A linear free energy relationship for an equilibrium isotope effect

The solution electron affinities of a series of monosubstituted phenyl-d 5 benzenes relative to their perprotiated analogues were measured via electron paramagnetic resonance studies. The equilibrium constant for the electron transfer X-C 6 H 5 .− +X-C 6 D 5 ⇔X-C 6 H 5 +X-C 6 D 5 .− , where X=H, C 6 D 5 , CN, NO 2 , OCH 3 , 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 sysrems that have an electron-donating group as the substituent