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Article

Atmospheric Photochemical Oxidation of 4-Nitroimidazole

by
Nayan Kondapalli
,
Oliver Cernero
,
Aaron Welch
and
Aaron W. Harrison
*
Department of Chemistry, Austin College, Sherman, TX 75090, USA
*
Author to whom correspondence should be addressed.
Atmosphere 2025, 16(5), 624; https://doi.org/10.3390/atmos16050624
Submission received: 22 March 2025 / Revised: 16 May 2025 / Accepted: 17 May 2025 / Published: 20 May 2025
(This article belongs to the Special Issue Recent Advances in Photochemistry and Spectroscopy of Atmospheric Aerosols)
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Abstract

:
Nitro-functionalized heterocycles, such as nitroimidazoles, are significant environmental contaminants and have been identified as components of secondary organic aerosols (SOA) and biomass-burning organic aerosols (BBOA). Their strong absorption in the near-UV (300–400 nm) makes photochemistry a critical aspect of their atmospheric processing. This study investigates both the direct near-UV photochemistry and hydroxyl radical (OH) oxidation of 4-nitroimidazole (4-NI). The atmospheric photolysis rate of 4-NI in the near-UV (300–400 nm) was found to be J4-NI = 4.3 × 10−5 (±0.8) s−1, corresponding to an atmospheric lifetime of 391 (±77) min under bulk aqueous conditions simulating aqueous aerosols and cloud water. Electrospray ionization mass spectrometry (ESI-MS) analysis following irradiation indicated loss of the nitro group, while NO elimination was observed as a more minor channel in direct photolysis. In addition, the rate constant for the reaction of 4-NI with OH radicals, kNI+OH, was determined to be 2.9 × 109 (±0.6) M−1s−1. Following OH oxidation, ESI-MS results show the emergence of a dominant peak at m/z = 130 amu, consistent with hydroxylation of 4-NI. Computational results indicate that OH radical addition occurs with the lowest barrier at the C2 and C5 positions of 4-NI. The combined results from direct photolysis and OH oxidation experiments suggest that OH-mediated degradation is likely to dominate under aerosol-phase conditions, where OH radical concentrations are elevated, while direct photolysis is expected to be the primary loss mechanism in high-humidity environments and bulk cloud water.

