Brahim Samirab,
Carmen Kalaliana,
Estelle Rotha,
Rachid Salghib and
Abdelkhaleq Chakir*a
aGroupe de Spectrométrie Moléculaire et Atmosphérique GSMA, UMR CNRS 7331, Université de Reims, Moulin de la Housse B.P. 1039, 51687 Reims Cedex 2, France. E-mail: abdel.chakir@univ-reims.fr
bLaboratory of Environmental Engineering and Biotechnology, ENSA, University Ibn Zohr, PO Box 1136, 80000 Agadir, Morocco
First published on 30th August 2019
In this work, we report the gas phase UV absorption spectra and the kinetics of the OH-oxidation of 1H-1,2,3-triazole and pyrazole. UV spectra were determined between 200 and 250 nm, at 350 ± 2 K and at pressures between 0.09 and 0.3 Torr. The reported maximal UV absorption cross sections are (cm2 per molecule): σ206 nm, 1H–1H-1,2,3-triazole = 2.04 × 10−18 and σ203 nm, pyrazole = 5.44 × 10−18. The very low absorption capacity of these compounds beyond 240 nm indicates that their atmospheric photodissociation is negligible. The OH-oxidation of these species was performed in an atmospheric simulation chamber coupled to an FTIR spectrometer and to a GC/MS over the temperature range 298–357 K and at atmospheric pressure. Experiments were conducted in relative mode using benzaldehyde, trans-2-hexenal and heptane as references. The obtained rate constants at 298 K were (×10−11 cm3 per molecule per s): k(OH + 1H-1,2,3-triazole) = 2.16 ± 0.41; k(OH + pyrazole) = 2.94 ± 0.42. These results were compared to those available in the literature and discussed in terms of structure-reactivity and temperature dependency. Their tropospheric lifetimes with respect to reaction with OH radicals were then estimated.
Palmer et al.20 measured the vacuum ultraviolet photoabsorption spectrum of 1H-1,2,3-triazole in the spectral range of 115–245 nm at 35 °C. The obtained measurements were analyzed using the comparison of the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. However, the reported cross sections were very noisy beyond 220 nm. Thus, more accurate UV spectra of these species are required. To understand the atmospheric reactivity and to enrich kinetic and spectroscopic databases regarding these species further laboratory measurements are needed. For this purpose, in this work the UV spectra of the simplest five-membered nitrogen heterocycles, namely 1H-1,2,3-triazole and pyrazole alongside their kinetic degradation by OH radicals are investigated. This work provides the first kinetic data for the reactions of 1H-1,2,3-triazole and pyrazole with OH as a function of temperature. The obtained spectra are compared with previous studies, and their atmospheric lifetimes with respect to OH radicals are estimated. The chemical structures of the investigated compounds are given below.
(I) |
Experiments were carried out under static regime, in the spectral range 200–300 nm, at temperature T = 350 ± 2 K to avoid any condensation and at pressures 0.09–0.25 Torr for 1H-1,2,3-triazole and 0.09–0.3 Torr for pyrazole. The concentration of the studied compound was chosen in such way to obtain optical density values between 0.05 and 2.5, conditions for which the linearity of Lambert Beer's law is respected.
An Equinox 55 FTIR spectrometer was used to monitor the consumption of the reactants and reference compounds. The spectral resolution was in the range of 2–0.5 cm−1 in the spectral domain 600–4000 cm−1. 24 UV lamps symmetrically arranged emitting from 300 to 400 nm were used to generate OH by the photolysis of nitrous acid which was produced in a drop-wise by the addition of 10% of sulphuric acid to a solution of 0.2 M of sodium nitrite. A small flow of nitrogen gas was used to carry the generated nitrous acid into the reactor. In this work, benzaldehyde, heptane and trans-2-hexenal were used as reference compounds. The reference compounds were chosen in such way that at least one absorption band of the studied compound does not exhibit any interference with those of the chosen references and vice versa. Measured amounts of reagents were flushed from calibrated bulbs into the reactor through a stream of ultra-pure air. The reactor was then filled at atmospheric pressure with ultra-pure air. The experimental conditions used for the kinetic study are summarized in Table 1. As can be seen in Table 1, the initial reagents concentrations were chosen to minimize any secondary reactions and to be discernable analytically. It should be noted that, during an experiment, infrared spectra were recorded every 5 minutes. Each spectrum constitutes the average of 20 accumulated spectra with a resolution of 2 cm−1. The kinetic monitoring of the reaction medium was conducted until more than 30% consumption of the studied compound and the used reference. In this study the duration of experiments goes from 4 to 6 hours.
