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

We investigated the simplest alkylperoxy radical, CHOO, formed by reacting photolytically generated CH radicals with O, 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 CHOO generates the CH fragment ion at the appearance energy of 11.164 ± 0.010 eV. Combining these two values with ΔH(CH) yields ΔH(CHOO) = 22.06 ± 0.97 kJ mol, reducing the uncertainty of the previously determined value by a factor of 5. Statistical simulation of the CHOO breakdown diagram provides a molecular thermometer of the free radical's internal temperature, which we measured to be 330 ± 30 K.