1. Introduction

Nitroaromatic compounds such as nitrophenols, nitro-polycyclic aromatic hydrocarbons, and nitro-functionalized heterocycles are a diverse and atmospherically relevant class of pollutants that significantly influence air quality and climate through their photochemical behavior [1,2,3,4,5,6,7,8]. These compounds are significant emission products from biomass burning and can also form as photochemical oxidation products of volatile organic compounds (VOCs) in urban areas with elevated concentrations of NOx species [9,10]. Nitroaromatic compounds are estimated to account for approximately 25% of UV absorption by gas- and aerosol-phase brown carbon (BrC) in the boundary layer of urban environments, primarily due to the strong near-UV absorption of the nitro-functional group [11]. Furthermore, photochemical oxidation of nitroaromatic compounds in the atmosphere leads to the formation of a range of reactive gas-phase products [12] and contributes to the growth of secondary organic aerosols (SOA) [13]. Recent field measurements have confirmed that nitroaromatics, particularly nitrated phenolic compounds, are present across all major atmospheric phases: gas, particle, and cloud water, with distributions and abundances influenced by seasonal conditions and cloud processing [14].
Recent studies have revealed that the photolysis of nitroaromatics produces reactive nitrogen species such as nitrite (NO2⁻), nitric oxide (NO), and nitrous acid (HONO), which can act as important precursors to atmospheric OH radicals [12,15]. For instance, Yang et al. demonstrated that the sunlight-induced photolysis of solid-phase nitrophenols and NPAHs results in HONO and NO emissions, highlighting their potential as daytime sources of reactive nitrogen in urban environments [12]. Similarly, Barsotti et al. showed that the irradiation of aqueous and viscous films containing nitrophenols can generate HONO and nitrite, with quantum yields of ~10−4 comparable to those in gas-phase reactions [15]. These findings highlight the role of nitroaromatics in modifying aerosol composition and serving as photochemical sources of radical precursors. Understanding their photochemical oxidation pathways is crucial for improving models of SOA formation and assessing the broader environmental impact of nitroaromatic compounds.
Nitrogen-containing heterocycles, such as pyroles and imidazoles, have been increasingly identified as key constituents of SOA, particularly in regions impacted by anthropogenic emissions [16,17,18,19,20] as well as primary emission products from biomass burning [7]. Their nitrated derivatives, including nitroimidazoles, have been detected as secondary oxidation products formed during SOA aging processes [21], and as transformation products within biomass-burning organic aerosol (BBOA) [7,8]. The presence of these nitro-functionalized heterocycles in atmospheric particles, coupled with the low vapor pressures of nitroaromatic compounds [22], suggests that these compounds can influence both the chemical evolution and optical properties of SOA and BrC. However, the atmospheric photochemistry of nitro-functionalized heterocycles, including nitroimidazoles, is not well characterized, leaving gaps in our knowledge of their environmental fate and potential impact.
In addition to their role in the atmosphere, nitroimidazoles represent an important chemical moiety in a variety of antibiotics widely used in farming and agriculture [23]. Consequently, metabolites of these antibiotics contribute to nitroimidazole-based environmental contaminants [24]. Previous studies have primarily focused on the photochemical degradation of antibiotics such as metronidazole [25], dimetridazole, tinidazole, and ronidazole, with an emphasis on their breakdown under UV-C (254 nm) for wastewater treatment [26]. However, photochemical reaction rates under actinic UV conditions relevant to atmospheric degradation have not been measured.
In this study, we investigated the photochemical oxidation kinetics of 4-nitroimidazole through direct photolysis and OH radical oxidation, providing estimates of its atmospheric lifetimes for each decay pathway. Mass spectrometry analysis of irradiated samples revealed nitro group elimination, particularly through HONO and NO2 loss, as an important photochemical fragmentation pathway. Furthermore, mass spectrometry results following OH oxidation indicate hydroxylation of the imidazole ring, with computational analysis identifying the C2 and C5 positions as the most favorable sites for OH addition. These insights clarify the photochemical behavior of nitroimidazoles under atmospheric conditions.

2. Materials and Methods

2.1. Experimental

UV-vis absorption measurements were performed using a Horiba Duetta spectrofluorometer (Horiba Instruments, Irvine, CA USA). Photochemical degradation rates were assessed under simulated sunlight using an Oriel LCS-100 solar simulator (Newport Corporation; Irvine, CA USA), which is calibrated to closely reproduce the AM 1.5 solar spectrum [27,28]. Samples were prepared in 1 cm pathlength quartz cuvettes, positioned horizontally to ensure uniform light exposure during irradiation. To quantify the incident UV photon flux, o-nitrobenzaldehyde (Sigma-Aldrich St. Louis, MO USA) was employed as a chemical actinometer following established protocols [29,30]. Based on these measurements, the estimated power density of the light source was approximately 49.2 W/m2, assuming an average photon energy near 350 nm (~3.5 eV). This value represents a slight enhancement (~5%) relative to the standard AM 1.5 solar flux within the near-UV range [27], confirming that the solar simulator provides similar flux in the near-UV and confirming that the experimental setup closely mimics atmospheric photolysis conditions. Results for this experiment are included in the Supporting Information (Figure S1).
To measure the kinetics of OH radical oxidation, hydrogen peroxide (H2O2, 30% reagent grade; Carolina Chemicals, Charlotte, NC USA) was introduced into the cuvette as a photolytic OH precursor. The decay of 4-nitroimidazole (Sigma-Aldrich) was monitored during irradiation in the presence of H2O2 and compared against control samples lacking the oxidant. Additional control experiments were conducted by adding H2O2 without light exposure [31]. Under dark conditions, the compound exhibited minimal degradation over the 1–2 h duration of the experiment, indicating that H2O2 alone does not significantly contribute to oxidative decay in the absence of photolysis.
The steady-state concentration of hydroxyl radicals ([OH]ss) was estimated by tracking the decay of salicylic acid (SA), using fluorescence detection at an excitation wavelength of 300 nm. The bimolecular rate constant (kSA+OH) for the decay rate given by: kSA+OH [OH][SA] is estimated to be 1.1 × 1010 M−1 s−1 [32,33]. Assuming steady-state conditions for [OH], the decay of salicylic acid (SA) can be described by pseudo-first-order kinetics, where the rate is expressed as k′[SA]. Here, k′ represents the pseudo-first-order rate constant and is defined as ksa+oh [OH]ss [34]. The experimental value for [OH]ss is then calculated by dividing k′ by the known bimolecular rate constant for the SA + OH reaction:
O H s s = k ′ k S A + O H
Triplicate measurements resulted in an estimated [OH]ss ~5.2 (± 0.4) × 10−14 M. A complete tabulation of these results is provided in the Supporting Information (Table S1).
Electrospray ionization mass spectrometry (ESI-MS; Thermo-Finnegan LCQ; Thermo_Fisher Scientific, Waltham, MA USA) was performed in positive-ion mode to analyze aqueous 4-nitroimidazole (4-NI) samples before and after 3 h of irradiation. Analytes were detected primarily as protonated molecular ions ([M + H]⁺). Prior to analysis, samples were diluted 1:1 (v/v) with methanol to enhance ionization efficiency and ensure signal stability. The instrument was calibrated using a standard LTQ positive-ion calibration solution (Thermo Fisher Scientific). Mass spectra were recorded over the m/z range of 50 to 150 amu.

