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

Abstract

Enthalpies of formation and entropies are calculated for 13 carbonyl radicals. Radicals of atmospheric importance are selected for study. The CBS-4 (Complete Basis Set 4) and G2(MP2,SVP) ab initio molecular orbital methods are used to obtain the thermodynamic properties of the radicals. In addition to the enthalpy of formation determinations based on atomization energy computations, enthalpies of formation are derived for CH₃CO, CF₃CO and CCl₃CO by studying the isodesmic reactions CH₃ + CH₃CHO = CH₄ + CH₃CO, CH₃ + CF₃CHO = CH₄ + CF₃CO and CH₃ + CCl₃CHO = CH₄ + CCl₃CO. Good agreement is found between the results obtained by these two approaches. For comparison, carbonyl radical enthalpies of formation are estimated by using recent experimental room temperature rate coefficients for reactions of a bromine atom with a series of aliphatic aldehydes. Systematic comparison is made of the theoretical results with these experimentally based data and recommendations for the enthalpies of formation of carbonyl radicals and the formyl C–H bond dissociation energies 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.