Abstract
The 298 K heat of formation for the propionyl cation (C~2~H~5~CO^+^) 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 $\Delta H^{\circ}_{{\rm f},298}(\hbox{C}_{2}\hbox{H}_{5}\hbox{CO}^{+})$, 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.