DFT/B3LYP study of the substituent effects on the reaction enthalpies of the antioxidant mechanisms of Indole-3-Carbinol derivatives in the gas-phase and water

In this paper the reaction enthalpies of three antioxidant action mechanisms, HAT, SET-PT and SPLET, for mono-substituted Indole-3-Carbinols were calculated in gas-phase and water. Results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP) and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In comparison to gas-phase, water attenuates the substituent effect on all reaction enthalpies. Results show that IP and BDE values can be successfully correlated with the indolic NH bond length after electron abstraction, R(NH+), and the partial charge on the indolyl radical nitrogen atom, q(N). Furthermore, calculated IP and PA values for mono-substituted Indole-3-Carbinols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO) of studied molecules in the two environments. SPLET represents the thermodynamically preferred mechanism in water.