B. J.
Finlayson-Pitts‡
*,
A.
Anderson‡
,
P. S. J.
Lakey
,
W.
Wang
,
M. J.
Ezell
,
X.
Wang
,
L. M.
Wingen
,
V.
Perraud
and
M.
Shiraiwa
*
Department of Chemistry, University of California Irvine, Irvine, California 92697-2025, USA. E-mail: bjfinlay@uci.edu; m.shiraiwa@uci.edu; Fax: +1-949-824-2420; Tel: +1-949-824-7670
First published on 24th November 2022
Neonicotinoids (NNs) are commonly found throughout the environment on surfaces such as seeds, soil, vegetation, and blowing dust particles. However, there is a paucity of data on the kinetics and oxidation products formed on contact with the atmosphere which limits understanding of their potentially far-reaching impacts. In this study, in situ attenuated total reflectance (ATR) FTIR spectroscopy was used to investigate the OH oxidation of thin films of three solid NNs, imidacloprid (IMD), dinotefuran (DNF) and clothianidin (CLD) at 295 ± 3 K. The experimentally measured reaction probabilities based on initial rates of NN loss are (1.6 ± 0.8) × 10−2 for IMD, (1.5 ± 0.6) × 10−2 for DNF and (0.9 ± 0.2) × 10−2 for CLD (±1σ), suggesting initial NN lifetimes with respect to OH of 10–17 days. The kinetics were interpreted using a multiphase kinetics model, KM-SUB, which showed that the OH uptake and reaction occurred primarily in the surface layer. Products identified by mass spectrometry included carbonyl-, alcohol- and olefin-containing species formed via hydrogen abstraction from aliphatic C–H groups. Additionally, carbonyl-containing desnitro and urea derivative products were observed from secondary reactions of the initially formed photodegradation products. Reaction with OH will contribute to NN loss both during the day as well as at night when there are non-photolytic sources of this radical. Thus, OH reactions with both the parent neonicotinoid and its photodegradation products should be considered in assessing their environmental impacts.
Environmental significanceNeonicotinoids (NNs) are the most widely used insecticides globally and their use is increasingly implicated in negative impacts on ecosystem services, particularly pollinator health. However, their environmental fates and effects remain undetermined. Here, the reaction rate constants and mechanisms of thin films of three solid NNs with gas-phase OH radicals, a primary oxidant in air, were investigated. Our study shows that OH oxidation contributes to the loss of the parent NN over days, and to subsequent loss of their desnitro and urea photolysis products. This oxidation will also play a significant role in NN loss at night when photolysis is not occurring but there are non-photolytic sources of OH. |
Due to their systemic nature and widespread use, both the parent NNs and their degradation products have accumulated in the environment; for example, studies conducted in Europe showed that NNs were still present in the soil even five years after the moratorium, with no clear declining trend.21 Globally, NNs have been found in 75% of honey samples,5 in non-target species further up the food chain such as birds,22,23 and perhaps not surprisingly then, are often detected in human biological samples.24–27 Therefore, NNs are a persistent threat to non-target species via continual exposure at sub-lethal concentrations.2,28
A major advantage of the NNs is that they have lower mammalian toxicity than previously used pesticides. While there is increasing concern regarding potential impacts on long-term exposure, there are at present relatively few data available on their health effects.25,28 However, there are some studies that suggest that at least some of the metabolites and oxidation products are significantly more toxic than the parent NN. For example, the binding of the desnitro-derivative of imidacloprid to the nicotinic acetylcholine receptor in mouse brain membranes, which is related to toxicity, is much stronger than that of imidacloprid itself.29
Of crucial importance in understanding the environmental impact of NNs is identifying the environmental degradation products so that a complete toxicity map on non-target species, including humans, can be assessed. An abundance of work is available on the reactions and fate of NNs in aqueous systems.2–4,26,30–41 In addition, previous studies from this lab have reported the heterogeneous photolysis of solid NNs42–45 as would be the case for example for NNs on leaf, soil, and dust surfaces, and the oxidation of the NN nitenpyram by gas phase ozone.46 However, little is known about the heterogeneous reactions of solid NNs with the hydroxyl (OH) radical, which is a major atmospheric oxidant that is ubiquitous in air.
