Mechanisms, energetics and dynamics of a key reaction sequence during the decomposition of nitromethane: HNO+HNO->N2O+H2O

Quantum chemical calculations (MP4/6-311 + G(2df,2pd)) demonstrate that four alternative routes exist for the title reaction. Common to all routes is a first step which involves the formation of the dimer (HNO)(2). Starting from the dimer, the completion of the reaction requires two consecutive hydrogen migrations to the same oxygen atom, one from each of the ni trogen atoms. The two main routes (A and B) differ in the order in which the two hydrogens are transferred. Further separation of route B into subroutes B-1 and B-2/B-3 is the result of two conformational alternatives for the transition structure of the last hydrogen migration. Routes B-2 and B-3 are common until the final step where they separate. Of the four routes, routes B-2 and B-3 have the lowest overall calculated activation energy (44 kJ mol(-1)). The rate-determining step corresponds to the initial formation of the dimer. Ab initio dynamics calculations of the final step of each route confirm that the anticipated products are formed, and show that all four routes give rise to the fast and efficient conversion of potential energy into relative translational energy of the two product molecules, which in all cases acquire high relative velocities. (C) 1997 Elsevier Science B.V.