HEAT OF FORMATION OF THE CH3CO RADICAL

The heat of formation of the CH3CO radical has been determined on several occasions(1-16) (see Table 1). The experimental literature before 1992 supports a value of about -5 kcal/mol for the heat of formation. Yadav and Goddard(17) studied acetaldehyde and its dissociation using relatively low levels of theory. While the calculations illustrated the character of the potential energy surface, they were incapable of accurately determining the heat of formation of CH3CO. The more accurate calculations of Francisco and Abersold(15) support a heat of formation of around -5 kcal/mol, especially if one takes their value from scheme 1 (-4.9 kcal/mol) in preference to their average value. That is, their reaction which involves breaking a C-H bond is expected to be more accurate than their scheme which involved breaking a C-Cl bond, because it is easier to describe a C-H bond than a C-Cl bond. Also in 1991, Radom and co-workers(16) computed the C-H bond energy in acetaldehyde using the G1 approach.(18) Their bond energy (at 0 K) was 3.8 kcal/mol larger than the experimental value (derived from a heat of formation(13) at 298 K of -5.4 kcal/mol). Because the G1 approach is usually accurate to +/-2 kcal/mol, they suggested that the acetyl radical heat of formation was several kcal/mol smaller in magnitude than experiment. Unfortunately, they did not pursue this suggestion as the acetyl radical was only a minor aspect of their study. Recently, Niiranen et al.(14) determined a heat of formation of -2.39 +/- 0.29 kcal/mol for CH3CO from a kinetics study of the reaction CH3CO + HBr. This value supports the suggestion of Radom and co-workers that the older values are too large in magnitude. In this work we determine the heat of formation of CH3CO using high levels of theory in conjunction with large basis sets. In addition, we determine all of the other bond energies in CH3CHO using the G2(MP2) approach.(19) The G2(MP2) approach combines a highly accurate method in small basis sets with a more approximate method in a large basis set and an empirical correction, and it is therefore a very cost effective method of computing bond energies accurate to about +/-2 kcal/mol.