Thermochemistry of acetonyl and related radicals

Density functional and ab initio calculations at CBS-QB3 levels of theory were employed with a series of isodesmic reactions to determine the thermochemistry of the 2-oxopropyl or acetonyl radical ((CH3COCH2)-H-.). In turn, this was used to determine formation enthalpies of 2-oxoethyl or formylmethyl ((CH2CHO)-H-.), 2-oxobutyl ((CH2COC2H5)-H-.), 1-methyl-2-oxopropyl or methylacetonyl ((CH)-H-.(CH3)COCH3), 1-methyl-2-oxobutyl ((CH)-H-.(CH3)COC2H5), and 3-oxopentyl ((CH2CH2COC2H5)-H-.). Our computed standard enthalpy of formation of -34.9 +/- 1.9 kJ mol(-1) and a resonance stabilization energy of similar to 22 kJ mol(-1) for acetonyl are in good agreement with recent re-determinations, which have indicated a substantial lowering in the long-established value for Delta H-f(o) (298.15 K). A bond dissociation energy of 401 kJ mol(-1) is suggested for the C-H bond in acetone with consistent values for the others. The calculations support the enthalpy of formation of acetaldehyde obtained from combustion experiments of -166.1 kJ mol(-1) rather than the figure of -170.7 kJ mol(-1) extracted from enthalpies of reduction and, in addition, serve to reduce the uncertainty in Delta H-f(o) of the 2-oxoethyl radical to + 13 +/- 2 kJ mol(-1).