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Wavelength dependent mechanism of phenolate photooxidation in aqueous solution.

ABSTRACT: Phenolate photooxidation is integral to a range of biological processes, yet the mechanism of electron ejection has been disputed. Here, we combine femtosecond transient absorption spectroscopy, liquid-microjet photoelectron spectroscopy and high-level quantum chemistry calculations to investigate the photooxidation dynamics of aqueous phenolate following excitation at a range of wavelengths, from the onset of the S₀-S₁ absorption band to the peak of the S₀-S₂ band. We find that for λ ≥ 266 nm, electron ejection occurs from the S₁ state into the continuum associated with the contact pair in which the PhO˙ radical is in its ground electronic state. In contrast, we find that for λ ≤ 257 nm, electron ejection also occurs into continua associated with contact pairs containing electronically excited PhO˙ radicals and that these contact pairs have faster recombination times than those containing PhO˙ radicals in their ground electronic state.

SUBMITTER: Robertson K

PROVIDER: S-EPMC10034221 | biostudies-literature | 2023 Mar

REPOSITORIES: biostudies-literature

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Wavelength dependent mechanism of phenolate photooxidation in aqueous solution.

Robertson Kate K Fortune William G WG Davies Julia A JA Boichenko Anton N AN Scholz Michael S MS Tau Omri O Bochenkova Anastasia V AV Fielding Helen H HH

Chemical science 20230220 12

Phenolate photooxidation is integral to a range of biological processes, yet the mechanism of electron ejection has been disputed. Here, we combine femtosecond transient absorption spectroscopy, liquid-microjet photoelectron spectroscopy and high-level quantum chemistry calculations to investigate the photooxidation dynamics of aqueous phenolate following excitation at a range of wavelengths, from the onset of the S0-S1 absorption band to the peak of the S0-S<sub ...[more]

PMID: 36970081

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Project description:The bromine-selenite reaction at strongly acidic conditions was investigated by monitoring the absorbance-time traces at the isosbestic point of the bromine-tribromide system at a constant ionic strength (0.5 M adjusted by sodium perchlorate) and temperature. Despite the simplicity of the stoichiometry, the kinetics was found to be very complex. Although the formal kinetic orders of the reactants bromine and selenite are strictly 1, that of the hydrogen ion varies from -2 to less than -3 and notably depends on the initial bromide concentration as well. The bromide ion also inhibits the reaction, making the whole system as a sound example of efficient autoinhibition. We have clearly shown that the inhibitory effect of the bromide ion cannot be explained quantitatively by either exclusively considering the unreactivity of the tribromide ion over elemental bromine or driving the reaction via hypobromous acid formed from the well-known hydrolysis of bromine in aqueous solutions. Instead of that, bromonium ion transfer initiating equilibrium is suggested between the selenium(IV) and bromine species to produce bromide ion and SeO3Br- followed by the hydrolysis of this short-lived intermediate. This hydrolytic transformation was found to be catalytic with respect to hydroxide and bromide ions as well. We have also demonstrated that, among the wide variety of selenium species present in the acidic aqueous solution, the best result can be obtained by considering HSeO3 - as the kinetically active species toward bromine. The proposed mechanism containing 10 acid-base equilibria with known equilibrium constants, the above-mentioned initiating equilibrium, and the hydrolysis of SeO3Br- is able to fit all 49 kinetic absorbance-traces simultaneously, taking into account properly the most important characteristics of the measured data at strongly acidic conditions. Furthermore, this kinetic model was further extended by the direct reactions of hypobromous acid with selenium(IV) species suggested previously with reasonably modified rate coefficients to describe the pH dependence of the apparent second-order rate coefficients over the pH = 1-13 range, providing a useful tool to predict more accurately the kinetic behavior of selenium(IV) species in water treatment process conditions.

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Wavelength dependent mechanism of phenolate photooxidation in aqueous solution.

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Wavelength dependent mechanism of phenolate photooxidation in aqueous solution.

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