Reactions of phloroglucinols with radical species, a theoretical study in different media

Abstract

A theoretical study on the reactions of phloroglucinol (FG) and phloroacetophenone (THAP) with •OH and •OOH has been performed through hydrogen atom transfer (HAT), single electron transfer-proton transfer (SET-PT), sequential proton-loss electron-transfer (SPLET) and the oxidation mechanisms. The aim of the investigation has been to determine the preferred reaction mechanism relating to the radical scavenging activity of the compounds. The objectives of the study have been to determine the reaction enthalpies for the HAT, SET-PT and SPLET mechanisms, geometric, electronic, energetic and kinetic properties for the HAT and oxidation mechanisms. The DFT/M06•2X, DFT/BHHLYP and DFT/MPW1K methods have been utilised in conjunction with either the standard 6-31++G(d,p) basis set or the extended 6-311++G(3df,2p) basis set. The selected DFT functionals have been benchmarked using the CBS-QB3 compound method for their ability to estimate barrier heights. The study has been performed in vacuo, in benzene and in ethanol media. Analysis of the reaction enthalpies suggests that the preferred mechanism is the HAT mechanism; reactions involving the studied compounds with •OH are exothermic in nature while reactions involving •OOH are slightly endothermic in nature. THAP has a higher radical scavenging ability than FG; this result is in agreement with the experimental findings. The preferred reactive site of THAP for the abstraction of the free phenolic H atom is the ortho position. The direct hydrogen abstraction mechanism provides the smallest branching ratio with respect to •OH addition mechanism, indicating that hydrogen atom transfer mechanism occurs largely through the addition mechanism. More importantly, the phenoxyl radical, forming through the addition-eliminination mechanism, prefers to form under basic conditions. The oxidation mechanism, resulting in tetrahydroxybenzene for reactions involving FG and THAP with •OH, prefers to occur under neutral conditions in the presence O2. The reactions involving phloroglucinols and •OOH largely occur through direct hydrogen abstraction mechanism, forming a phenoxyl radical and hydrogen peroxide. The spin density and branching ratio values indicate that the most reactive site for the THAP + •OOH reaction is the ortho position with respect to the substituted acyl chain. Reactions performed in polar medium are more kinetically preferred than those performed in vacuo and in non-polar media. The DFT/M06 •2X method provides barrier heights which are closer to the barrier heights determined using the CBS-QB3 method.