Computational Study of the Thermochemistry of N₂O₅ and the Kinetics of the Reaction N₂O₅ + H₂O → 2 HNO₃

The multistructural method for torsional anharmonicity (MS-T) is employed to compute anharmonic conformationally averaged partition functions which then serve as the basis for the calculation of thermochemical parameters for N₂O₅ over the temperature range 0–3000 K, and thermal rate constants for the hydrolysis reaction N₂O₅ + H₂O → 2 HNO₃ over the temperature range 180–1800 K. The M06-2X hybrid meta-GGA density functional paired with the MG3S basis set is used to compute the properties of all stationary points and the energies, gradients, and Hessians of nonstationary points along the reaction path, with further energy refinement at stationary points obtained via single-point CCSD­(T)-F12a/cc-pVTZ-F12 calculations including corrections for core–valence and scalar relativistic effects. The internal rotations in dinitrogen pentoxide are found to generate three structures (conformations) whose contributions are included in the partition function via the MS-T formalism, leading to a computed value for S°_298.15(N₂O₅) of 353.45 J mol^–1 K^–1. This new estimate for S°_298.15(N₂O₅) is used to reanalyze the equilibrium constants for the reaction NO₃ + NO₂ = N₂O₅ measured by Osthoff et al. [Phys. Chem. Chem. Phys. 2007, 9, 5785−5793] to arrive at Δ_fH_298.15^°(N₂O₅) = 14.31 ± 0.53 kJ mol^–1 via the third law method, which compares well with our computed ab initio value of 13.53 ± 0.56 kJ mol^–1. Finally, multistructural canonical variational-transition-state theory with multidimensional tunneling (MS-CVT/MT) is used to study the kinetics for hydrolysis of N₂O₅ by a single water molecule, whose rate constant can be summarized by the Arrhenius expression 9.51 × 10^–17 (T/298 K)^3.354 e^(−7900K/T) cm³ molecule^–1 s^–1 over the temperature range 180–1800 K.