1H-1,2,3-Triazole | Pyrazole | |
---|---|---|
Temperature (K) | 298–357 | 298–357 |
Pressure (Torr) | 760 ± 5 | 760 ± 5 |
Reference compound | Benzaldehyde, heptane, trans-2-hexenal | Benzaldehyde, heptane |
Optical path (m) | 56–64 | 56–64 |
[Triazole or pyrazole] molecules per cm3 | (2–4) × 1014 | (7–8) × 1014 |
[Reference] molecules per cm3 | (4–6) × 1014 | (5–9) × 1014 |
Spectral range (cm−1) | (905–943); (3480–3552) | (3466–3549); (3480–3552) |
Spectral range (cm−1) (reference) | Benzaldehyde: (2700–2760); (2770–2832) | Benzaldehyde: (2700–2760); (2770–2832) |
Heptane: (1430–1500) | Heptane: (1337–1403); (1430–1500) | |
trans-2-Hexenal: (2660–2760) |
The reaction medium was also sampled using solid phase microextraction fibers (SPME) mainly polydimethylsiloxane/divinylbenzene fibers (PDMS/DVB) exposed for 1 min into the chamber. The SPME fiber was then introduced into the GC/MS injector operating at 220 °C. The GC/MS analysis were performed in TIC mode (Total Ion Current).
The measurements were performed in purified air provided by Air Liquide (>99.9999%). The reagents: 1H-1,2,3-triazole (97%), pyrazole (98%), benzaldehyde (99.5%), trans-2-hexenal (98%) and heptane (97%) were provided by Sigma-Aldrich. They were further purified by distillation and by repeated freeze–pump–thaw cycles before use in both the spectroscopic and kinetic studies.
Fig. 1 shows the absorption spectra of 1H-1,2,3-triazole and pyrazole. These spectra consist of a broad continuum with a strong absorption band between 200–240 nm. They are very similar in terms of widths, however pyrazole absorbs 3 times more than 1H-1,2,3-triazole. The absorption maximum of 1H-1,2,3-triazole is localized at 206 nm (σ206, 1H-1,2,3-triazole = 2.04 × 10−18 cm2 per molecule) and at 203 nm (σ203, pyrazole = 5.44 × 10−18 cm2 per molecule) for pyrazole. This absorption band is attributed to the π–π* electronic transitions band, characteristic of aromatic compounds. Nevertheless, the n–π* transition was not observed probably due to its low intensity, so that it is completely drowned in the π–π* band.
The addition of a nitrogen atom to the heterocycle exerts an hypsochromic effect of about 4 nm for 1H-1,2,3-triazole with respect to the pyrazole and an hypochromic effect since the absorption intensity of 1H-1,2,3-triazole is 3 times lower than that of pyrazole.
The absorption of these two compounds beyond 290 nm, spectral range which corresponds to radiation reaching the troposphere, is very low (≤10−21 cm2 per molecule). Thus, their atmospheric photodissociation processes are negligible.
The main error sources are attributed to the low vapor pressure of the studied compounds and their tendency to stick to the wall of the absorption cell, which distorts the pressure measurements and entails an error on the concentration calculation. To minimize this error, we increased the temperature of the cell (353–357 K) so that the uncertainty on the concentration measurements did not exceed 15%.
Other sources of errors in the spectral measurements can result the calibration wavelength, the temperature, the optical length and the absorbance. However, the uncertainty due to these parameters does not exceed 5%.
For pyrazole, only one experimental study exists in literature. Walker et al.19 determined the UV-Vis spectrum of pyrazole between 106 and 250 nm with an increment of 0.05 nm. The obtained spectrum is in a good agreement with that of literature, with cross sections deviations less than 20% in the domain 200–220 nm. Nevertheless, beyond 230 nm, Walker et al.19 have cross sections 2 to 10 times higher than ours. Beyond 220 nm the cross sections of Walker et al.19 oscillates around 5 × 10−19 cm2 per molecule including negative values and are very noisy explaining our discrepancies (Fig. 3).
The rate coefficients of the reaction of OH radicals with the reference compounds used are (in cm3 per molecule per s):
k(OH+benzaldehyde) (T) = 5.33 × 10−12exp((2020 ± 710)/RT) (ref. 25) | (1) |
k(OH+trans-2-hexenal) (298 K) = (4.44 ± 0.94) × 10−11 | (2) |
k(OH+n-heptane) (298 K) = (6.68 ± 0.48) × 10−12 | (3) |
The compound and the reference are simultaneously subjected to oxidation by OH radicals. The reactions taking place in the reactor are:
Analyte + OH → products, kOH |
Analyte → products, kp |
Reference + OH → products, kref |
Secondary processes such as wall loss and photolysis under UV radiation can occur. In order to evaluate these losses, experiments were conducted in the absence of UV radiation and/or HONO. These experiments showed that the photolysis of these compounds and their reaction with HONO are negligible. However, wall losses were (2.50 ± 0.50) × 10−5 s−1 for 1H-1,2,3-triazole and (3.50 ± 0.50) × 10−5 s−1 for pyrazole.
