The standard one-electron reduction
potentials of halogen atoms, E°′(X•/–), and many
other radical or unstable species, are not accessible through standard
electrochemical methods. Here, we report the use of two Ir(III) photoredox
catalysts to initiate chloride, bromide, and iodide oxidation in organic
solvents. The kinetic rate constants were critically analyzed through
a derived diffusional model with Marcus theory to estimate E°′(X•/–) in propylene
carbonate, acetonitrile, butyronitrile, and dichloromethane. The approximations
commonly used to determine diffusional rate constants in water gave
rise to serious disagreements with the experiment, particularly in
high-ionic-strength dichloromethane solutions, indicating the need
to utilize the exact Debye expression. The Fuoss equation was adequate
for determining photocatalyst–halide association constants
with photocatalysts that possessed +2, +1, and 0 ionic charges. Similarly,
the work term contribution in the classical Rehm–Weller expression,
necessary for E°′(X•/–) determination, accounted remarkably well for the stabilization
of the charged reactants as the solution ionic strength was increased.
While a sensitivity analysis indicated that the extracted reduction
potentials were all within experimental error the same, use of fixed
parameters established for aqueous solution provided the periodic
trend expected, E°′(I•/–)