Chemical Substitution Effect on Energetic and Structural Differences between Ground and First Electronically Excited States of Thiophenoxyl Radicals

Effect of chemical substitution at the para-position of the thiophenoxyl radical has been theoretically investigated in terms of energetics, structures, charge densities and orbital shapes for the ground and first electronically excited states. It is found that the adiabatic energy gap increases when CH3 or F is substituted at the para-position. This change is attributed to the stabilization of the ground state of thiophenoxyl radical through the electron-donating effect of F or CH3 group as the charge or spin of the singly-occupied molecular orbital is delocalized over the entire molecule especially in the ground state whereas in the excited state it is rather localized on sulfur and little affected by chemical substitutions. Quantitative comparison of predictions based on four different quantum-mechanical calculation methods is presented.