2.2. Theoretical

Electronic structure calculations were performed with Gaussian 16 (Wallingford, CT USA) [35] to characterize the 4-NI near-UV absorption spectrum in the gas and aqueous phase. Equilibrium structures were optimized with the ωB97 functional using the Dunning-type cc-pVDZ basis set augmented with diffuse functions [36]. To include the role of the aqueous solvent, these calculations included the SMD solvent model [37]. The vertical excitation energies (VEEs) were calculated using time-dependent density functional theory (TD-DFT).
The reaction pathways for the addition of OH radical to 4-NI were also explored. Equilibrium and transition state structures were optimized within the CBS-QB3 compound method that utilizes the B3LYP functional for geometry optimization, followed by complete basis limit extrapolation based on coupled cluster calculations [38]. Minima and transition states were confirmed by frequency analysis. CBS-QB3 was chosen as it has been shown to provide reliable thermochemistry for reactions involving the main group elements at lower computational cost [39]. Cartesian coordinates and images of the optimized geometries of the transition states can be found in the Supporting Information.

3. Results

3.1. Near-UV Absorption Spectroscopy

The near-UV absorption spectrum of 4-NI (~150 uM) is shown in Figure 1 in black. 4-NI exhibits a strong peak in the near-UV at approximately 300 nm that decays to zero as the wavelength approaches 400 nm. Shown in red in Figure 1 is the AM 1.5 solar spectrum to indicate the overlap between the two.
The nitro-functional group substantially decreases the basicity of 4-NI with respect to imidazole [23]. Therefore, under the typical pH conditions of cloud water (pH 4–6) [40], 4-NI predominantly exists in its neutral form. Previous studies have determined the pKa for the formation of the 4-nitroimidazolate ion to be approximately 9–9.5, depending on the ionic strength of the solution [23]. In this study, experiments were conducted at pH ~7, where neutral 4-NI remains the dominant species.
The vertical excitation energies (VEE) of 4-NI were computed using the ωB97xd functional both in the gas phase and using the SMD continuum model for the aqueous solvent. Under the mildly acidic condition of cloud water, 4-NI is primarily neutral, and as such, we focused on this species. Using this methodology, the optically active near-UV transition is calculated to occur at approximately 244 and 282 nm in the gas and aqueous phase, respectively. The addition of the continuum solvent model results in a red shift (~0.86 eV), resulting in a VEE very similar to the experimental values of ~298 nm (Figure 1). These results are summarized in Table 1 below.
We also performed natural bond orbital (NBO) analysis to visualize the HOMO and LUMO involved in this transition. As can be seen in Figure 2, this transition corresponds to a n − π* transition in the nitro-functional group consistent with optical activity of a wide range of nitroaromatic species [41].

