Gas‐phase OH oxidation kinetics of pyrazine, pyrimidine, and pyridazine

Abstract

The gas-phase kinetics of the OH-oxidation of three six-membered nitrogen heterocycles C 4 N 2 H 4 isomers, namely pyrazine, pyrimidine, and pyridazine, were determined in a photochemical reactor coupled to a FTIR spectrometer and a SPME-GC/MS, at atmospheric pressure over the temperature range 298-353 K. Experiments were conducted using the relative rate method where two different references were employed for each studied compound. The obtained Arrhenius expressions are (in cm 3 molecule -1 s -1 ) OH+pyrazine ( ) = (7.21 ± 1.57) × 10 −13 exp (− (166 ± 71) ∕ ) , OH+pyrimidine ( ) = (1.18 ± 0.57) × 10 −12 exp (− (362 ± 155) ∕ ) , OH+pyridazine ( ) = (9.82 ± 7.20) × 10 −12 exp (− (1063 ± 248) ∕ ) .These results were compared with available literature data for other heterocyclic compounds and discussed in terms of structure-reactivity and temperature dependency. Their tropospheric lifetimes with respect to reaction with OH radicals were then calculated and found to be of the order of 29, 34, and 37 days for pyrazine, pyrimidine, and pyridazine, respectively.

A theoretical study on the mechanism and kinetics of the reactions between diazine isomers and OH radicals

“…It is also found that hydrogen abstraction channels always dominate. The obtained rate constants are close to the experimental ones obtained by Samir et al 4 and full agreement can be obtained by adjustments of the barrier heights by less than 1 kcal mol -1 .…”

“…5 Field measurements to assess the atmospheric concentration of diazine isomers are lacking, but there is information related to the persistence of pyrimidine and its derivatives. 6 The recent experimental research done by Samir et al 4 employed a rigid atmospheric simulation chamber coupled to an FTIR spectrometer and SPME-GC/MS over the temperature range 298-353 K at atmospheric pressure, providing rate constants for OH oxidation of the diazine isomers, which are as follows in cm 3 molecule -1 s -1 at 298 K: 𝑘 𝑃𝑦𝑟𝑎𝑧𝑖𝑛𝑒 = 4.11 × 10 −13 , 𝑘 𝑃𝑦𝑟𝑎𝑑𝑖𝑧𝑖𝑛𝑒 = 3.12 × 10 −13 , 𝑘 𝑃𝑦𝑟𝑎𝑚𝑖𝑑𝑖𝑛𝑒 = 3.48 × 10 −13 . Accordingly, tropospheric lifetimes of 29, 37, and 34 days were found for pyrazine, pyridazine, and pyrimidine, respectively.…”

“…Therefore, in this article transition state theory (TST) calculations are performed in order to obtain thermal reaction rate constants of the title reactions. The calculated rate constants are compared to the ones obtained by Samir et al 4 as the only available experimental study on the title reactions. The atmospheric implications are also discussed.…”

“…3 It is also used as a precursor in synthesis of more complex molecules. 4 Lastly, pyrimidine (or 1,3-diazine, 𝐶 2𝑣 ) is like pyridazine also used in pesticides and has in addition clinical applications. 4 Excesses of the diazine isomers can be transferred to the atmosphere, where they can react with OH radicals, NO 3 radicals, and O 3 molecules.…”

“…4 Lastly, pyrimidine (or 1,3-diazine, 𝐶 2𝑣 ) is like pyridazine also used in pesticides and has in addition clinical applications. 4 Excesses of the diazine isomers can be transferred to the atmosphere, where they can react with OH radicals, NO 3 radicals, and O 3 molecules. 5 Field measurements to assess the atmospheric concentration of diazine isomers are lacking, but there is information related to the persistence of pyrimidine and its derivatives.…”