A non-empirical LCAO–MO–SCF study on the site of protonation of keten

β-Carbon-protonated keten (the methyloxocarbenium or acetyl ion) is computed to be more stable than its α-carbon- or oxygen-protonated isomers. The proton affinities of C2O2–, HC2O–, and H2C2O are computed to be 651·7, 448·4, and 216·8 kcal mol–1 respectively.