Understanding the mechanism of hydrazine oxidation reaction by OH radical along with the rate constants of all possible pathways leads to explain the fate of hydrazine in the atmosphere. In this article, the comprehensive mechanisms and kinetics of the hydrazine plus hydroxyl radical reaction have been investigated theoretically at different temperatures and pressures. To achieve the main goals, a series of high levels of quantum chemical calculations have been widely implemented in reliable channels of the H-abstraction, SN2, and addition/elimination reactions. The energy profile of all pathways accompanied by the molecular properties of the involved stationary points has been characterized at the MP2, M06-2X, and CCSD(T)/CBS levels. To estimate accurate barrier energies of the H-abstraction channels, large numbers of the CCSD (T) calculations in conjunction with various augmented basis sets have been implemented. The direct dynamic calculations have been carried out using the validated M06-2X/maug-cc-pVTZ level, and also by the CCSD(T) (energies) + MP2 (partition functions) level. The pressure-dependent rate constants of the barrierless pathways have been investigated by the strong collision approach. Therefore, the main behaviors of the N2H4 + OH reaction have been explored according to the influences of temperature and pressure on the computed rate coefficients within the well-behaved theoretical frameworks of the TST, VTST, and RRKM theories. It has been found that the H-abstraction mechanism (to form N2H3) is dominant relative to the SN2 reaction and OH-addition to the N center of N2H4 moiety (to form H2NOH + NH2). The computed high pressure limit rate constant of the main reaction pathway, k(298.15) = 7.31 × 10–11 cm3 molecule−1 s−1, has an excellent agreement with the experimental value (k (298.15) = (6.50 ± 1.3) × 10–11 cm3 molecule−1 s−1) recommended by Vaghjiani. Also, the atmospheric lifetime of hydrazine degradation by OH radicals has been demonstrated to be 32.80 to 1161.11 h at the altitudes of 0–50 km. Finally, the disagreement in the calculated rate constants between the previous theoretical study and experimental results has been rectified.

Understanding the mechanism of hydrazine oxidation reaction by OH radical accompanied by the rate constants of all possible pathways is important. They are key parameters to explain the fate of hydrazine in the atmosphere. To reach the mentioned parameters, higher-level calculations by using quantum chemical methods have been implemented comprehensively for reliable channels such as H-abstraction, SN2, and addition/elimination reactions. To estimate the barrier energies of H-abstraction channels accurately, large numbers of the CCSD(T)/X calculations (where X denotes the augmented Dunning and Pople double zeta or triple zeta basis sets) have been applied to the optimized geometries of the MP2/aug-cc-pVTZ, MP2/maug-cc-pVTZ, and M062X/maug-cc-pVTZ levels. Contributions of excited states on the computed potential energy surface have been considered by the MR-MP2 (multi-reference) method in conjunction with the large augmented quadruple zeta, aug-cc-pVQZ, basis sets. The direct dynamic calculations have been carried out using the accurate energies of the CCSD(T) method and the partition functions of the second-order MØller-Plesset perturbation theory, and also by the validated M06-2X method with the aug-cc-pVTZ, and maug-cc-pVTZ basis sets. Finally, The VTST and TST theories have been used to calculate the temperature dependence of rate constants of the considered pathways. Also, the pressure-dependent rate constants of the barrierless pathways have been investigated by the strong collision master equation/RRKM theory.