An ab initio molecular orbital study of potential energy surface of the NH ₂ +NO ₂ reaction

Potential energy surface of the reaction of NH₂ with NO ₂ has been studied at the QCISD(T)/6-311G(d,p)//MP2/6-311G(d,p) +ZPC[MP2/6-311G(d,p)] and GAUSSIAN-2 (G2) levels of calculation. The reaction is shown to give three different groups of products. H₂NO+NO can be produced by two different channels: (i) the barrierless association of the reactants to form H₂NNO₂ 1, followed by the nitro-nitrite rearrangement into H₂NONO 3 and the ON bond scission and (ii) the association of H₂N with ONO directly forming 3 without barrier, followed by the dissociation 3. The barrier for the nitro-nitrite rearrangement at the transition state (TS) 2, 31.2 kcal/mol with respect to 1, is 20.8 kcal/mol lower than the reactants at the best G2 level. The TS 2 is found to lie significantly lower and to have much tighter structure than those previously reported. The thermodynamically most stable N₂O+H₂O products can be formed from 1 by the complex mechanism (iii), involving 1,3-hydrogen shift from nitrogen to oxygen, rotation of the OH bond, H shift from one oxygen to another and migration of the second H atom from N to O leading to elimination of H₂O. The rate-determining step is the 1,3-H shift at TS 4 which is 12.5 kcal/mol lower than NH₂+NO₂, but 8.3 kcal/mol higher than the barrier for the nitro-nitrite isomerization at TS 2 at the G2 level. N₂+H₂O₂ cannot be formed in the reaction, but several channels are shown to produce N₂+2OH. All of them have as the rate-determining step the second 1,3-hydrogen shift from nitrogen to oxygen at TS 11 or 16, lying by 6.9 kcal/mol higher than NH ₂+NO₂, and are not expected to compete with the reaction mechanisms producing H₂NO+NO and N₂O+H₂O.