Iionic I 1 PHOTODI SSOClATlON and the substitution takes place via an ion pair as the intermediate analogous to other systems, studied by McCullough, I2 Arr~ old,'~ and Lewis, I4 one should expect a dependence of the mode selectivity on the electron transfer enthalpies AG. These AG values can be easily calculated by means of the Rehm-Weller eq~ ation.'~ A simplified version of this equation for 1, 4-dioxane (dielectric constant, c 2.2) as solvent is ...