Previous work on aqueous phase oxidation of NNs has largely focused on imidacloprid (IMD), the most widely used NN. These studies have applied a variety of oxidation techniques, in which OH radicals were generated from chemical reactions involving O3, H2O2, and photolysis,33–35,40 photo-Fenton reactions,38,39,41,47–50 and heterogeneous photocatalysis.36–38,41,51–54 While these studies have documented mineralization of IMD for removal of the NN from wastewater, the oxidation methods chosen were often not solely due to OH but rather to a combination of processes such as photolysis and catalysis. As a result, extrapolation of the kinetics, products and mechanisms to the reactions of NNs as a solid on environmental surfaces is fraught with uncertainty.
This work provides kinetics and products data on the heterogeneous reactions of three structurally unique solid-phase NNs, imidacloprid (IMD), dinotefuran (DNF), and clothianidin (CLD), with gas phase OH, a major atmospheric oxidant.55 While these three NNs are all nitroguanidines, they have a variety of C–H bonds, amino groups, and ring structures:
These three NNs are of interest as they are widely used, with relative amounts of each varying by location. For example, in Japan, dinotefuran has historically had the highest use, followed by imidacloprid and clothianidin, while in California, imidacloprid is the major NN used, with smaller amounts of clothianidin and dinotefuran.1 Similarly, they are found in a variety of environmental media worldwide in varying proportions, including rivers and streams,4 pollen and food such as honey.2,5 From the kinetics data, atmospheric lifetimes with respect to the OH reaction are estimated and these, along with the products and proposed mechanisms, provide a guide for future toxicity assessments of NNs in the environment.
The number of monolayers (ML) on the ATR crystal was calculated from the thickness and the surface area per molecule estimated using the volume per molecule derived from the densities (1.54 g cm−3 for IMD, 1.40 g cm−3 for DNF and 1.61 g cm−3 for CLD) and molecular masses of the NNs (255.7 g mol−1 for IMD, 202.2 g mol−1 for DNF and 249.7 g mol−1 for CLD). Assuming the average dimension was the same for all three directions, the average thickness of one molecule was the cube root of its volume and calculated to be ∼0.65–0.62 nm with ∼(2.4–2.6) × 1014 molecules cm−2 in one monolayer. This is similar to the surface concentrations that are calculated based on the crystal structures for IMD and DNF.57 From the total number of molecules on the crystal, the film thicknesses and number of monolayers were estimated to be 0.04–0.22 μm and 60–350 ML. All film thicknesses were less than the depth of penetration, assuring the entire film was probed by ATR-FTIR. The ATR crystal was mounted in a custom reaction cell58 (Fig. S1†) and held in a stream of O2 carrier gas for 15 minutes in the dark before reaction.
(CH3)2CHONO + hν → (CH3)2CHO˙ + NO | (R1) |
(CH3)2CHO˙ + O2 → CH3C(O)CH3 + HO2 | (R2) |
HO2 + NO → OH + NO2 | (R3) |
During the experiment, a high pressure Xe lamp (Ushio 300 W bulb, UXL-302-O) mounted in a housing with a power source (Newport model 69911) was used to photolyze the flow of IPN in the cell. The UV light beam first passed through a water filter to absorb infrared/heat and was directed at an angle onto the quartz window via a reflecting mirror. A 360 nm optical filter (Newport longpass colored glass alternative filter, 20CGA-360) was placed on top of the quartz window to remove light below 360 nm in order to reduce the photolysis of the NN in this region. All experiments were carried out at 295 ± 3 K.
