The propionyl cation heat of formation revisited

The 298 K heat of formation for the propionyl cation (C₂H₅CO⁺) has been measured previously by dissociative photoionization mass spectrometry. However, recent theoretical and experimental studies involving methylketene suggest that this may be significantly underestimated, resulting in a methylketene proton affinity that is too high by ∼30 kJ mol⁻¹. In this study, the previous m/z 57 appearance energies were carefully re-evaluated, with various possible sources of error being investigated. These include factors such as sample purity, carbon-13 contamination from lower energy m/z 56 processes, kinetic and/or competitive shifts, reverse activation energies, ionizing energy calibration errors and the availability of accurate supplementary thermochemical data. In addition, high-level ab initio calculations are used to model the relevant unimolecular fragmentation processes for each of the ionized precursor molecules. As a result, it is found that only the 2-butanone appearance energy can be used to provide a reliable value for the propionyl cation heat of formation. From the 298 K m/z 57 appearance energy of 10.199 ± 0.003 eV for 2-butanone measured here, a value of 617.8 ± 0.9 kJ mol⁻¹ is derived for , which corresponds to 845.4 ± 4.8 kJ mol⁻¹ for the proton affinity of methylketene. This is in good agreement with previous theoretical calculations and thermokinetic proton affinity measurements, indicating that a significant upward revision to the propionyl cation heat of formation is warranted. Copyright © 2004 John Wiley & Sons, Ltd.