On the Electrophilicity of Hydroxyl Radical:  A Laser Flash Photolysis and Computational Study

• Published in: Journal of the American Chemical Society Vol. 127; no. 19; pp. 7094 - 7109
• Main Authors: DeMatteo, Matthew P, Poole, James S, Shi, Xiaofeng, Sachdeva, Rakesh, Hatcher, Patrick G, Hadad, Christopher M, Platz, Matthew S
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 18.05.2005
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Reaction intermediate; Monte Carlo method; Nitrogen heterocycle; Inorganic free radical; Theoretical study; Activation parameter; Flame ionization detector; Experimental study; Gas chromatography; Near ultraviolet radiation; Laser beam; Six membered ring; Thermodynamic parameter; Density functional method; Kinetic parameter; Rate constant; Flash photolysis; Ultraviolet visible spectrometry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja043692q

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.