A high level theoretical investigation of the N2O4→2 NO2 dissociation reaction: Is there a transition state?

The N2O4→2 NO2 dissociation reaction was investigated at a high level of theory using the couple cluster with all single and double excitations and connected triples [CCSD(T)] and complete active space self-consistent field approaches, and the cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. Only at the coupled cluster level a first-order saddle point was found connecting reactant and products. Collectively, structural, vibrational, and thermodynamic data for the three stationary points represent the best theoretical description of this reaction system to date, and are in good agreement with available experimental results. Unimolecular transition state theory rate constants (k∞) were also evaluated at 250, 298.15, and 350 K. At the CCSD(T)/cc-pVTZ level of calculation these results are 0.62×101, 1.90×103, and 1.66×105 s−1, respectively. Known experimental results at 298 K vary from 1.7×105 to 1.0×106 s−1. Including an estimate for basis set superposition error, we predict ΔH2980 for the dissociation reaction to be 12.76 kcal/mol (Expt. 13.1–13.7 kcal/mol).