A LINEAR FREE-ENERGY RELATIONSHIP FOR AN EQUILIBRIUM ISOTOPE EFFECT

The solution electron affinities of a series of monosubstituted phenyl-d5 benzenes relative to their perprotiated analogues were measured via electron paramagnetic resonance studies. The equilibrium constant for the electron transfer X-C6H5.- + X-C6D5 half-arrow-right-over-half-arrow-left X-C6H5 + X-C6D5.-, where X = H, C6D5, CN, NO2, OCH3, tert-butyl, was found to be less than unity for all cases. However, this equilibrium constant is closer to unity for those systems where the substituent is electron withdrawing in nature, and it deviates further from unity for those systems that have an electron-donating group as the substituent. A linear free energy relationship was found, and the positive rho value (0.34) is presumably due to the fact that there is a smaller excess charge and spin density from the antibonding electron in the area of the molecule where isotopic substitution takes place, for those systems where the sigma value of the substituent is positive.