Non-Singlet Oxygen Kinetic Solvent Isotope Effects in Aquatic Photochemistry.

Caroline A Davis,Elisabeth M.-L Janssen,Kristopher Mcneill

doi:10.1021/acs.est.8b01512

Abstract

The kinetic solvent isotope effect (KSIE) is typically utilized in environmental photochemistry to elucidate whether a compound is susceptible to photooxidation by singlet oxygen (1O2), due to its known difference in lifetime in water (H2O) versus heavy water (D2O). Here, the overall indirect photodegradation rates of diarylamines in the presence of dissolved organic matter (DOM) were enhanced in D2O to a greater extent than expected based on their reactivity with 1O2. For each diarylamine, the relative contribution of reaction with 1O2 to the observed KSIE was determined from high resolution data of 1O2 lifetimes by time-resolved infrared luminescence spectroscopy. The additional enhancement in D2O beyond reaction with 1O2 contributed significantly to the observed KSIE for diarylamines (8-65%) and diclofenac (100%). The enhancement was ascribed to slower reduction of transient radical species of the diarylamines due to H/D exchange at DOM's phenolic antioxidant moieties. A slower second-order reaction rate constant with a model antioxidant was verified for mefenamic acid radicals using transient absorption spectroscopy. Changes in lifetime and reactivity with triplet sensitizers were not responsible for the additional KSIE. Other pollutants with quenchable radical intermediates may also be susceptible to such an additional KSIE, which has to be considered when using the KSIE as a diagnostic tool.

Highlights

Singlet oxygen (1O2) is a reactive oxygen species present in sunlit surface waters at steady-state concentrations ranging from 10−12 to 10−14 M.1−4 1O2 is formed through an energy transfer reaction between ground state oxygen (3O2) and photochemically produced excited triplet states of ubiquitous chromophoric dissolved organic matter (CDOM)
Oxidation of several pollutants proceeds through radical intermediates, as observed for direct photochemical ionization reaction of tryptophan[29] and triplet sensitizer-mediated oxidation of anilines,[30−32] sulfonamide antibiotics,[33] beta blockers,[18] and various pesticides, including chloroacetamide and phenylurea herbicides[34] and diarylamines[35] that we focus on
We showed previously that diarylamines react with triplet state CDOM, and their indirect photochemistry proceeds through a radical intermediate that is quenchable by antioxidants.[35]

Summary

■ INTRODUCTION

Singlet oxygen (1O2) is a reactive oxygen species present in sunlit surface waters at steady-state concentrations ranging from 10−12 to 10−14 M.1−4 1O2 is formed through an energy transfer reaction between ground state oxygen (3O2) and photochemically produced excited triplet states of ubiquitous chromophoric dissolved organic matter (CDOM). When the triplet mediated oxidation involves hydrogen abstraction from the compound of interest, an increase in the bond strength for isotopologues with deuterium substitution results in higher dissociation energies and potentially slower reactivity.[26−28] thermally induced reorganization of the solvent for outer sphere electron transfers can depend on solvent polarization, which determines the free activation energy, and thereby the reaction rate Another reaction that could exhibit a KSIE is the reduction of radical intermediates. Of perinaphthenone and lumichrome showed a linear response with laser power from 1 to 3 μJ and 1−4 μJ, respectively (Figure S7)

Quenching of the radical intermediate was determined using

■ ACKNOWLEDGMENTS

■ REFERENCES