Thermochemistry and Bond Dissociation Energies of Ketones

Ketones are a major class of organic chemicals and solvents, which contribute to hydrocarbon sources in the atmosphere, and are important intermediates in the oxidation and combustion of hydrocarbons and biofiiels. Their stability, thermochemical properties, and chemical kinetics are important to understanding their reaction paths and their role as intermediates in combustion processes and in atmospheric chemistry. In this study, enthalpies (Delta H-f 298 degrees), entropies (S degrees(T)), heat capacities (C-p degrees(T)), and internal rotor potentials are reported for 2-butanone, 3-pentanone, 2-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone, and their radicals corresponding to loss of hydrogen atoms. A detailed evaluation of the carbon hydrogen bond dissociation energies (C-H BDEs) is also performed for the parent ketones for the first time. Standard enthalpies of formation and bond energies are calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBS-QB3, and G3MP2B3 levels of theory using isodesmic reactions to minimize calculation errors. Structures, moments of inertia, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) density functional level and are used to determine the entropies and heat capacities. The recommended ideal gas-phase Delta H-f 298 degrees, from the average of the CBS-QB3 and G3MP2B3 levels of theory, as well as the calculated values for entropy and heat capacity are shown to compare well with the available experimental data for the parent ketones. Bond energies for primary, secondary, and tertiary radicals are determined; here, we find the C-H BDEs on carbons in the alpha position to the ketone group decrease significantly with increasing substitution on these alpha carbons. Group additivity and hydrogen-bond increment values for these ketone radicals are also determined.