Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical

We investigated the simplest alkylperoxy radical, CH 3 OO, formed by reacting photolytically generated CH 3 radicals with O 2 , using the new combustion reactions followed by photoelectron photoion coincidence (CRF-PEPICO) apparatus at the Swiss Light Source. Modeling the experimental photoion mass-selected threshold photoelectron spectrum using Franck-Condon simulations including transitions to triplet and singlet cationic states yielded the adiabatic ionization energy of 10.265 ± 0.025 eV. Dissociative photoionization of CH 3 OO generates the CH 3 + fragment ion at the appearance energy of 11.164 ± 0.010 eV. Combining these two values with Δ f H 0K ° (CH 3 ) yields Δ f H 0K ° (CH 3 OO) = 22.06 ± 0.97 kJ mol -1 , reducing the uncertainty of the previously determined value by a factor of 5. Statistical simulation of the CH 3 OO breakdown diagram provides a molecular thermometer of the free radical's internal temperature, which we measured to be 330 ± 30 K.