Exploring the transfer of hydrogen atom from kaempferol-based compounds to hydroxyl radical at ground state using PCM-DFT approach

Thermodynamic and kinetic studies of the hydrogen atom transfer (HAT) from hydroxyl (OH) groups of four kaempferol-based compounds, namely kaempferol, morin, morin-5^*-sulfonate and morin-7-O-sulfate to hydroxyl radical (·OH), have been carried out using density functional theory (DFT) methods at the CAM-B3LYP/6–311++G(d,p) level equipped with polarizable continuum model (PCM) of solvation. All HAT reactions in aqueous solution are exothermic and spontaneous. For most compounds, the most preferable OH group for HAT is situated at position C3 (O3-H3) on the pyrone ring. The reaction potential of such a reactive group is found to be highest in morin-7-O-sulfate. The rate constants for the HAT reactions at different OH groups of each compound have been determined based on the transition state theory. The presence of substituents leads to the variation in either the characteristic interactions at the reactive site or the charge distribution on transition-state geometries, hence significantly affecting the kinetics of HAT. The highest rate of HAT is resulted for the OH group at position C4^* (O4^*-H4^*) on the phenyl ring (ring B) of morin-5^*-sulfonate because a hydrogen bond between ·OH and the sulfonate group favors the formation of transition state. However, for most compounds under study, the HAT reaction at O3-H3 initiated by ·OH is highly favorable both thermodynamically and kinetically.