To take into account to the wall loss, the following relation was used to determine the rate constants of the reaction of OH radicals with 1H-1,2,3-triazole and pyrazole:
(II) |
According to eqn (II), the plot of as a function of is a straight line whose slope is equal to R = kOH/kRef. Knowing the value of kRef from literature, we can deduce kOH for the studied compounds. Fig. 4 and 5(a) and (b) show an example of this plot for 1H-1,2,3-triazole and pyrazole at room temperature obtained by FTIR and GC/MS with different references. Good linearity is observed with a correlation coefficient greater than 94%.
Fig. 4 Relative kinetics of the reaction of the OH radicals with 1H-1,2,3-triazole for different references obtained by FTIR at 298 K. |
The rate constants of the reaction of 1H-1,2,3-triazole and pyrazole with OH radicals obtained using different references at ambient temperature are very close with a difference ≤15%. Given the low vapor pressure of these compounds, temperature studies were carried out between 298 and 357 K, using benzaldehyde as a reference. The rate coefficients obtained at different temperatures by FTIR (1H-1,2,3-triazole and pyrazole) and GC/MS (pyrazole) are summarized in Table 2. Each experiment was performed three times under the same experimental conditions. The relative error on the slope corresponds to one standard deviation.
References | T (K) | 1H-1,2,3-Triazole | Pyrazole | ||
---|---|---|---|---|---|
R = kOH/kRef | k1H-1,2,3-triazole+OH (10−11 cm3 per molecule per s) | R = kOH/kRef | kpyrazole+OH (10−11 cm3 per molecule per s) | ||
a Results obtained in SPME-GC/MS.b Uncertainty on R is 1σ.c Uncertainty on k1H-1,2,3-triazole+OH and kpyrazole+OH calculated with error propagation method. | |||||
Benzaldehyde | 298 | 1.66 ± 0.14 | 2.00 ± 0.60 | 2.43 ± 0.10 | 2.93 ± 0.84 |
313 | 1.61 ± 0.10 | 1.86 ± 0.51 | 2.36 ± 0.10 | 2.73 ± 0.78 | |
333 | 1.46 ± 0.10 | 1.61 ± 0.41 | 2.13 ± 0.13 | 2.36 ± 0.62 | |
357 | 1.40 ± 0.10 | 1.47 ± 0.35 | 2.14 ± 0.10 | 2.25 ± 0.54 | |
Heptane | 298 | 3.31 ± 0.02 | 2.22 ± 0.16 | 4.27 ± 0.10 | 2.91 ± 0.20 |
4.38 ± 0.11a | 2.98 ± 0.22a | ||||
trans-2-Hexenal | 298 | 0.51 ± 0.01 | 2.26 ± 0.48 | — | — |
The Arrhenius diagram was then determined by plotting ln(kOH) as a function of 1/T (Fig. 6). The Arrhenius equations for the OH oxidation of 1H-1,2,3-triazole and pyrazole were as follows (in cm3 per molecule per s):
kOH+1H-1,2,3-triazole (T) = (2.93 ± 0.20) × 10−12exp((570 ± 43)/T) |
kOH+pyrazole (T) = (5.42 ± 0.60) × 10−12exp((499 ± 74)/T) |
Uncertainties on the Arrhenius parameters (activation energy Ea and pre-exponential factor A) were determined by the least square minimization method.
The obtained kinetic data show that the OH-oxidation of 1H-1,2,3-triazole and that of pyrazole exhibit a low negative temperature dependence. The rate constants decrease by about 26% for 1H-1,2,3-triazole and 28% for pyrazole when the temperature goes from 298 K to 357 K.
(III) |
Considering (III)) the main error sources in this study are:
• The uncertainty on the rate constant of the reference given by the literature kRef varies between 5 and 20%.
• The experimental error corresponds to the uncertainty on the determination of the slope which generally depends on the integration accuracy of the IR bands and chromatographic peaks. This error is minimized by the repetition of several experiments, and the kinetic tracking of IR bands that do not interfere with parasitic peaks. Indeed, this error is estimated between 5% to 15%.
We can note that the kinetic measurements performed with different references and analytical techniques (IR and GC/MS) were reproducible. An excellent agreement between the different determinations was observed with a difference that does not exceed 8%.
