Theoretical study on the mechanism of the N2H4 plus O2 reaction on the singlet and triplet potential energy surfaces

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Kinetic and mechanism of atmospheric reaction of hydrazine (N2H4) and molecular oxygen (O2) on the singlet and triplet potential energy surfaces have been investigated in details using ab initio and DFT methods. All stationary points involved in the title reaction were optimized at the B3LYP, MP2 and G3B3 methods of computation in connection with the 6-311++G (3df, 3pd) basis set. For calculation of accurate energies, the CCSD(T) method is applied. Also, thermodynamic parameters and rate constant are calculated at M06-2x method with the mentioned basis set. The results show that direct hydrogen abstraction mechanism is the most important pathways of reaction. Three pre-reactive complexes, 1C1, 1C2, 3C1, on the singlet and triplet potential energy surfaces were formed between hydrazine and molecular oxygen. Seven different products are suggested which all of them have enough thermodynamic stability. The production of HNNH+H2O2 and H2N2(OH)2 are the main reaction channels in thermodynamic viewpoint with standard Gibbs free energy of ∆G0=−41.9 and −54.1kcal/mol at M06-2x level, respectively. In kinetic point of view, H2NN+H2O2 adduct (on singlet state) and 2N2H3+2HO2 adducts (on singlet and triplet states) after passing one corresponding low level transition state are the most favor pathways of reaction. In this reaction, N2H3+HO2 adducts in singlet and triplet states are the main kinetically products. So, the rate constants are calculated at the 300–2500K temperature range for the reliable pathway of the mentioned adducts in both states at M06-2X/6-311++(3df, 3pd) method.