Properties and reactions of ketene in the gas phase by ion cyclotron resonance spectroscopy and photoionization mass spectrometry. Proton affinity, site specificity of protonation, and heat of formation of ketene

The properties and reactions of ketene and ketene-d2 are investigated using the techniques of ion cyclotron resonance spectroscopy and photoionization mass spectrometry. The major reaction sequence initiated by the ketene molecular ion in its ground vibrational state at thermal energies involves sequential incorporation of CH2 into the ionic product with loss of CO. The species C2H3O+ (presumably the acyl cation) is a major product at long times or high pressure and does not react further with ketene neutrals. The energetics of ionization of ketene and ketene-d2 and the effect of vibrational energy on reactivity are examined using photoionization mass spectrometry and photoelectron spectroscopy. The threshold determined for the endothermic process CH2CO+ + CH2CO → C2H4+ + 2CO provides confirmation of the recently redetermined heat of formation of ketene (ΔH/sub f/ = -11.4 +- 0.4 kcal/mol). Equilibrium proton transfer reactions in a number of mixtures lead to a determination of the proton affinity of ketene, PA(CH2CO) = 195.9 +- 2 kcal/mol, relative to PA(NH3) = 202.3 +- 2 kcal/mol. The site of protonation is shown to be the methylene carbon. These results yield ΔH/sub f/(CH3CO+) = 159.9 +- 2 kcal/mol, which is discussed in light of other measurements of the heat of formation of the acyl cation. With sufficiently acidic species (e.g., CH5+), oxygen protonation occurs. 5 tables, 4 figures, 48 references