Phenol derivatives are distinguished as successful free radical scavengers. We present a detailed analysis of hydroxyl hydrogen abstraction from hydroquinone by hydroxyl and hydroperoxyl radical with emphasis on changes that take place in the vicinity of the transition state. Quantum theory of atoms in molecules is employed to elucidate the sequence of positive and negative charge transfer by studying selected properties of the three key atoms (the transferring hydrogen, the donor atom, and the acceptor atom) along intrinsic reaction path. The presented results imply that in both reactions, which are examples of proton coupled electron transfer, proton, and electron get simultaneously transferred to the radical oxygen atom. The fact that the hydrogen's charge and volume do not monotonously change in the vicinity of the transition state in the product valley results from the adjacency of the proton and the electron to the donor and the acceptor oxygen atoms. Obtaining a detailed understanding of mechanisms by which free radicals are disarmed is of paramount importance given the effects of those highly reactive species on biological systems. A comprehensive analysis of hydroxyl hydrogen abstraction from hydroquinone by hydroxyl and hydroperoxyl radicals, based on changes of selected electronic properties of the three most relevant atoms (hydrogen donor, hydrogen acceptor, and the hydrogen itself), along the reaction coordinate, can be obtained by first-principles calculations. K E Y W O R D S density functional theory, free radical scavenging, hydroquinone, proton coupled electron transfer, quantum theory of atoms in molecules 1 | I N TR ODU C TI ONDue to their ability to cause oxidative stress in living organisms, free radicals are held responsible for aging process. Although certain natural products have antioxidative properties, scientists still search for efficient tools to trap those extremely reactive species. Phenol derivatives are wellknown antioxidants [1,2] that terminate radical chain reactions by donating a hydroxyl hydrogen atom. Hydrogen atom could also be abstracted from either of phenolic CAH bonds, although such processes are characterized with higher energy barrier due to lower acidity of the transferring atom.In addition to hydrogen transfer, free radicals can be blocked through formation of an adduct with the scavenger.Hydrogen transfer can, in principle, proceed via several reaction mechanisms [3][4][5][6] due to the fact that hydrogen atom consists of two entities-a proton and an electron. They can be transferred either simultaneously or separately, and the process might also involve solvent molecules. All those mechanisms are known as formal hydrogen transfer (FHT). [6] (i) If hydrogen transfer takes place in a single elementary step, it can be either hydrogen atom transfer (HAT) when the proton and the electron are transferred from one species to another as one entity, or proton-coupled electron transfer (PCET) if the proton and the electron travel separately between different parts of molec...
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