Absolute Proton Affinity for Acetaldehyde

2008-01-17T00:00:00Z (GMT) by John C. Traeger
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.