3.2. Direct Photochemistry

To characterize the direct photochemistry of 4-NI in the atmosphere, aqueous 4-NI samples were irradiated with a solar simulator for 2–3 h. This experiment was performed in triplicate, with a complete tabulation of results available in the Supporting Information. Figure 3 presents representative absorption spectra over time, along with the first-order decay of absorbance at 300 nm. For the photolysis experiments, more dilute solutions with lower peak absorbance were used to ensure uniform illumination within the cuvette. We observe relatively uniform decay from 300 to 400 nm; however, from 240 to 265 nm, we observe an increase in absorption consistent with the formation of a product that more strongly absorbs in this region. As discussed further below, the near-UV decay is consistent with the loss of the nitro-functional group chromophore from 4-NI and the increased absorption from 240 to 265 nm could be due to imidazole photoproducts following NO2 elimination.
Based on these results, we determine the direct photolysis rate in the near-UV (300–400 nm) to be J4-NI = 0.0026 (±0.0005) min−1 or 4.3 × 10−5 (±0.8) s−1. This corresponds to an atmospheric lifetime (1/J4-NI) of ~391 ± 77 or approximately 6–7 h in aqueous aerosols and cloud water with respect to direct photolysis. With the measured photolysis rate, we can also estimate the atmospheric photolysis quantum yield for 4-NI in the aqueous phase. Using the molar extinction coefficient at 350 nm of ε = 0.75 mM−1cm−1 [23], which corresponds to an absorption cross-section of approximately 2.9 × 10−18 cm2/molecule and the measured near-UV photon flux of 8.7 × 1015 photons/cm2/s, we find the photolysis quantum yield (φhv) to be ~0.0017 (±0.0003). However, this value should be regarded as a conservative estimate. A more accurate determination of the photolysis quantum yield would require a wavelength-resolved measurement of the photon flux to account for variations in the spectral output of the light source and also include the wavelength-dependent absorption of 4-NI from 300 to 400 nm.
In order to investigate the direct photochemical products, aliquots of the aqueous 4-NI samples were diluted with methanol and analyzed by direct infusion ESI-MS before and after 3 h of irradiation. The mass spectrum for a representative dataset is shown in Figure 4.
In Figure 4a, the parent ion peak [M+H]+ is observed at m/z = 114 amu. In the inset showing the m/z range below 100 amu, only two significant peaks are evident at m/z = 84 and 57 amu. The peak at m/z = 84 is consistent with NO elimination (−30 amu), likely due to a small amount of fragmentation in the ESI source. This type of fragmentation has been previously reported in mass spectrometric studies of nitroimidazoles [42,43].
Following irradiation, the parent ion peak remains the dominant feature from m/z = 100 to 150 amu, but numerous additional peaks appear within the m/z 50–100 range (Figure 4b). We are not able to unambiguously identify all observed peaks, but the dominant feature appears at m/z = 67 amu, with accompanying peaks at 68 and 69 amu. The peaks at m/z = 67 and 68 amu are consistent with the loss of HONO (−47 amu) and NO2 (−46 amu), respectively, from the parent ion, while the peak at m/z = 69 amu corresponds to protonated imidazole (C3H4N2). However, due to potential differences in ionization efficiency, the peak intensities do not necessarily reflect the relative abundances of these fragments. By comparison, the NO elimination channel (peak at m/z = 84 amu) does not appear to be a major pathway, as it is observed at low intensity both before and after irradiation. The photochemical mechanisms underlying these observations are discussed in Section 4.2.