The flow of O2 (AirGas, Ultrahigh purity, >99.993%) was controlled at 500 mL min−1 using a mass flow controller (Alicat, Model # MC500-SCCM-D/5M) and could either flow directly into the cell, or be routed over the headspace of a trap containing liquid IPN at 263 K. The gas phase IPN concentration was measured before and after each experiment using infrared spectroscopy (Mattson Galaxy 5020 FTIR) in a separate 10 cm gas cell with 16 coadded scans at a resolution of 0.5 cm−1. An absorption cross section of 1.61 × 10−18 cm2 molecule−1 (base e) at 1664 cm−1 was used,59 giving IPN concentrations ranging from (1.5–2.8) × 1016 molecules cm−3. The concentration of OH was measured by adding 13CO with an IPN/O2 mixture into the cell and monitoring formation of 13CO2 (Fig. S2†) from the reaction of 13CO with OH as a function of time under static conditions using the same reaction cell (Fig. S1†) in the transmission IR mode. Details of the OH measurement are described in the ESI.† The average steady state OH concentration was determined to be (8.2 ± 2.0) × 108 molecule cm−3.
The loss of NN and formation of surface-bound products were followed using ATR-FTIR with 16 coadded scans at a resolution of 0.5 cm−1. FTIR spectra were recorded before irradiation and at 20 or 30 minute intervals during the irradiation while IPN was continuously flowing over the NN films. Absorbance data were converted to a total number of NN molecules on the ATR crystal using calibrations generated by adding a known number of molecules (10 μL aliquots of 4–40 mM solutions) to the ATR crystal and recording the absorbance of the thin films after evaporation of the solvent.
After the reaction, the ATR crystal was removed from the holder, and the reacted film was analyzed via mass spectrometry, either directly on the ATR crystal by DART-MS or by UPLC-HESI-HRMS on extracted samples, as described below.
NN + OH → Products1-OH | (R4) |
NN + hν → Productshν | (R5) |
Productshν + OH → Products2-OH | (R6) |
The ensemble of products from the initial NN-OH reaction is designated Products1-OH, and that from the reactions of the photolysis products Products2-OH. Parameters used in the model are summarized in Table S1† and include the gas-diffusion coefficient, the boundary layer thickness adjacent to the surface, the surface accommodation coefficient of OH, the desorption lifetime of OH, the partitioning coefficient of OH, the bulk diffusion coefficient of OH, NNs and products as well as the surface and bulk reaction rate coefficients of reactions (R4)–(R6). Parameters were determined based on literature values and by fitting to the measurements. Explanations of the final parameter values are summarized in Table S1.† Rapid OH surface uptake and slow gas diffusion may cause depletion of OH concentrations in the near-surface gas phase. The true uptake coefficient is calculated as the ratio of net uptake and gas collision from near-surface NN bulk to the surface using modeled near-surface gas-phase concentrations, while the effective uptake coefficient is calculated as the ratio of net uptake and gas collision from the gas phase using the average gas-phase concentration determined in the experiments.
To test for the loss of the NN in the absence of OH, difference spectra were also acquired in control studies of flowing IPN/O2 in the dark or photolysis of NN in the absence of IPN. No significant changes were detected under the NN + IPN/O2 flow in the dark but some small loss and product formation were observed under NN + UV with the 360 nm filter (yellow traces, Fig. 1).
The loss of the –NO2 asymmetric stretch (Fig. 1) during the OH reaction and the photolysis only was followed to generate plots of the number of NN molecules cm−2 (NNt) as a function of time (Fig. 2). To estimate the initial rate of loss as t → 0 where the surface is only NN, the data were fit using a third order polynomial fit of the form NNt = at3 + bt2 + ct + d, where a, b, c and d were obtained from the best fit to each data set. The initial slope (R0 = d(NNt)/dt) was obtained from the derivative of this polynomial with respect to time evaluated at t → 0 (i.e., the coefficient c):
(I) |
To determine the reaction probability (γ) for the NN + OH reaction, which is equal to the rate of loss of the NN normalized to the rate of gas-surface collisions of OH,55 the initial number of reactions cm−2 s−1 (R0), corrected for the loss due to photolysis in the absence of OH, is divided by the gas phase concentration of OH and the number of collisions per second from gas kinetic molecular theory:55,62
(II) |
In eqn (II), [OH] is the steady state gas concentration of OH radicals (molecules cm−3), R is the gas constant, T is temperature (K) and M is molar mass of OH (g mol−1). As seen in Table 1, the average reaction probabilities (± one standard deviation) range from (0.9 ± 0.2) × 10−2 for CLD to (1.6 ± 0.8) × 10−2 for IMD. Note that these are likely lower limits to the initial reaction probabilities since the first data points are taken at 30 min. In addition, as discussed below, the multiphase kinetics modeling suggests that the reaction is sufficiently fast that the OH concentration in the near-surface gas phase is limited by gas diffusion so that the actual near-surface concentration is somewhat smaller than the measured bulk gas phase concentration. This also leads to these values being lower limits.
