HYDRIDE AFFINITIES OF CARBENIUM IONS IN ACETONITRILE AND DIMETHYL-SULFOXIDE SOLUTION

Hydride affinities of organic cations in solution, defined as the free energy change for the reaction (R+)s + (H-)s reversible (RH)s, where the subscript s refers to the solvent, can be determined using either of two equations (eqs 5 and 7) derived from thermochemical cycles. The degree of uncertainty in DELTAG(hydride)(R+)s depends primarily upon the errors associated with the determination of the reversible electrode potentials for the (R./R-) and (R+/R.) couples. In this study, DELTAG(hydride)(R+)s values have been derived for a number of stable carbenium ions in dimethyl sulfoxide solution applying eq 7 with reversible electrode potentials. Values with a higher degree of uncertainty were also obtained from ring-substituted toluenes, 9-substituted fluorenes, and alpha- or 10-substituted 9-methylanthracenes, employing irreversible electrode potentials. The fact that the potentials for the (R./R-) and (R+/R.) are summed in eq 7 gives rise to a partial cancellation of errors due to kinetic shifts of the experimental potentials from the reversible values. Structural effects on free energies of hydride transfer are discussed, comparisons with available gas-phase values are made, and ionic solvation energies are calculated in some cases.