Thermochemistry of the smallest QOOH radical from the roaming fragmentation of energy selected methyl hydroperoxide ions

Abstract

The dissociative photoionization processes of methyl hydroperoxide (CH₃OOH) have been studied by imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments as well as quantum-chemical and statistical rate calculations. Energy selected CH₃OOH⁺ ions dissociate into CH₂OOH⁺, HCO⁺, CH₃⁺, and H₃O⁺ ions in the 11.4–14.0 eV photon energy range. The lowest-energy dissociation channel is the formation of the cation of the smallest “QOOH” radical, CH₂OOH⁺. An extended RRKM model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH₂OOH⁺ ion, and a 74.2 ± 2.6 kJ mol⁻¹ mixed experimental-theoretical 0 K heat of formation for the CH₂OOH radical. The proton affinity of the Criegee intermediate, CH₂OO, was also obtained from the heat of formation of CH₂OOH⁺ (792.8 ± 0.9 kJ mol⁻¹) to be 847.7 ± 1.1 kJ mol⁻¹, reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the complex web of possible rearrangement-dissociation processes was used to model the higher-energy fragmentation. Supported by Born–Oppenheimer molecular dynamics simulations, we found that the HCO⁺ fragment ion is produced through a roaming transition state followed by a low barrier. H₃O⁺ is formed in a consecutive process from the CH₂OOH⁺ fragment ion, while direct C–O fission of the molecular ion leads to the methyl cation.