NN (# experiments) | R 0 (initial # rxns cm−2 s−1 ± 1σ) | γ (±1σ) | γ ratio to IMD (±1σ) |
---|---|---|---|
a Corrected for the decay due to photolysis alone. | |||
IMD (7) | (1.9 ± 1.0) × 1011 | (1.6 ± 0.8) × 10−2 | 1 |
DNF (6) | (1.8 ± 0.7) × 1011 | (1.5 ± 0.6) × 10−2 | 0.94 ± 0.6 |
CLD (4) | (1.1 ± 0.2) × 1011 | (0.9 ± 0.2) × 10−2 | 0.56 ± 0.15 |
Abstraction of hydrogens from C–H groups is expected to be the primary reaction step. Using known structure–activity relationships (SAR) from gas phase OH reactions with alkanes, relative reactivities for the hydrogen abstraction can be calculated for the three NNs,55,63,64 with the caveat that this assumes that the relative reactivities in the gas–solid reactions follow those in the gas phase. The SAR predict that = 1.36 × 10−13 cm3 molecule−1 s−1, = 9.34 × 10−13 cm3 molecule−1 s−1 and = 1.95 × 10−12 cm3 molecule−1 s−1 at 298 K, with neighboring group factors (F) for –CH2–, 〉CH– and 〉C〈 of 1.23, F(〉N–) of 10 and F(–O–) of 6.1. For a C–H in a 5-membered ring, F is 0.64.63,65 Using the sum of the reactivity of the aliphatic C–H groups predicts gas phase rate constants of 2.40 × 10−11, 2.50 × 10−11, and 1.07 × 10−11 cm3 molecule−1 s−1 for IMD, DNF, and CLD respectively. This gives ratios to IMD of 1.0 for DNF and 0.45 for CLD. These ratios for predicted gas phase reaction rate constants are within experimental uncertainties (1σ) of the experimentally measured ratios of the reaction probabilities of 0.94 ± 0.6 for DNF and 0.56 ± 0.15 for CLD (Table 1). This calculation ignores the potential reactivity of the N–H groups as there was no evidence from identified products (see below) for attack on the amino groups. The C–H groups in the aromatic ring structures of IMD and CLD were also not included as SAR predicts them to be less reactive, and no ring-opening products were observed.
Sensitivity simulations by varying each parameter (Table S1†) indicated that the most sensitive parameters include the NN surface rate coefficient due to photolysis in the absence of OH, the NN bulk diffusion coefficient and the boundary layer thickness. Gas-phase diffusion of OH radicals across a boundary layer limits the decay of NN at early times (approximately <1000 seconds) as it causes OH radical concentrations close to the surface to be significantly reduced compared to the bulk average gas-phase concentrations. Hydroxyl radical concentrations in the bulk of the solid NN are small and therefore the majority of the NN loss in the presence of OH occurs via reaction at the surface rather than in the bulk. This is seen by the common curves in Fig. 3 for the first ∼1000 s regardless of the diffusion coefficients of NN, DNN, in the bulk. NN loss at later times is limited by the rate at which NN can diffuse to the surface, and hence the curves in Fig. 3 diverge for different modelled NN diffusion coefficients. Our results are consistent with the study by Arangio et al.66 where the decay of levoglucosan was limited by bulk-to-surface diffusion of the organic under high OH concentrations.
