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 (CH₃GOC ^̇H₂). In turn, this was used to determine formation enthalpies of 2-oxoethyl or formylmethyl (C^̇H₂CHO), 2-oxobutyl (C^̇H₂COC₂H₅), 1-methyl-2-oxopropyl or methylacetonyl (C^̇H(CH ₃)COCH₃), 1-methyl-2-oxobutyl (C^̇H(CH ₃)-COC₂H₅), and 3-oxopentyl (C ^̇H₂CH₂COC₂H₅). Our computed standard enthalpy of formation of -34.9 ± 1.9 kJ mol ^-1 and a resonance stabilization energy of ∼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 Δ 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 ΔH _f^o of the 2-oxoethyl radical to +13 ± 2 kJ mol ^-1.