Moderately Strong Phenols Dissociate by Forming an Ion-Pair Kinetic Intermediate

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2013-12, Vol.117 (51), p.13976-13987
• Main Authors: Iftimie, Radu, Tremblay, Marie-Hélène, Thomas, Vibin, Hétu, Sebastien, de Lasalle, Francis, Rivard, Ugo
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Bonding; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; Fluctuation; Forming; Hydrogen-Ion Concentration; Ions; Kinetics; Molecular Dynamics Simulation; Oxygen - chemistry; Oxygen atoms; Phenols; Phenols - chemistry; Physical chemistry; Physics; Protons; Solvation; Temperature; Time; Water - chemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp410858d

We show computational evidence that ground-state moderately strong hydroxyarenes (Ar–OH, pK a ∼ 0) dissociate by forming an ion-pair intermediate that lives for 3–5 ps. The concentration of this intermediate is approximately 2 times smaller than that of the un-ionized acid at pH ∼ 0.6 and is characterized by average C–O bond lengths (1.30 Å) that are intermediate between those of un-ionized (1.29 Å) and fully dissociated (1.34 Å) species. During the lifetime of the ion-pair intermediate the excess proton fluctuates between the oxygen atom of the phenolic moiety and those of water molecules in the first and second solvation shells on a subpicosecond time scale (∼100–300 fs).