An ab initio molecular orbital study of potential energy surface of the NH2+NO2 reaction

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Potential energy surface of the reaction of NH2 with NO2 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 NH2+NO2, and are not expected to compete with the reaction mechanisms producing H2NO+NO and N2O+H2O.