title: Structural and energetic aspects of the protonation of phenol, catechol, resorcinol, and hydroquinone.
abstract: The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol.
authors.affiliation: Département de Chimie Laboratoire des Mécanismes Réactionnels Ecole Polytechnique 91128 Palaiseau, France. bouchoux@dcmr.polytechnique.fr
authors.forename: Guy
authors.initials: G
authors.lastname: Bouchoux
authors.forename: Dirk
authors.initials: D
authors.lastname: Defaye
authors.forename: Terrance
authors.initials: T
authors.lastname: McMahon
authors.forename: Alexander
authors.initials: A
authors.lastname: Likholyot
authors.forename: Otilia
authors.initials: O
authors.lastname: Mó
authors.forename: Manuel
authors.initials: M
authors.lastname: Yáñez
mesh_terms: D001665:Binding Sites; D002396:Catechols / Q000737:chemistry; D006873:Hydroquinones / Q000737:chemistry; D008958:Models, Molecular; D019800:Phenol / Q000737:chemistry; D010636:Phenols / Q000737:chemistry*; D011522:Protons; D012118:Resorcinols / Q000737:chemistry
chemical_list: D002396:Catechols; D006873:Hydroquinones; D010636:Phenols; D011522:Protons; D012118:Resorcinols; D019800:Phenol; C034221:catechol; C031927:hydroquinone; C031389:resorcinol