There are a number of uncertainties that impact the comparison between the experimental data and model predictions. Inputs to the KM-SUB model assumed that the film was evenly distributed on the ATR crystal surface, i.e., had a uniform thickness, which is likely not to be the case. Measuring the OH concentration under the same conditions as the NN photolysis is also challenging, and we estimate the uncertainty in the OH concentration to be ±25%. Experimental measurements show that direct photolysis is not occurring throughout the entire bulk as this would lead to more molecules being lost for thicker films (Fig. S3†). (This is in contrast to earlier studies of the photolysis of thin films of the NN43–45 where the light source was different and irradiation was directly from above the film; in the present studies, the light beam from a different lamp was reflected from a mirror at an angle onto the film which was likely quite irregular at the surface.) Thus, it was assumed in the model that all of the photolytic loss occurred in the surface layer. However, it should be noted that even if the photolysis occurs throughout the entire bulk, the impact on the best fit diffusion coefficients of NN is relatively small. For example, the best fit to the data for this scenario was obtained by decreasing the NN bulk diffusion coefficient by approximately a factor of 1.5–3, and at the same time decreasing the photolysis rate coefficients in both the bulk and the surface by a factor of 200–400 compared to the values in Table S1.†
The scatter in the experimental data combined with different combinations in the model inputs that can provide a reasonable match to the data preclude assigning unique, accurate values to some model input parameters such as bulk diffusion coefficients. However, these uncertainties do not affect the overall conclusions and insights from the model that the reaction occurs mainly at the surface and is initially limited by gas-phase diffusion of OH to the surface and then limited by diffusion of NN from the bulk into the surface region.
Table 2 summarizes the measured accurate masses of the [M + H]+ ions from the IMD-OH reaction, the corresponding molecular formulae, the errors between the measured accurate masses and the exact masses, and the molecular structures of the products proposed based on the MS/MS fragmentation patterns summarized in the table and reasonable reaction mechanisms. In all chromatograms, the [M + H]+ ion of the parent IMD at m/z 256 is observed. Products of the OH reaction include the IMD structure with a carbonyl (m/z 270) or alcohol (m/z 272) group, desnitro (m/z 211) and urea (m/z 212) derivatives that are also seen to a smaller extent than under UV irradiation only, and their carbonyl oxidation products (m/z 225 and 226). Additionally, products observed at m/z 131 and 158 are identified in HRMS as nitroguanidine and 6-chloronicotinic acid which have been observed as oxidation products in a number of aqueous oxidation studies.34,37–39,47,51 Products with alkene groups are also observed (m/z 209 and 254).
Detected ion [M+ H]+ | Ion formula | Error (ppm) | Proposed product structurea | MS/MS fragments |
---|---|---|---|---|
a Only one isomer shown for simplicity. The MS/MS for the other isomers were consistent with the alternate structures. b Also reported in Schippers and Schwack73 on tomato plant surfaces. c IMD photolysis product. d MS/MS matched authentic standard. e MS/MS is consistent with the fragmentation reported by Fusetto et al.85 | ||||
256.0594 | IMD, [C9H10O2N5Cl + H]+ | −0.75 | 209 (–HNO2), 175 (–NO2, –Cl), 128, 126, 84 | |
131.0562 | [C3H6O2N4 + H]+ | −1.17 | 85 (–NO2) | |
158.0003 | 6-Chloronicotinic acid, [C6H4O2NCl + H]+ | −0.27 | 140 (–H2O), 112 (–H2O, –CO), 78 | |
185.0592b | [C7H9N4Cl + H]+ | 1.66 | 168 (–NH3), 143 (–C(NH)2), 126, 107 | |
209.0585 | [C9H9N4Cl + H]+ | 1.47 | 173 (–HCl), 126 | |
211.0745b,c,d | Desnitro IMD, [C9H11N4Cl + H]+ | 0.11 | 175 (–HCl), 133, 126, 84 | |
212.0585b,c,d | IMD urea, [C9H10ON3Cl + H]+ | 0.01 | 175, 128, 126, 99 | |
225.0535 | [C9H9ON4Cl + H]+ | −0.11 | 126 | |
226.0378b | [C9H8O2N3Cl + H]+ | −0.20 | 169, 126 | |
254.0438e | [C9H8O2N5Cl + H]+ | −0.72 | 236 (–H2O), 207 (–HNO2), 206 (–H2O, –NO), 205 (–H2O, –HNO), 171 (–H2O, –NO, –Cl), 167, 126 | |