3.3. OH Oxidation

In addition to direct photolysis, we also measured the decay of 4-NI in the presence of OH radical under pseudo-first-order conditions (see Section 2.1). Representative absorbance decay data in the presence and absence of H2O2 are shown in Figure 5.
For these experiments, measurements were performed over a shorter time scale (60 min) as the decay of absorbance at 300 nm during the irradiations with added H2O2. Based on the measured pseudo-first-order rate of ~1.5 × 10−4 s−1 and the estimated [OH]ss, we determine the rate constant for OH radical reaction with 4-NI to be kNI+OH ~2.9 × 109 (±0.6) M−1s−1. See Supporting Information for full tabulation of results. This value is similar to the OH oxidation rate constants measured for other nitroimidazole-based pharmaceutical species, such as metronidazole, dimetridazole, ornidazole, etc., with reported rate constants ranging from ~2.3–3.7 × 109 M−1 s−1 [44,45].
However, it should be noted that the reaction products exhibit overlapping absorption with 4-NI, which may introduce some uncertainty in the determination of the OH radical rate constant. While the absorption at 300 nm decays over the irradiation period, there is a concurrent increase in absorbance in the 370–400 nm range. This spectral evolution is illustrated in the time-dependent absorption spectra shown in Figure 6.
Following the OH oxidation experiment, the solution was directly infused into the ESI-MS to investigate product formation. In the direct photolysis-only experiments (Section 3.2), the dominant peak in the ESI-MS spectrum remains the parent ion at m/z = 114 amu, both before and after irradiation. In contrast, the dominant peak following OH oxidation is at m/z = 130 amu, consistent with OH addition followed by hydrogen abstraction, resulting in the formation of a hydroxy-4-NI isomer. The ESI-MS results are shown in Figure 7. However, peak heights do not necessarily reflect their relative concentrations. Based on computational results outlined below, this product likely includes positional isomers resulting from OH addition at the C2 and C5 positions of the imidazole ring, where the reaction barriers are lowest.
To gain further insight into the underlying chemical mechanism, we also used computational methods to investigate the OH addition at each carbon site in 4-NI. These three additional pathways are shown in Figure 8. Other pathways are possible, such as hydrogen abstraction, but previous work on the OH oxidation of imidazole revealed the abstraction reactions to be higher in energy than OH addition [18,21]. Also, addition at the nitrogen centers of the imidazole ring has been shown to be significantly higher in energy in analogous nitroimidazole systems [24]. The transition states and reaction products were calculated at the CBS-QB3 level of theory to determine the relative energies of the different reaction pathways in the gas phase. It should be noted that there is typically an entry reactant complex involved in this type of addition reaction, but the transition state will be the highest energy point along the reaction coordinate [46].
The results indicate that the lowest barrier occurs via addition at the C2 and C5 positions. For the C2 and C5 reaction pathway, the TS is submerged with respect to the separated reactants in terms of energy; however, the entropic contribution to the Gibbs free energy results in free energy barriers ranging from ΔG~6.0 to 6.5 kcal mol–1. Addition at the C4 position proceeds via a slightly higher free energy barrier of 9.8 kcal mol–1 and is expected to be the least favorable carbon center for OH addition. The overall reaction energetics are qualitatively similar to previous theoretical studies on the reaction of OH with imidazole and that of other nitroimidazole isomers [21,47], as discussed further below in Section 4.2.

4. Discussion

4.1. Atmospheric Lifetime of 4-NI

To quantify the major loss pathways of 4-NI in the atmosphere, we measured its direct photolysis rates under solar irradiation and its degradation via OH radical oxidation. Based on the measured direct photolysis rate, we estimate a photolysis lifetime of approximately 391 min. However, these measurements are more representative of cloud water conditions or high-relative-humidity (RH) aqueous aerosols. Under low RH conditions, aerosols exhibit significantly different physicochemical properties, such as high ionic strength and increased viscosity. These changes can strongly influence photodegradation rates by altering light absorption, reaction kinetics, and the diffusion of reactants within the aerosol phase [3].
For OH radical oxidation, the atmospheric lifetime will depend on the steady-state OH concentration in the troposphere and the bimolecular rate constant. The lifetime of 4-NI with respect to OH oxidation can be calculated with the following equation:
τ = 1 k O H [ O H ]
Estimates of [OH] radical concentration can be as high as 10−13 M in aerosols and 10−15 M in atmospheric waters (e.g., cloud droplets) [48,49,50]. Based on the 10−13–10−15 M concentration range and measured rate constant (2.9 × 109 M−1s−1), the OH radical lifetime would be estimated to span from ~58 to 5800 min, with shorter lifetimes expected in aerosols where OH concentrations are higher.
When comparing the direct photolysis and OH oxidation pathways, direct photolysis is expected to be the dominant sink under cloud water or high-RH conditions due to the lower OH radical concentrations in these environments. However, in conditions where OH levels are elevated, such as in highly reactive aerosols or polluted regions, OH oxidation would become the more rapid degradation pathway. The relative importance of each process may shift depending on aerosol composition, phase state, and the presence of other reactive species that can influence photochemical oxidation.

