Toward a microscopic model of light absorbing dissolved organic compounds in aqueous environments: theoretical and experimental study

Enhanced HONO Formation from Aqueous Nitrate Photochemistry in the Presence of Marine Relevant Organics: Impact of Marine-Dissolved Organic Matter (m-DOM) Concentration on HONO Yields and Potential Synergistic Effects of Compounds within m-DOM

Nitrous acid (HONO) is a key molecule in the reactive nitrogen cycle. However, sources and sinks for HONO are not fully understood. Particulate nitrate photochemistry has been suggested to play a role in the formation of HONO in the marine boundary layer (MBL). Here we investigate the impact of marine relevant organic compounds on HONO formation from aqueous nitrate photochemistry. In particular, steady-state, gas-phase HONO yields were measured from irradiated nitrate solutions at low pH containing marine-dissolved organic matter (m-DOM). m-DOM induces a nonlinear increase in HONO yield across all concentrations compared to that for pure nitrate solutions, with rates of HONO formation increasing by up to 3-fold when m-DOM is present. Furthermore, to understand the potential synergistic effects that may occur within complex samples such as m-DOM, mixtures of chromophoric (light-absorbing) and aliphatic (non-light-absorbing) molecular proxies were utilized. In particular, mixtures of 4-benzoylbenzoic acid (4-BBA) and ethylene glycol (EG) in acidic aqueous solutions containing nitrate showed more HONO upon irradiation compared to solutions containing only one of the molecular proxies. This suggests that synergistic effects in the HONO formation can occur in complex organic samples. Atmospheric implications of the results presented here are discussed.

Interpreting pH-Dependent Differential UV/VIS Absorbance Spectra to Characterize Carboxylic and Phenolic Chromophores in Natural Organic Matter

Natural organic matter (NOM) is critical for the biogeochemical cycles of energy and many elements in terrestrial and aquatic ecosystems, and protonation-active functional groups in NOM molecules, notably carboxylic and phenolic groups often mediate these critical environmental functions. Molecular heterogeneity, polydispersity and dynamic behavior of NOM complicate achieving an unambiguous description of its molecular properties and reactivity. This study demonstrates that differential ultraviolet-visible (UV/VIS) absorbance spectra (DAS) of NOM acquired at varying pH values exhibit several distinct features associated with the deprotonation of NOM molecules, independent of the environmental provenance of NOM (e.g., surface water, seawater, sediment, and wastewater). The protonation-active functionalities that contribute to the Gaussian distribution bands present in the DAS were identified here by comparing characteristic properties of the bands with the stoichiometries of NOM molecules ascertained by Ultrahigh-Resolution Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). The protonation-active individual chromophores universally present in NOM molecules were identified by a genetic molecular network analysis. The observed DAS features were closely modeled via superimposing DAS spectra of 51 individual protonation-active chromophores. Molecular orbital theory was applied to further interpret the deprotonation of these chromophores, their molecular structure, electron distribution, and electron transitions measured using DAS. The high sensitivity and easy implementation of the DAS approach allows using it as a powerful tool to quantify the molecular properties and reactivity of NOM at environmental concentrations.

Reduction and Photoreduction of NO2 in Humic Acid Films as a Source of HONO, ClNO, N2O, NO X , and Organic Nitrogen

Atmospheric nitrous acid (HONO), a trace atmospheric gas, is often underestimated in global atmospheric models due to the poor understanding of its daytime sources and sinks. HONO is known to accumulate during nighttime and undergo rapid photodissociation during the day to form NO and highly reactive OH radical, making it important to have accurate atmospheric HONO estimations. Despite its rapid photolysis, recent field observations have found quasi-steady-state concentrations of HONO at midday, suggesting photolytic HONO formation pathways to replenish daytime atmospheric HONO. Recent studies suggest that the presence of complex organic photosensitizers in atmospheric aerosols converts atmospheric NO₂ into HONO. To better understand the effect of environmental photosensitizers in daytime mechanisms of HONO formation, we present here laboratory studies on the heterogeneous photolytic reduction of NO₂ by humic acid films, a proxy for organic chromophoric compounds. The effect of pH and Cl^- in the photosensitized formation of HONO and other nitrogen-containing gases is also investigated. A dual Fourier transform infrared (FTIR) system is utilized to simultaneously perform in situ analysis of condensed-phase reactants and gas-phase products. We find that the rate of HONO formation is faster at lower pHs. Nitrogen incorporation in the complex organic chromophore is observed, suggesting a competing pathway that results in suppressed daytime formation of nitrogenous gases. Significantly, the presence of chloride ions also leads to the organic-mediated photolytic formation of nitrosyl chloride (ClNO), a known precursor of HONO. Overall, this work shows that organic acid photosensitizers can reduce adsorbed NO₂ to form HONO, ClNO, and NO while simultaneously incorporating nitrogen into the organic chromophores present in aerosol.

