Free radical scavenger capacity of 1,2,5-trihydroxyanthraquinone and 1,2,5-trihydroxythioxanthone: a theoretical comparative study

In this contribution are estimated and compared free radical scavenger capacity of 1,2,5- trihydroxyanthraquinone (AN) and 1,2,5-trihydroxythioxanthone (TX). For this purpose, $\mathbf{M06}-\mathbf{2X/6}-311++\mathbf{G}(\mathbf{d, p})$ method is used. Scavenger capacities of both molecules are determined in benzene and water. It is found that both antioxidants generate stable radicals in water following SPLET mechanism. On the other hand, the most plausible mechanism for that purpose in benzene is HAT. In the presence of three selected free radicals $(\mathbf{HO}^{\bullet},\mathbf{HOO}^{\bullet}\mathbf{and}\ \mathbf{CH_{3}OO}^{\bullet})$ these molecules manifest their scavenger capacity following HAT and SPLET competitively in both estimated environment conditions. The reactivity of observed molecules toward free radicals decreases following the order: $\mathbf{HO}^{\bullet}\gg\mathbf{HOO}^{\bullet}>\mathbf{CH_{3}OO}^{\bullet}$. Comparing thermodynamic parameters that describe homolytic O-H cleavage for estimated antioxidants, it is concluded that TX shows somewhat higher scavenger capacity.