270.0387 | [C9H8O3N5Cl + H]+ | −0.54 | 226 (–N2O), 223 (–HNO2), 189 (–NO2, –Cl), 161 (–NO2, –Cl, –CO), 126 | |
272.0543b | [C9H10O3N5Cl + H]+ | −0.52 | 228 (–N2O), 209 (–NO2, –OH), 173 (–NO2, –Cl, –H2O), 144 |
Multiple peaks often appear at the same mass due to different isomers. These can be more clearly seen in extracted ion chromatograms (EIC) for selected values of m/z. For example, Fig. 5 shows the EIC for m/z at 256 for the parent IMD as well as 270, 272, 254 and 225 due to products of the OH reaction. One significant peak is seen for m/z 254, two for m/z 272 and 225, and three for m/z 270. Attack of OH at the C–H groups as discussed above could occur at three different sites, giving three different isomeric products. However, the isomers representing attack on the two sites on the five-membered ring are sufficiently similar that the products may co-elute, leading to two peaks rather than three. One of the peaks for m/z 225 has a larger area, indicating either one isomer is formed preferentially and the third is not formed at all, or more likely, the larger peak intensity is due to the contribution of both 5-membered ring isomers.
Fig. 5 ESI(+) extracted ion chromatograms (EIC) of species with m/z 256 (IMD), 270, 272, 254 (offset for clarity) and 225 for IMD + OH reaction. |
Most of the product peaks are also seen in DART-MS spectra (Fig. 6). There is not an exact correspondence in terms of their mass-to-charge ratios and their relative intensities, as some compounds may not survive the heating used in DART-MS to volatilize the sample. Thus, DART-MS detects gas phase species while ESI detects soluble species. For example, the peak at m/z 272 assigned to the alcohol derivative of IMD is the largest product peak in the UPLC chromatogram (Fig. 4a) but is much smaller in the DART-MS spectrum (Fig. 6). This is likely due to fragmentation of the alcohol at the higher temperatures in DART to yield an ion at m/z 254 due to [M + H − H2O]+. However, the EIC data showed that the alcohol (m/z 272) and alkene (m/z 254) chromatographically separated (Fig. 5), indicating that the olefin is formed as its own unique product of the OH reaction. To the best of our knowledge, this is the first observation of this alkene product from OH reactions, although the desnitro-olefin product (Table 2, m/z 209) has been reported in oxidations in aqueous solution.33,37,47 From an environmental impacts perspective, it is interesting that the IMD–olefin has been widely observed as a plant metabolite and is more toxic to insects than IMD itself.67–69 This product is expected to undergo additional oxidation processes in air. Many of the oxidation products identified here have been observed in soils at pesticide manufacturing sites in China.70
The ATR-FTIR spectra are consistent with the mass spectrometry data. The difference spectrum on reaction with OH (Fig. 1a) shows loss of the parent compound and the formation of oxidized products with absorption bands in the 1600–1800 cm−1 region. These infrared bands are assigned to CN and CO groups, the latter group being present in many of the products identified by HRMS and DART-MS. The broad feature in the 3000–3300 cm−1 region reflects a hydrogen-bonded OH, for example from 6-chloronicotinic acid (m/z 158) which was also identified from HRMS and DART-MS.
There are a great deal of data for oxidation of NN in aqueous media. For example, the most commonly observed aqueous product for IMD is 6-chloronicotinic acid,34,37–39,47,51 which is also observed in this work. The carbonyl products at m/z 270 were observed in several other studies.33,34,51 Alcohol products at m/z 272 have also been observed in the aqueous studies.34,36,37,47,51 IMD urea is frequently observed in aqueous reactions,33,37,38,41,47,51 and desnitro IMD has also been reported.47 However in the aqueous reactions, there are other potential processes occurring in addition to OH oxidation, such as photolysis, ozonolysis, and catalytic reactions that may be contributing to product formation. Many smaller products (MW 100–157) are commonly observed in the aqueous reactions,37,39–41 as the goal is often complete mineralization of the NN.