4.2. Photochemical Oxidation Mechanisms

The possible unimolecular fragmentation pathways of nitroimidazoles on the ground state have been extensively characterized in prior studies using both experimental and computational methods [25,42,43]. These investigations have established two principal dissociation channels: the NO elimination through a three-center nitro-nitrite rearrangement transition state, and the direct, barrier-less dissociation of NO2.
Previous work under low-energy collision-induced dissociation (CID) conditions demonstrated that in the case of 4-NI, the loss of NO2 dominates over NO elimination [42]. The CID spectra showed a strong preference for C–N bond cleavage leading to NO2 loss, supported by calculated dissociation energies, with the NO elimination exhibiting lower intensity in experimental mass spectra. This trend is supported by the electron ionization (EI) mass spectra, where NO and NO2 were observed as the major neutral losses in the dissociative ionization of 2-nitroimidazole [43]. The EI-derived mass spectrum reveals prominent fragment ions at m/z = 67 amu (C3H3N2+) and 68 amu (C3H4N2+), which the authors attribute to HONO and NO2 losses, respectively. In addition, photoionization mass spectra of 4-NI at 60 eV also indicated NO2-type loss channels to be more prominent for 4-NI [25].
In our present study, the mass spectral results obtained following irradiation (Figure 4b) further support the conclusion that NO elimination is a relatively minor fragmentation route compared to HNO2/NO2 loss. Although a peak at m/z = 84 corresponding to NO loss is observed, its intensity remains low both before and after irradiation. This is in contrast to the peaks at m/z = 67 and 68, which we interpret as indicative of HONO and NO2 loss channels. The appearance of the imidazole fragment at m/z = 69 likely reflects a sequential process involving NO2 loss and subsequent hydrogen abstraction by the imidazole radical.
These findings are in line with earlier CID and EI studies and suggest that the NO elimination channel is not the preferred dissociation route under either low-energy activation or photochemical conditions [25,42,43]. However, the exact structures of the observed fragment ions remain uncertain. In previous studies, these fragments were assigned to radical species, which are less likely to persist in solution and be detected by ESI-MS. While some radical cation formation during ESI is possible, it is relatively uncommon due to the low-energy, soft ionization conditions characteristic of ESI. Further investigation, including tandem MS or high-resolution mass spectrometry studies, is needed to better characterize the structures and formation mechanisms of these photochemical fragments and the other major fragments observed, for instance, at m/z = 81 and 95 amu, to more precisely quantify the relative amounts of the channels. Overall, the results presented here contribute additional evidence that the NO2-related dissociation channels dominate over NO elimination in the unimolecular fragmentation chemistry of 4-NI.
Understanding the underlying chemical mechanism of OH oxidation for nitroimidazole species is also of significant interest. Indeed, the rate constant measured for 4-NI in this study closely aligns with those reported for other nitroimidazole compounds, suggesting a common initiation mechanism. It is clear from the ESI-MS results (Figure 7) that hydroxylated 4-NI is a major product formed in this reaction. Previous computational studies on imidazole and its derivatives have estimated the OH addition barrier at the C4 position of 4-NI to be 11.4 and −18.5 kcal mol−1 (M06-2X with SMD) [21]. However, energy barriers at the other carbon positions were not reported. Additional investigations on 2-nitroimidazole and 5-nitroimidazole using B3LYP/6-31G(d,p) with a PCM solvent model found that OH addition to carbon sites in the imidazole ring occurs with free energy barriers ranging from 3 to 7 kcal mol−1 [47]. These previous findings and the CBS-QB3 results reported here indicate that OH addition at the C2 and C5 positions of the imidazole ring is likely the favored initiation pathway for nitroimidazole oxidation by the OH radical.
Based on the ESI-MS results and on the observed increase in absorbance between 370 and 400 nm following OH oxidation (Figure 6), we calculated the UV-Vis absorption spectra for the likely hydroxylated products, specifically 2-hydroxy-4-NI and 5-hydroxy-4-NI. Notably, 2-hydroxy-4-NI will also undergo tautomerization to its keto form: 2-keto-4-nitroimidazole. The calculated absorption spectra for these three species are presented in Figure 9. Each has a red-shifted VEE compared to 4-NI (Table 1), consistent with the experimentally observed increase in absorption in the 370–400 nm region.
OH addition, followed by the elimination of nitrite (NO2⁻), has been observed in other nitroaromatic compounds [4,51]. This pathway is well-documented as a key degradation mechanism, suggesting that a similar process may occur for 4-NI. This suggests that a similar pathway may be operative for 4-NI. In the present work, a low-intensity peak at m/z = 85 amu is observed, which could be attributed to hydroxyimidazole products formed via this mechanism. However, because OH radicals are photolytically generated from H2O2 in these experiments, both direct photochemical and OH-initiated reaction pathways occur simultaneously. As a result, the ESI-MS data may reflect contributions from both processes, making it challenging to unambiguously attribute this product solely to OH oxidation.

