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

Abstract

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.

Theoretical investigation on the kinetics and mechanisms of hydroxyl radical-induced transformation of parabens and its consequences for toxicity: Influence of alkyl-chain length

“…Thus, the hydroxylation of the aromatic ring of parabens via OH is expected to occur in ortho to phenolic OH group of parabens, namely, meta to the ester group of parabens. This preference result is in agreement with previous data obtained in similar system DeMatteo et al, 2005;Tay et al, 2010b). However, there is a slight different with the experimental research that the hydroxylation on the ortho to the ester group (Daghrir et al, 2014 Based on the discussion above, the single-electron transfer route (R set ), two OH-addition routes (R add 3 and R add 7) and four benzene-H-abstraction routes (Rabs1,2,4,5) could be ruled out from the degradation pathways of corresponding parabens in OH-based processes because of the endothermic reactions and high energy barriers.…”

Unusual Stability of Polycyclic Aromatic Hydrocarbon Radical Cations in Amorphous Water Ices up to 120 K: Astronomical Implications

“…Nonpolar species such as CO and N 2 readily rearrange in, and escape through, the amorphous ice pores at much lower temperatures as it is warmed from 10 to 20 K to the crystallization point (Bar-Nun et al 1985Laufer et al 1987;Allamandola et al 1988). The special interactions possible between aromatic species with water molecules and subsequent changes in chemistry and molecular physics of these species is now a topic of interest in chemical physics (e.g., DeMatteo et al 2005;Ibrahim et al 2005;Woon & Park 2004) and will influence our understanding of the chemistry and physics of cosmic ices.…”

Kinetic studies of the reaction between pesticides and hydroxyl radical generated by laser flash photolysis

“…Systematic photophysical studies such as LFP can be employed to establish the role of the hydroxyl radical in the degradation of pesticides under various oxidative conditions . Although hydroxyl radicals do not absorb in the visible region (300–700 nm) and cannot be directly detected by conventional LFP, they can be detected indirectly via their reaction with a compound that generates a spectroscopically detectable intermediate . A suitable system for the production of hydroxyl radicals by LFP is the photolysis MNO using a 355 nm Nd‐YAG laser pulse, which also produces the pyrithiyl radical absorbing at 490 nm [Eqn ]: …”

“…In the presence of trans ‐stilbene (TS), the hydroxyl radical adds to TS with an absolute second‐order rate constant of 6.1 ± 0.2 × 10 9 M −1 s −1 at 298 K in acetonitrile to generate an adduct that absorbs at 390 nm [Eqn ]: …”

“…LFP is a technique used to study transient species generated by an intense, several nanosecond‐long light pulse from a pulsed laser source, monitoring the decay and spectra of these species by absorption spectroscopy. A particularly convenient LFP method for studying the reactivity of HO • is to generate HO • by direct photolysis of 2‐mercaptopyridine‐ N ‐oxide in acetonitrile and use trans ‐stilbene to capture HO • , which results in the formation of an adduct with a characteristic absorption peak at 390 nm . Advantages of this method are that it avoids the use of hydrogen peroxide as a radical promoter, does not generate other interfering radicals and allows indirect monitoring of the generation of HO • by the concomitant formation of the readily detectable pyridinethiyl radical at 490 nm .…”