Dissociative photoionization mass spectrometry has been used to measure the PhCO⁺ appearance energies for several benzoyl compounds. Based on these threshold measurements, an experimental 298 K heat of formation of 738.8 ∓ 3.3 kJ mol^–1 is obtained for the benzoyl cation. The experimental results are supported by high-level ab initio calculations.

Keywords: benzoyl cation, photoionization, heat of formation, appearance energy