Effects of Hydrogen Bonding to Amines on the Phenol/Phenoxyl Radical Oxidation

Abstract

Theoretical calculations were performed to study the effects of hydrogen bonding to various amines on the oxidation of phenol to phenoxyl radical. It was found that with ammonia as the hydrogen bond acceptor the phenol oxidation process was a barrierless proton-coupled electron transfer and the shift of the adiabatic phenol oxidation potential by ammonia in the gas phase was as large as about 1 eV. For other amines, it was found that depending on the basicity of the amine, the effects of hydrogen bonding to different amines on the phenol oxidation varied. For those amines that had a proton affinity larger than ca. 204 kcal/mol, the oxidation of the phenol−amine complexes caused a proton transfer and the proton-transferred structure was the only minimum found on the potential surface after oxidation. When the proton affinity of the amine was located in the range of ca. 190−197 kcal/mol, both the proton-transferred and nonproton-transferred structures were found to be minima for the oxidized complex. However, when the proton affinity of the amine was smaller than ca. 189 kcal/mol, no proton transfer occurred in the oxidation. The shift of the adiabatic oxidation potential was found to be roughly in linear correlation with the proton affinity of the amine. For substituted aminoacetylenes, the shift of the adiabatic oxidation potential was also found to be in linear correlation with the Hammett σ p substituent constants. Finally, it was found that the phenol−imidazole−formate complex was not a good model for the tyrosine oxidation in photosystem II, because this complex had a too low oxidation potential. In fact, because of the strong electron donating effects of formate, in addition to phenol the imidazole moiety in the complex could also be oxidized, which was not observed in the enzymatic systems. Therefore, the Glu189 residue in photosystem II was proposed to be protonated under the physiological condition.

Computational Investigation of 1,3‐H and 1,5‐H Shifts in Isomerization of Enol Acetate of 2‐Aceto‐1,3‐cyclohexanedione

“…However, the situation under trimethylamine is different, the proton transfer is facilitated by the following mechanism, The shift H is firstly abstracted from methyl by N of trimethylamine molecule, and then shifted to carbonyl oxygen. The transition state structure is stabilized by a hydrogen bonded ion-pair 19 consisting of the protonated trimethylamine and deprotoned reactant, in which the shift H combines with N of trimethylamine not by H-bond but a single bond of 0.10560 mm, leading to the lower energy barrier. It has been verified that the unique imaginary vibrational frequency is linked to a proton transfer from the carbon toward the carbonyl oxygen in the transition state of ion-pair.…”

Synthesis and Cyclic Voltammetry Studies of 3,4-Methylenedioxymethamphetamine (MDMA) Human Metabolites

“…Probably, the most obvious reason for this event may be the difference in the pK a of 4, a primary amine, versus 3, a secondary amine, bearing higher basicity. 41) It's known from literature 42,43) that OH-N hydrogen bonding can have a profound effect in altering the oxidation potential of enols to lower values. At the pH of 7.4 used for the cyclic voltammetry experiments, 3 should exist preferentially as its hydrochloride salt and 4, partially in its free amine form, could be involved in intermolecular hydrogen bonding responsible for lowering the E p .…”

Correlation analysis of quantum chemical data and the polarizability effect in H-complexes

“…Besides, the σ α constants are normalized to the induc tive (σ I ) and resonance (σ R , σ R + , σ R -) constants of substitu ents X. This allows linear free energy relationships (LFER) and the assumption of independence and additivity of the effects of substituents X (for more detail, see, e.g., Refs 5 and 16) to be 10 (series V) 10 …”