Abstract
Sorbic acid (2,4-hexadienoic acid; HDA) isomerization is frequently used to probe triplet-state dissolved organic matter (3CDOM*) reactivity, but there remain open questions about the reaction kinetics of 3CDOM* with HDA due to the difficulties of directly measuring 3CDOM* quenching rate constants. Using our recently developed approach based on observing the radical cation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) formed through oxidation of TMPD by 3CDOM*, we studied 3CDOM* quenching kinetics with HDA monitored via transient absorption spectroscopy. A competition kinetics-based approach utilizing formation yields of TMPD•+ was developed, validated with model sensitizers, and used to determine bimolecular rate constants between 3CDOM* oxidants and HDA for diverse DOM isolates and natural waters samples, yielding values in the range of (2.4-7.7) × 108 M-1 s-1. The unquenchable fraction of TMPD-oxidizing triplets showed that, on average, 41% of 3CDOM* oxidants cannot be quenched by HDA. Conversely, cycloheptatriene quenched nearly all TMPD•+-forming triplets in CDOM, suggesting that most 3CDOM* oxidants possess energies greater than 150 kJ mol-1. Comparing results with our companion study, we found slight, but noticeable differences in the 3CDOM* quenching rate constants by HDA and unquenchable triplet fractions determined by oxidation of TMPD and energy transfer to O2 (1O2 formation) methods.
Highlights
Produced reactive intermediates (PPRI) are central for natural element cycling processes, atmospheric chemistry, and the degradation of environmental contaminants.[1−8] Among the Photochemically produced reactive intermediates (PPRI), triplet state chromophoric dissolved organic matter (3CDOM*) is notably important, both because of its role as an oxidant, and as a precursor to other reactive intermediates
Produced reactive intermediates (PPRI) are central for natural element cycling processes, atmospheric chemistry, and the degradation of environmental contaminants.[1−8] Among the PPRI, triplet state chromophoric dissolved organic matter (3CDOM*) is notably important, both because of its role as an oxidant, and as a precursor to other reactive intermediates. It is formed from the excited singlet state of CDOM by intersystem crossing and is known or suspected to be the precursor of other downstream PPRI like singlet oxygen (1O2) and reactive halogen species.[9,10]
Electron transfer probes utilize the fact that 3CDOM* oxidizes different classes of organic molecules, especially those with phenol and aniline moieties,[11] which is largely driven by the free energy of the reaction[12] and the reduction potential of 3CDOM*
Summary
Produced reactive intermediates (PPRI) are central for natural element cycling processes, atmospheric chemistry, and the degradation of environmental contaminants.[1−8] Among the PPRI, triplet state chromophoric dissolved organic matter (3CDOM*) is notably important, both because of its role as an oxidant, and as a precursor to other reactive intermediates. Several probes have been developed for 3CDOM* and, based on their photophysical and redox properties, different pools of triplets are captured. They can be separated into two classes: electron and energy transfer probes. HDA is the most frequently used energy transfer 3CDOM* probe, but there is high uncertainty about the fraction of the captured 3CDOM* pool as well as quenching rate constants with. The reaction of oxidizing CDOM triplets with energy transfer quenchers was studied by competition kinetics with TMPD through monitoring TMPD+ yields from 3CDOM*. The same sensitizers were used as in the experiments in the companion study to determine rate constants with HDA.[26]. Details about data evaluation are given in SI Section S4
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