Ab initio chemical kinetics for the N₂H₄+NO_x (x=1-3) reactions and related reverse processes

The kinetics and mechanisms for N₂H₄+NO_x (x=1-3) reactions and the related reverse reactions have been investigated by ab initio molecular orbital theory based on the CCSD(T)/CBS//CCSD/6-31G(d,p), CCSD(T)//B3LYP and CCSD(T)//BH&HLYP methods with the 6-311++G(3df,2p) basis set. These reactions are important to the propulsion chemistry of the N₂H₄-N₂O₄ propellant system. The results show that the reactions of N₂H₄ with NO and NO₂ producing N₂H₃+HNO and N₂H₃+c-HONO by H-abstraction with 33.7 and 10.3kcal/mol barriers, respectively, are dominant. For the N₂H₄+NO₃(D_3h) reaction via two pre-reaction van der Waals complexes with 0.5kcal/mol and -1.6kcal/mol binding energies produces HNO₃+N₂H₃ by H-abstraction and t-HONO+N₂H₃O by concerted O- and H-atom transfers, respectively. The predicted enthalpies of formation of various products at 0K are in good agreement with available experimental data within reported errors. Furthermore, the rate constants for the forward and some key reverse reactions have been predicted in the temperature range 300-2000K with tunneling corrections using transition state theory (for direct abstraction) and variational Rice-Ramsperger-Kassel-Marcus theory (for association/decomposition) by solving the master equation.