In a study carried out by Samuni et al.28 on the reaction of OH radicals with pyrazole in aqueous phase, it was proved that this reaction mainly occurs by adding the OH radical to the adjacent carbon of the nitrogen atom. In addition, Dillon et al.17 observed that the gas phase oxidation of pyrrole, a compound having a chemical structure similar to the compounds studied in this work, by OH radicals follows the same trend concerning the variation of the rate constant with the temperature: kOH slightly varies with temperature exhibiting a negative temperature coefficient. These authors also performed a theoretical study to elucidate the mechanism of the reaction between pyrrole and OH radicals. The results showed that the reaction is preferably carried out by adding the OH radical to the pyrrole ring (C4H5N). This process leads to the formation of a pre-reactive complex where the OH radical forms a hydrogen bond with the aromatic π system, slightly excentric on the carbon side of C4H5N. The most favorable addition concerns the carbons adjacent to the nitrogen atom. By analogy to the mechanism established for the OH-oxidation of pyrazole in the aqueous phase28 and those of pyrrole in the gas phase,17 we can suppose the OH-reaction of 1H-1,2,3-triazole and pyrazole follows the same mechanism as that described above.
Compounds | T (K) | k (cm3 per molecule per s) | Techniquesa | References |
---|---|---|---|---|
a RM: relative method; RE-MS: flow reactor mass spectrometry; FP-FR: flash photolysis resonance fluorescence; PL-FIL: laser-induced fluorescence laser photolysis.b Reference: propene.c Reference benzaldehyde, analysis by FTIR.d Reference heptane, analysis by FTIR.e Reference heptane, analysis in GC/MS.f Reference: n-hexane. | ||||
Pyrrole | 298 ± 2 | (1.20 ± 0.04) × 10−10 | RMb | 16 |
298 ± 2 | (1.05 ± 0.06) × 10−10 | FP-FR | 17 | |
298 ± 2 | (1.28 ± 0.10) × 10−10 | PL-FIL | 18 | |
1,2,3-Triazole | 298 ± 2 | (2.00 ± 0.60) × 10−11 | RMc | This work |
298 ± 2 | (2.22 ± 0.20) × 10−11 | RMd | This work | |
298 ± 2 | (2.26 ± 0.48) × 10−11 | RMe | This work | |
Pyrazole | 298 ± 2 | (2.93 ± 0.84) × 10−11 | RMc | This work |
298 ± 2 | (2.91 ± 0.2) × 10−11 | RMd | This work | |
298 ± 2 | (2.98 ± 0.22) × 10−11 | RMe | This work | |
Thiophene | 298 | (9.6 ± 0.3) × 10−12 | RMf | 31 |
Furan | 298 | (4.1 ± 0.3) × 10−11 | RMf | 32 |
The data presented in this table highlight the following points:
• The rate constants of the heterocyclic compounds with five atoms are of the same order of magnitude.
• Pyrrole is the most reactive towards OH radicals with a ten times higher rate constant than 1H-1,2,3-triazole and pyrazole. Thus, replacing a carbon atom by a nitrogen in 5 atoms aromatic ring induces a deactivating effect on the reaction: k(OH + pyrrole) ≫ k(OH + pyrazole) > k(OH + 1H-1,2,3-triazole).
According to the SAR method29 which estimates OH reaction rate coefficient for gaseous phase of compounds based on molecular structure, a difference of 15% is observed between the experimental value and the estimated one for the OH-reaction of pyrazole. However, for 1H-1,2,3-triazole the difference is very large, the experimental value being 200 times higher than the estimated value. This discrepancy can be explained by the fact that, for this compound the SAR procedure uses an addition factor of 0.1 × 10−12. This value is based on the kinetic of the reaction between the OH radicals and 1,2,3 triazine,30 a compound whose structure is not similar to that of 1H-1,2,3-triazole. Consequently, the experimental value determined in this work for the rate coefficient of OH-reaction with 1H-1,2,3-triazole could be used as an addition factor to estimate the rate constants between OH radicals and chemical compounds having a chemical structure close to that of 1H-1,2,3-triazole.
τ = 1/kx[X] | (IV) |
The lifetime values τ of 1H-1,2,3-triazole and pyrazole with respect to OH radicals are 14 h and 9 h respectively. A 24 hours average concentration of OH radicals in the atmosphere equal to 1 × 106 molecule per cm3 was used in the calculation.32
The atmospheric lifetimes of these compounds are relatively short (few hours) indicating that these compounds are non-persistent and can undergo a fast-photochemical transformation close to their emission source contributing therefore to the photochemical pollution mainly at the local scale.
Finally, it is noteworthy to mention that information regarding the atmospheric oxidation mechanisms of these species are still required to better evaluate their atmospheric impacts. Quantum chemistry calculations concerning the OH-oxidation kinetic of these compounds are yet necessary to explore in depth the reactions mechanisms and to fully explain the present experimental findings regarding their structural effect on their reactivity.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra04235k |
This journal is © The Royal Society of Chemistry 2019 |