Novel bimolecular reactions between NH₃ and HNO₃ in the gas phase

High-level molecular orbital calculations have been performed in the framework of the G2M method to explore the reactivity between NH₃ and HNO₃, key molecular reactants in ammonium nitrate and ammonium nitramide systems. Two nonionic molecular reaction channels have been identified with a similar reaction barrier, 46 kcal/mol. One channel occurring via a four-member-ring transition state produces H₂NNO₂ + H₂O (1), and the other, taking place via a five-member-ring transition state, yields H₂NONO + H₂O (2). A transition-state theory calculation employing the predicted energies and molecular parameters gave rise to the rate constants k₁ = 0.81T^3.47e^-21670/T and k₂ = 23.27^3.50e^-22610/T for the temperature range 300-3000 K in units of cm³(mol's). In addition to the reactants, products, and transition states associated with the two reaction channels, several local minima (or molecular complexes) and secondary reaction products derived from the structural rearrangement of some of the molecular complexes, such as H₃NO and H₂NOH, have been identified and their energies calculated at the G2M level of theory.