Effect of Methyl Substitution on the Thermochemistry of Ketene

The heats of formation of ketene, methylketene, and dimethylketene have been measured by energy-selected electron-induced dissociation of phenyl acetate, propanoate, and isobutyrate, respectively. The values are Δf H°[CH2CO] = −54 ± 5 kJ mol-1, Δf H°[CH3CHCO] = −95 ± 5 kJ mol-1, and Δf H°[(CH3)2CCO] = −137 ± 5 kJ mol-1. For ketene, the new result was in excellent agreement with literature data and recent high-level ab initio calculations. The values for the methyl-substituted analogues differed significantly from those obtained by the recent calculations. The heats of formation of ionized methyl- and dimethylketene were also measured to be 765 and 683 ± 5 kJ mol-1, respectively. These experimental results show that the effect of methyl substitution in neutral ketene decreases the heat of formation by ca. 40 kJ mol-1 per methyl group, which is in keeping with the effect of methyl substitution in structurally related systems.