Absolute Proton Affinity for Acetaldehyde

Dissociative photoionization mass spectrometry has been used to measure appearance energies for the 1-hydroxyethyl cation (CH₃CHOH⁺) formed from ethanol and 2-propanol. Molecular orbital calculations for these two unimolecular fragmentation reactions suggest that only methyl loss from ionized 2-propanol does not involve excess energy at the threshold. The experimental appearance energy of 10.31 ± 0.01 eV for this latter process results in a 298 K heat of formation of 593.1 ± 1.2 kJ mol^-1 for CH₃CHOH⁺ and a corresponding absolute proton affinity for acetaldehyde of 770.9 ± 1.3 kJ mol^-1. This value is supported by both high-level ab initio calculations and a proposed upward revision of the absolute isobutene proton affinity to 803.3 ± 0.9 kJ mol^-1. A 298 K heat of formation of 52.2 ± 1.9 kJ mol^-1 is derived for the tert-butyl radical.