EFFECT OF METHYL SUBSTITUTION ON THE THERMOCHEMISTRY OF KETENE

The heats of formation of ketene, methylketene, and dimethylketene hav e been measured by energy-selected electron-induced dissociation of ph enyl acetate, propanoate, and isobutyrate, respectively. The values ar e Delta(f)H degrees[CH2=C=O] -54 +/- 5 kJ mol(-1), Delta(f)H degrees[C H3CH=C=O] = -95 +/- 5 kJ mol(-1), and Delta(f)H degrees-[(CH3)(2)C=C=O ] = -137 +/- 5 kT mol(-1). For ketene, the new result was in excellent agreement with literature data and recent high-level ab initio calcul ations. The values for the methyl-substituted analogues differed signi ficantly from those obtained by the recent calculations. The heats of formation of ionized methyl-and dimethylketene were also measured to b e 765 and 683 +/- 5 kT mol(-1), respectively. These experimental resul ts show that the effect of methyl substitution in neutral ketene decre ases the heat of formation by ca. 40 kT mol(-1) per methyl group, whic h is in keeping with the effect of methyl substitution in structurally related systems.