Enthalpy of formation of selected carbonyl radicals from theory and comparison with experiment

Enthalpies of formation and entropies are calculated for 13 carbonyl radica ls. Radicals of atmospheric importance are selected for study. The CBS-4 (C omplete Basis Set 4) and G2(MP2,SVP) ab initio molecular orbital methods ar e used to obtain the thermodynamic properties of the radicals. In addition to the enthalpy of formation determinations based on atomization energy com putations, enthalpies of formation are derived for CH3CO, CF3CO and CCl3CO by studying the isodesmic reactions CH3+CH3CHO=CH4+CH3CO, CH3+CF3CHO=CH4+CF 3CO and CH3+CCl3CHO=CH4+CCl3CO. Good agreement is found between the results obtained by these two approaches. For comparison, carbonyl radical enthalp ies of formation are estimated by using recent experimental room temperatur e rate coefficients for reactions of a bromine atom with a series of alipha tic aldehydes. Systematic comparison is made of the theoretical results wit h these experimentally based data and recommendations for the enthalpies of formation of carbonyl radicals and the formyl C-H bond dissociation energi es in the appropriate aldehydes are presented. Finally, theoretical results are used to obtain reaction heats for carbonyl radical decomposition by CO elimination, and the atmospheric implications of the results are discussed .