Ketene and ketene radical cation: The unusual effect of methyl and dimethyl substitution

Abstract

The heats of formation ( ΔH f 298) for ketene (1, −52 kJ mol −1), methylketene (2, −68 kJ mol −1), dimethylketene (3, −92 kJ mol −1), and their radical cations 1 + (875 kJ mol −1), 2 + (797 kJ mol −1) and 3 + (726 kJ mol −1) have been determined at high levels of theory including variants of the G2 and CBS procedures. Agreement with experiment is good for the ionization energies of 1, 2 and 3 and for the heats of formations of 1 and 1 + but not for the heats of formation of 2, 3, 2 + and 3 +. This disagreement is puzzling and suggests that a re-examination of the experimental estimates would be desirable. The effect of a single methyl substituent on the heat of formation is predicted to be considerably lower for ketene (our best estimate being −16 kJ mol −1) than for ethylene (−31 kJ mol −1), allene (−27 kJ mol −1) or formaldehyde (−54 kJ mol −1).