Most relevant to the current studies are photodegradation reactions on tomato plant surfaces that were conducted outdoors in ambient air.71 Under these conditions, it is likely that OH was present and contributed to the oxidation. The most important products identified were the IMD-alcohol and a nitroso-derivative, along with the desnitro and urea products which were likely formed at least in part by direct photolysis. Other studies of the photodegradation on solid surfaces72,73 identified very similar products to those observed here (Table 2).
Fig. 7 summarizes proposed mechanisms for the formation of the major products. Both the kinetics and products point to abstraction of a hydrogen from a C–H bond as being the initial step. This produces an alkyl radical which in the presence of O2 forms a peroxy radical.55 Both RO2 radicals and NO are generated in the gas phase during the IPN/O2 photolysis, and the NN peroxy radicals may react with these gas phase species to form alkoxy radicals, alcohols, and carbonyls. The oxidized desnitro and urea carbonyl products identified in these studies are likely due to secondary reactions of the OH with the desnitro and urea products.
Detected ion [M + H]+ | Ion formula | Error (ppm) | Proposed structurea | MS/MS fragments |
---|---|---|---|---|
a Only one isomer shown for simplicity. The MS/MS for the other isomers were consistent with the alternate structures. b DNF photolysis product. | ||||
203.1142 | DNF, [C7H14O3N4 + H]+ | 1.4 | 129, 114, 100, 87, 73 | |
154.0976 | [C7H11ON3 + H]+ | 0.2 | 69 | |
156.1128b | [C7H13ON3 + H]+ | −1.14 | 83 | |
158.1284b | Desnitro DNF, [C7H15ON3 + H]+ | −1.09 | 102 | |
159.1124b | DNF urea, [C7H14O2N2 + H]+ | −1.13 | 102, 85, 67 | |
172.108 | [C7H13O2N3 + H]+ | −0.83 | 155, 116, 57 | |
189.0982 | [C6H12O3N4 + H]+ | 0.42 | 131, 115, 99, 73 | |
201.0988 | [C7H12O3N4 + H]+ | 2.30 | 187 (–CH2), 155 (–NO2), 127, 112 | |
217.0928 | [C7H12O4N4 + H]+ | −1.09 | 171 (–NO2), 127, 112, 87, 73 | |
219.1083 | [C7H14O4N4 + H]+ | −1.33 | 144, 129, 114, 87 |
Detected ion [M + H]+ | Ion formula | Error (ppm) | Proposed structurea | MS/MS fragments |
---|---|---|---|---|
a Only one isomer shown for simplicity. The MS/MS for the other isomers were consistent with the alternate structures. b CLD photolysis product. | ||||
250.0158 | CLD, [C6H8O2N5ClS + H]+ | −0.89 | 169 (–NO2, –Cl), 132, 113 | |
205.0311b | Desnitro CLD, [C6H9N4ClS + H]+ | 0.93 | 132, 113 | |
206.0152b | CLD urea, [C6H8ON3ClS + H]+ | 1.27 | 175, 132, 120, 113, 86 | |
219.0101 | [C6H7ON4ClS + H]+ | −0.66 | 202, 163, 146, 75, 57 | |
219.9940 | [C6H6O2N3ClS + H]+ | −1.04 | 189, 163, 146, 120 | |
236.0002 | [C5H6O2N5ClS + H]+ | −0.64 | 155 (–NO2, –Cl), 132, 113, 96 | |
263.9950 | [C6H6O3N5ClS + H]+ | −1.13 | 183 (–NO2, –Cl), 145 | |
266.0108 | [C6H8O3N5ClS + H]+ | −0.42 | 222, 203, 191, 167, 147, 129, 86 |
Fig. 8 Proposed mechanisms for formation of OH + DNF reaction products (where more than one isomer is possible, only one is shown for simplicity). |
Fig. 9 Proposed mechanisms for formation of OH + CLD reaction products (where more than one isomer is possible, only one is shown for simplicity). |