Changes in light absorption and composition of chromophoric marine-dissolved organic matter across a microbial bloom

Marine chromophoric dissolved organic matter (m-CDOM) mediates many vital photochemical processes at the ocean's surface. Isolating m-CDOM within the chemical complexity of marine dissolved organic matter has remained an analytical challenge. The SeaSCAPE campaign, a large-scale mesocosm experiment, provided a unique opportunity to probe the in situ production of m-CDOM across phytoplankton and microbial blooms. Results from mass spectrometry coupled with UV-VIS spectroscopy reveal production of a chemodiverse set of compounds well-correlated with increases in absorbance after a bacterial bloom, indicative of autochthonous m-CDOM production. Notably, many of the absorbing compounds were found to be enriched in nitrogen, which may be essential to chromophore function. From these results, quinoids, porphyrins, flavones, and amide-like compounds were identified via structural analysis and may serve as important photosensitizers in the marine boundary layer. Overall, this study demonstrates a step forward in identifying and characterizing m-CDOM using temporal mesocosm data and integrated UV-VIS spectroscopy and mass spectrometry analyses.

How Robust Is the Reversible Steric Shielding Strategy for Photoswitchable Organocatalysts?

A highly appealing strategy to modulate a catalyst's activity and/or selectivity in a dynamic and noninvasive way is to incorporate a photoresponsive unit into a catalytically competent molecule. However, the description of the photoinduced conformational or structural changes that alter the catalyst's intrinsic reactivity is often reduced to a handful of intuitive static representations, which can struggle to capture the complexity of flexible organocatalysts. Here, we show how a comprehensive exploration of the free energy landscape of N-alkylated azobenzene-tethered piperidine catalysts is essential to unravel the conformational characteristics of each configurational state and explain the experimentally observed reactivity trends. Mapping the catalysts' conformational space highlights the existence of false ON or OFF states that lower their switching ability. Our findings expose the challenges associated with the realization of a reversible steric shielding for the photocontrol of Brønsted basicity of piperidine photoswitchable organocatalysts.

Absorption Spectra and the Electronic Structure of Gallic Acid in Water at Different pH: Experimental Data and Theoretical Cluster Models

Gallic acid (GA) has been characterized in terms of its optical properties in aqueous solutions at varying pH in experiments and in theoretical calculations by analyzing the protonated and deprotonated forms of GA. This work is part of a series of studies of the optical properties of different carboxylic acids in aqueous media. The experimental electronic spectra of GA exhibit two strong well-separated absorption peaks (B- and C-bands), which agree with previous studies. However, in the current study, an additional well-defined low-energy shoulder band (A-band) in the optical spectra of GA was identified. It is likely that the A-band occurs for other carboxylic acids in solution, but because it can overlap with the B-band, it is difficult to discern. The theoretical calculations based on density functional theory were used to simulate the optical absorption spectra of GA in water at different pH to prove the existence of this newly found shoulder band and to describe and characterize the full experimental optical spectra of GA. Different cluster models were tested: (i) all water molecules are coordinated near the carboxy-group and (ii) additional water molecules near the hydroxy-groups of the phenyl ring were included. In this study, we found that both the polarizable continuum model (dielectric property of a medium) and neighboring water molecules (hydrogen-bonding) play significant roles in the optical spectrum. The results showed that only an extended cluster model with water molecules near carboxy- and hydroxy-groups together with the polarizable continuum model allowed us to fully reproduce the experimental data and capture all three absorption bands (A, B, and C). The oscillator strengths of the absorption bands were obtained from the experimental data and compared with theoretical results. Additionally, our work provides a detailed interpretation of the pH effects observed in the experimental absorption spectra.