5. Conclusions

This study quantifies the major atmospheric loss pathways of 4-NI through direct photolysis in the near-UV (J4-NI = 4.3 × 10−5 (±0.8) s−1) and OH radical oxidation (kNI+OH, = 2.9 × 109 (±0.6) M−1s−1). The estimated photolysis lifetime of ~391 min suggests that, under cloud water or high-relative-humidity (RH) conditions, direct photolysis is the dominant degradation mechanism due to lower OH radical concentrations. However, in environments with elevated [OH] levels, OH oxidation can become a more significant loss pathway.
Further studies are needed to fully characterize the photochemical oxidation mechanisms of 4-NI. ESI-MS results presented here provide initial evidence that direct photolysis predominantly proceeds through nitro group elimination, with major fragment ions corresponding to HONO and NO2 loss. In addition, ESI-MS results indicate that hydroxylation of 4-NI is a major product channel following OH oxidation. Computational results suggest that the reaction likely proceeds via OH addition to the imidazole ring at the C2 and C5 positions, consistent with previous findings for related compounds. However, a more detailed understanding of the site-specific reactivity of nitroimidazoles and the subsequent transformation pathways of their hydroxylated products remains necessary to fully elucidate their atmospheric degradation mechanisms. Finally, additional research is required to assess how factors such as aerosol composition, viscosity, and other atmospheric conditions influence the photochemical behavior of nitroaromatic species more broadly.

Supplementary Materials

The following Supporting Information can be downloaded at https://www.mdpi.com/article/10.3390/atmos16050624/s1, Figure S1. Absorption spectra of 2-nitrobenzaldehyde as a function of irradiation time; Table S1. Pseudo-first-order OH Oxidation Rates of salicylic acid and calculated [OH]ss using the experimental value of kSA+OH = 1.1 × 1010 M−1s−1; Table S2. Direct Solar Photolysis Rates of 4-NI and calculated lifetime; Table S3. Pseudo-first-order OH Oxidation Rates of 4-NI and calculated bimolecular rate constants kNI+OH using the experimental value of [OH]ss; Cartesian Coordinates for Transition States (CBS-QB3).