While IMD has three potential locations for aliphatic C–H hydrogen abstraction, DNF has six (four CH2 groups, one CH3 group, and one CH group) and CLD has two, a secondary CH2 group and a primary CH3 group. The fact that the extracted ion chromatogram for a carbonyl product of CLD oxidation at m/z 264 shows two isomers of similar intensities (Fig. S6†) suggests that hydrogen abstraction occurs efficiently from both the CH2 and CH3 groups. Evidence of hydrogen abstraction on the CH3 group was also observed in the DNF–OH reaction with a product at m/z 189 (Fig. 4b). This was surprising because the SAR for gas phase reactions predict that the terminal CH3 group will be less reactive. However, results from a combination of experiments and theory show that the bond dissociation energy (BDE) of a primary C–H group adjacent to an amine nitrogen is about 4 kJ mol−1 larger than that for a secondary C–H adjacent to a nitrogen,77,78 much less than a difference of ∼13 kJ mol−1 in BDE for primary vs. secondary C–H groups in alkanes.77 This suggests that the difference between abstraction from a terminal –CH3 group and a secondary CH2 group may be less in the case of amines, resulting in relatively more oxidation of the terminal methyl groups in DNF and CLD.
The ATR-FTIR spectra of the reaction products (Fig. 1) are consistent with the mass spectrometry identification of products summarized in Tables 3 and 4. A variety of carbonyl-containing products lead to absorption bands in the 1700–1800 cm−1 region, but unlike IMD, there are no significant broad peaks in the 3000–3300 cm−1 region. This is consistent with the identification of 6-chloronicotinic acid as the source of a hydrogen-bonded carboxylic acid only in the IMD reaction since corresponding acids cannot be formed in the DNF and CLD reactions.
(III) |
In air, the NN peroxy radical intermediate on the surface, RO2(surf), is likely to react with gas phase R′O2 radicals or NO via reactions (R7) or (R8):
(R7) |
(R8) |
In California's agriculturally-dominant San Joaquin Valley, for example, the NO concentration varies seasonally with an average of ∼8.2 × 108 molecules cm−3 in the summer and ∼1.5 × 1011 molecules cm−3 in the winter.80 Typical R′O2(g) concentrations measured at various locations around the world peak at ∼5 × 108 molecules cm−3 at midday.81–84 Using typical rate constants for reactions (R7) and (R8),55 first order rates are k7[R′O2(g)] = 5 × 10−4 s−1 and k8[NO] = 0.007–1.35 s−1. Thus, the NO reaction with RO2 to form carbonyl products, of both the parent NN and the desnitro and urea photolysis products, is likely favored under environmentally realistic conditions, even at low NO concentrations. The reaction with NO forms an alkoxy radical that depending on its structure, can decompose, react with O2 or isomerize. In the solid phase, reaction of alkoxy with O2 to form the carbonyl compounds shown in Fig. 7–9 seems most likely. Although not shown in the mechanisms, some formation of organic nitrates from the RO2 reactions with NO may also occur.55
In short, studies of the environmental fates and impacts of NN should take into account not only the parent compound but also its oxidation and photolysis products.
Footnotes |
† Electronic supplementary information (ESI) available: Reaction cell (Fig. S1); details of OH measurements, including observed 13CO2 formation during photolysis of IPN in the presence of 13CO (Fig. S2); experimental details for UPLC-HESI-HRMS and DART-MS measurement; KM-SUB model results (Fig. S3) and summary of parameters used as model inputs (Table S1); extracted ion chromatograms (EIC) for multiply oxidized products measured using UPLC-HESI-HRMS and DART-MS for DNF + OH (Fig. S4 and S5), and CLD + OH (Fig. S6 and S7). See DOI: https://doi.org/10.1039/d2ea00134a |
‡ Co-first authors. |
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