title: Contribution of the intramolecular hydrogen bond to the shift of the pKa value and the oxidation potential of phenols and phenolate anions.
abstract: Intramolecularly OHO[double bond, length as m-dash]C hydrogen bonded phenols, 2-HO-C6H2-3,5-(t-Bu)2-CONH-t-Bu (1-OH), 2-HO-C6H2-5-t-Bu-1,3-(CONH-t-Bu)2 (2-OH) and 2-HO-C6H2-3,5-(t-Bu)2-NHCO-t-Bu (4-OH), were synthesized and their phenolate anions were prepared as tetraethylammonium salts (-1O-(NEt4+), 2-O-(NEt4+) and 4-O-(NEt4+)) with intramolecular NHO(oxyanion) hydrogen bonds. 4-HO-C(6)H(2)-3,5-t-Bu(2)-CONH-t-Bu (3-OH) and its phenolate anion, 3-O-(NEt4+), were synthesized as non-hydrogen bonded references. The presence of intramolecular hydrogen bonds was established through the crystallographic analysis and/or (1)H NMR spectroscopic results. Intramolecular NHO(phenol) hydrogen bonds shift the pK(a) of the phenol to a more acidic value. The results of cyclic voltammetry show that the intramolecular OH...O=C hydrogen bond negatively shifts the oxidation potential of the phenol. In contrast, the intramolecular NHO(oxyanion) hydrogen bond positively shifts the oxidation potential of the phenolate anion, preventing oxidation. These contributions of the hydrogen bond to the pKa value and the oxidation potentials probably play an important role in the formation of a tyrosyl radical in photosystem II.
authors.affiliation: Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Kanamori
authors.forename: Atsushi
authors.initials: A
authors.lastname: Furukawa
authors.forename: Taka-aki
authors.initials: TA
authors.lastname: Okamura
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Yamamoto
authors.forename: Norikazu
authors.initials: N
authors.lastname: Ueyama
mesh_terms: D000838:Anions / Q000737:chemistry; D018360:Crystallography, X-Ray; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D010636:Phenols / Q000737:chemistry*
chemical_list: D000838:Anions; D010636:Phenols; D006859:Hydrogen; D009584:Nitrogen; D010100:Oxygen