Misovich Maria, Folarin Hannah, Paul Suranjan K, Walter Ryan, Everly R Michael, Baltrusaitis Jonas, Slipchenko Lyudmila V, Laskin Alexander
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States.
Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States.
J Phys Chem A. 2025 May 15;129(19):4265-4274. doi: 10.1021/acs.jpca.4c08458. Epub 2025 May 1.
Photochemically produced triplet excited states of organic matter (C*) play a crucial role in establishing free radical pools that drive oxidation chemistry in atmospheric and terrestrial water systems. Chemical characterization and quantitative speciation of these free radical mixtures are challenging due to their short lifetimes and the inherent complexity of the mixtures. In this study, we demonstrate the use of electron paramagnetic resonance (EPR) spectroscopy, assisted with the spin trap reagent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), to measure concentrations of free radicals initiated by C* formed through the photoexcitation of 3,4-dimethoxybenzaldehyde (DMB). We evaluate the background radical contributions from the photolysis of aqueous DMPO under various conditions of single-wavelength and broadband irradiation. Subsequently, we characterize and quantify the DMPO-trapped radical products produced in irradiated aqueous mixtures containing DMB, struvite microcrystal colloids, and a mixture of both DMB and struvite, serving as environmentally relevant proxies. The identified DMPO-trapped radical products include the spin adduct with hydroxyl (OH) radicals (DMPO-OH), the spin adduct with hydrogen (H) radicals (DMPO-H), the DMPO dimer radical, and the oxidation product DMPOX. Among these, DMPO-OH was found to exist at the highest concentration. The concentration of DMPO products increased linearly with C* concentration up to 50 μM, with DMPO-OH existing in the highest concentration. Struvite microcrystals were observed to influence the relative proportions of the DMPO products, altering the radical pool composition. Our experimental observations and computational studies of DMPO excitation energies in an aqueous environment revealed its photolytic lability under irradiation with wavelengths of 365 nm or shorter. These findings indicate the necessity of accounting for background photolysis reactions of DMPO to avoid overestimating radical concentrations during quantitative EPR studies. This work provides valuable insights into the mechanisms of free radical formation mediated by C* and evaluates the utility and limitations of DMPO as a tool for investigating complex aqueous photochemical radical systems.
有机物的光化学产生的三重激发态（C*）在建立驱动大气和陆地水系统中氧化化学的自由基池方面起着关键作用。由于这些自由基混合物的寿命短以及混合物固有的复杂性，对其进行化学表征和定量形态分析具有挑战性。在本研究中，我们展示了使用电子顺磁共振（EPR）光谱，并辅以自旋捕获试剂5,5-二甲基-1-吡咯啉-N-氧化物（DMPO），来测量由3,4-二甲氧基苯甲醛（DMB）光激发形成的C引发的自由基浓度。我们评估了在单波长和宽带照射的各种条件下水溶液中DMPO光解产生的背景自由基贡献。随后，我们对在含有DMB、鸟粪石微晶胶体以及DMB和鸟粪石混合物的辐照水性混合物中产生的DMPO捕获的自由基产物进行表征和定量，这些混合物可作为与环境相关的替代物。鉴定出的DMPO捕获的自由基产物包括与羟基（OH）自由基的自旋加合物（DMPO-OH）、与氢（H）自由基的自旋加合物（DMPO-H）、DMPO二聚体自由基以及氧化产物DMPOX。其中，发现DMPO-OH的浓度最高。DMPO产物的浓度随C浓度线性增加，直至50μM，其中DMPO-OH的浓度最高。观察到鸟粪石微晶会影响DMPO产物的相对比例，改变自由基池的组成。我们在水环境中对DMPO激发能的实验观察和计算研究表明，在波长为365nm或更短的照射下，它具有光解不稳定性。这些发现表明，在定量EPR研究中，有必要考虑DMPO的背景光解反应，以避免高估自由基浓度。这项工作为C*介导的自由基形成机制提供了有价值的见解，并评估了DMPO作为研究复杂水性光化学自由基系统工具的实用性和局限性。
