Theoretical enthalpies of formation for atmospheric hydroxycarbonyls

The G3 and DFT (B3LYP) quantum chemical methods were applied to estimate enthalpies of formation for the closed-shell molecules glycolaldehyde (HOCH2C(O)H) and hydroxyacetone (HOCH2C(O)CH3) as well as for the related free radicals hydroxy-acetyl ((HOCH2CO)-O-.) and hydroxy-vinoxy ((HOCHC)-H-.(O)H). These hydroxycarbonyls are important reaction intermediates formed in the photo oxidation of isoprene which is the most abundant biogenic hydrocarbon in the atmosphere. Several isodesmic reactions were used in the computations as working chemical reactions, giving results in good agreement with each other. The following standard enthalpies of formation are recommended as best values (in kcal mol(-1)): Delta(f)Hdegrees(298) (glycolaldehyde)= -75.6 +/- 0.8, Delta(f)Hdegrees(298) (hydroxyacetone)= -89.1 +/- 0.8, Delta(f)Hdegrees(298) ((HOCH2CO)-O-.) = -37.7 +/- 1.2 and Delta(f)Hdegrees(298) ((HOCHC)-H-.(O)H) = -50.8 +/- 1.2, where the errors given are proposed as 1sigma. The Delta(f)Hdegrees(298) values obtained imply that the presence of the OH-group significantly reduces the C-H and C-C bond dissociation energies at the alpha position in the glycolaldehyde and hydroxyacetone molecules, but has no effect on the bond strengths at the beta position. Inferences for atmospheric chemistry are briefly discussed. (C) 2004 Elsevier B.V. All rights reserved.