Heats of Formation of the Acetyl Radical and Ion Obtained by Threshold Photoelectron Photoion Coincidence

The dissociative photoionization onsets for the production of CH 3 CO + + CH 3 • from acetone and CH 3 CO + + CH 3 CO • from butanedione have been measured by threshold photoelectron photoion coincidence (TPEPICO) in which time-of-flight (TOF) mass spectra are obtained as a function of the ion internal energy. The use of velocity focusing for threshold electrons and the subtraction of “hot” electron coincidences from the TPEPICO spectra allow the 0 K dissociation onset to be measured with a precision of 1 kJ/mol. The experimental onset for CH 3 • loss from CH 3 COCH 3 was measured to be 10.563 ± 0.010 eV and the onset for CH 3 CO • loss from CH 3 COCOCH 3 was found to be 10.090 ± 0.006 eV. A 298 K heat of formation of the CH 3 CO + of 659.4 ± 1.1 kJ/mol is obtained by combining the measured dissociation onset with the well-established heats of formation of acetone and the methyl radical. A 298 K heat of formation of the CH 3 CO • radical of −9.8 ± 1.8 kJ/mol is obtained by combining the measured dissociation onset with the well-known heat of formation of butanedione and the measured heat of formation of CH 3 CO + . The acetone and butanedione ionization energies were measured to be 9.708 ± 0.004 and 9.21 ± 0.05 eV, respectively.