Potential Nonstatistical Effects on the Unimolecular Decomposition of H₂O₂

Abstract

An attempt is made to evaluate the nonstatistical effects in the thermal decomposition of hydrogen peroxide (H 2 O 2 ). Previous experimental studies on this reaction reported an unusual pressure dependence of the rate constant indicating broader falloff behavior than expected from conventional theory. In this work, the possibility that the rate constant is affected by nonstatistical effects is investigated based on classical trajectory calculations on the global potential energy surfaces of H 2 O 2 and H 2 O 2 + Ar. The emphasis is on the intramolecular energy redistribution from the K-rotor, that is, the external rotor for rotation around the principal axis of least moment of inertia. The calculations for the H 2 O 2 molecules excited above the dissociation threshold suggest that the energy redistribution from the torsion and Krotor to vibrations can be competitive with dissociation. In particular, the slow redistribution of the energy associated with the K-rotor significantly affects the dissociation rate. The successive trajectory calculations for collisions of H 2 O 2 with Ar show that the energy associated with the K-rotor can be collisionally transferred more efficiently than the vibrational energy. On the basis of these results and several assumptions, a simple model is proposed to account for the nonstatistical effects on the pressure-dependent thermal rate constants. The model predicts significant broadening of the falloff curve of the rate constants but still cannot fully explain the experimental data.

Combustion in a Sustainable World: From Molecules to Processes

“…Increasing attention is being focused on key individual reactions that are potentially sensitive steps in fuel decomposition and oxidation, particularly regarding their temperature- and pressure-dependent behavior. The decomposition of H 2 O 2 into two OH radicals, an important chain-branching reaction at intermediate temperatures, has been found to behave unusually as a function of pressure, indicating that nonstatistical effects may play a role . Discrepancies between state-of-the-art theory and experiments have also been noted in the temperature dependence of the self-reaction of HO 2 + HO 2 leading to H 2 O 2 + O 2 ; these could be resolved using a multiscale informatics approach that takes into account experimental raw data and, compared to previous work, includes the formation of a HOOOOH intermediate .…”

“…The decomposition of H 2 O 2 into two OH radicals, an important chain-branching reaction at intermediate temperatures, has been found to behave unusually as a function of pressure, indicating that nonstatistical effects may play a role. 2 Discrepancies between state-of-the-art theory and experiments have also been noted in the temperature dependence of the self-reaction of HO 4 and the photodissociation dynamics of propyl derivatives. 5 Such techniques also support the identification of transient reaction intermediates, such as phenoxy radical, which is a key intermediate in benzene oxidation and biomass conversion.…”