Radical Thermometers, Thermochemistry, and Photoelectron Spectra: a PEPICO Study of the Methyl Peroxy Radical.

We investigated the simplest alkylperoxy radical, CH3OO, formed by reacting photolytically generated CH3 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 CH3OO generates the CH3+ fragment ion at the appearance energy of 11.164 +/- 0.010 eV. Combining these two values with Delta H-f(0K)degrees(CH3) yields Delta H-f(0K)degrees(CH3OO) = 22.06 +/- 0.97 kJ mol(1), reducing the uncertainty of the previously determined value by a factor of 5. Statistical simulation of the CH3OO breakdown diagram provides a molecular thermometer of the free radicals internal temperature, which we measured to be 330 +/- 30 K.