dissociation constants to an appropriate electrode potential difference. For class II reactions, an electrode potential difference can be related to the sum of the free energies of two isodesmic reactions involving the transfer of atoms, ions, or groups. The Cr-H bond dissociation energies of chromium hydride complexes which have recently been evaluated experimentally were selected as standard processes for the evaluation of other BDE. The reversible oxidation of the 2,6-di-tert-butyl-4-nitrophenoxide ion was used to relate thermochemical quantities determined in acetonitrile to those in dimethyl sulfoxide.