A b i n i t i o multireference configuration interaction study of CH2NNO2. HONO elimination vs NN bond fragmentation

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Abstract
Features of the potential energy surface pertinent to the thermal decomposition mechanism of methylenenitramine (CH2NNO2) are investigated. At issue are the activation energies both for concerted molecular dissociation leading to HCN and HONO and for the simple NN bond cleavage leading to CH2N and NO2. We use multiconfiguration (MC)SCF wave functions with double‐zeta and double‐zeta plus polarization quality basis sets to locate minimum energy and transition state structures as well as determine harmonic vibrational frequencies and resulting zero‐point vibrational energies. The energetics are determined using configuration interaction wave functions which include single and double excitations from a selected set of reference configurations constructed with the MCSCF molecular orbitals. Our best calculations, without adjustment, predict the molecular dissociation to have an activation energy slightly lower than that for simple bond rupture. Calculations at the level of those reported here are known, however, to underestimate bond dissociation energies by several kcal/mol. Based on trends from past calculations, and new calculations on a related compound, we estimate the activation energy for the HONO elimination path to be 31±4 kcal/mol and the NN bond dissociation energy to be 35±4 kcal/mol. This reverses the energetic ordering of previous theoretical predictions and is consistent with recent molecular beam experiments.