Author Contributions

Conceptualization, A.W.H.; methodology, N.K., O.C. and A.W.H.; formal analysis, N.K., O.C. and A.W.H.; writing—original draft preparation, A.W.H.; writing—review and editing, A.W.H.; visualization, N.K. and A.W.; supervision, A.W.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Welch Foundation under Grant No. AD-0007.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of this study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Atmosphere 16 00624 g001
Figure 1. Absorption spectrum of 4-nitroimidazole (~150 μM; black) and AM 1.5 solar spectrum (red).
Figure 1. Absorption spectrum of 4-nitroimidazole (~150 μM; black) and AM 1.5 solar spectrum (red).
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Atmosphere 16 00624 g002
Figure 2. NBO analysis of 4-nitroimidazole HOMO (a) and LUMO (b).
Figure 2. NBO analysis of 4-nitroimidazole HOMO (a) and LUMO (b).
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Atmosphere 16 00624 g003
Figure 3. Absorption spectra as a function of solar irradiation time (a) and first-order decay plot (b).
Figure 3. Absorption spectra as a function of solar irradiation time (a) and first-order decay plot (b).
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Atmosphere 16 00624 g004
Figure 4. Direct infusion positive-ion mass spectra (ESI-MS) of aqueous 4-NI before (a) and after (b) irradiation. Inset in (a,b) are zoomed in on the range spanning from m/z = 50 to 100 amu.
Figure 4. Direct infusion positive-ion mass spectra (ESI-MS) of aqueous 4-NI before (a) and after (b) irradiation. Inset in (a,b) are zoomed in on the range spanning from m/z = 50 to 100 amu.
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Atmosphere 16 00624 g005
Figure 5. Representative first-order decay of absorbance at λ = 300 nm with (black) and without (red) H2O2 added.
Figure 5. Representative first-order decay of absorbance at λ = 300 nm with (black) and without (red) H2O2 added.
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Atmosphere 16 00624 g006
Figure 6. Time-dependent UV-Vis absorption spectra recorded during OH oxidation experiments, shown over 250–400 nm (a), and magnified from 350 to 400 nm (b). Contour plots are presented in arbitrary units, with the color scale normalized to highlight relative changes in spectral features.
Figure 6. Time-dependent UV-Vis absorption spectra recorded during OH oxidation experiments, shown over 250–400 nm (a), and magnified from 350 to 400 nm (b). Contour plots are presented in arbitrary units, with the color scale normalized to highlight relative changes in spectral features.
Atmosphere 16 00624 g006
Atmosphere 16 00624 g007
Figure 7. Direct infusion positive-ion mass spectra (ESI-MS) following OH oxidation of 4-nitroimidazole.
Figure 7. Direct infusion positive-ion mass spectra (ESI-MS) following OH oxidation of 4-nitroimidazole.
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Atmosphere 16 00624 g008
Figure 8. Calculated transition state and product energies for OH addition at each carbon of 4-NI (CBS-QB3). Values are Δ(E + ZPE at 0 K) and ΔG (298 K) in kcal mol–1.
Figure 8. Calculated transition state and product energies for OH addition at each carbon of 4-NI (CBS-QB3). Values are Δ(E + ZPE at 0 K) and ΔG (298 K) in kcal mol–1.
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Atmosphere 16 00624 g009
Figure 9. Calculated UV-Vis absorption spectra (HWHM = 0.33 eV) of 2- and 5-hydroxy-4-nitroimidazole as well as 2-keto-4-nitroimidazole at ωB97xd/aug-cc-pVDZ + SMD level of theory.
Figure 9. Calculated UV-Vis absorption spectra (HWHM = 0.33 eV) of 2- and 5-hydroxy-4-nitroimidazole as well as 2-keto-4-nitroimidazole at ωB97xd/aug-cc-pVDZ + SMD level of theory.
Atmosphere 16 00624 g009
Table 1. Vertical Excitation Energy (VEE) and Oscillator Strength (f) of 4-nitroimidazole calculated at the ωB97xd/aug-cc-pVDZ + SMD level of theory.
Table 1. Vertical Excitation Energy (VEE) and Oscillator Strength (f) of 4-nitroimidazole calculated at the ωB97xd/aug-cc-pVDZ + SMD level of theory.
4-NIVEE (eV)λ (nm)f
4-NI: Gas Phase5.07244.40.1532
4-NI: SMD4.21282.00.2245
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Kondapalli, N.; Cernero, O.; Welch, A.; Harrison, A.W. Atmospheric Photochemical Oxidation of 4-Nitroimidazole. Atmosphere 2025, 16, 624. https://doi.org/10.3390/atmos16050624

AMA Style

Kondapalli N, Cernero O, Welch A, Harrison AW. Atmospheric Photochemical Oxidation of 4-Nitroimidazole. Atmosphere. 2025; 16(5):624. https://doi.org/10.3390/atmos16050624

Chicago/Turabian Style

Kondapalli, Nayan, Oliver Cernero, Aaron Welch, and Aaron W. Harrison. 2025. "Atmospheric Photochemical Oxidation of 4-Nitroimidazole" Atmosphere 16, no. 5: 624. https://doi.org/10.3390/atmos16050624

APA Style

Kondapalli, N., Cernero, O., Welch, A., & Harrison, A. W. (2025). Atmospheric Photochemical Oxidation of 4-Nitroimidazole. Atmosphere, 16(5), 624. https://doi.org/10.3390/atmos16050624

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