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 biofuels. 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 (ΔH°(f 298)), entropies (S°(T)), heat capacities (C(p)°(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 ΔH°(f 298), 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 α position to the ketone group decrease significantly with increasing substitution on these α carbons. Group additivity and hydrogen-bond increment values for these ketone radicals are also determined.
[Show abstract][Hide abstract] ABSTRACT: Thermochemistry of reactants, intermediates, transition state structures, and products along with kinetics on the association of CH2•C(=O)CH2CH3 (2-butanone -1yl) with O2 and dissociation of the peroxy adduct isomers are studied. Thermochemical properties are determined using ab initio (G3MP2B3 and G3) composite methods along with Density Functional (B3LYP/6-311g(d,p)). Entropy and heat capacity contributions versus temperature are determined from structures, vibration frequencies and internal rotor potentials. The CH2•C(=O)CH2CH3 radical + O2 association results in chemically-activated peroxy radical with 27 kcal mol-1 excess of energy. The chemically activated adduct can react to stabilized peroxy or hydroperoxide alkyl radical adducts, further react to lactones plus hydroxyl radical or form olefinic ketones and a hydroperoxy radical. Kinetic parameters are determined from the G3 composite methods derived thermochemical parameters and quantum Rice-Ramsperger-Kassel (QRRK) analysis to calculate k(E) with Master Equation analysis to evaluate fall-off in the chemically activated and dissociation reactions. One new, not previously reported, peroxy chemistry reaction is presented. It has a low barrier path and involves a concerted reaction resulting in olefin formation, H2O elimination and an alkoxy radical.
The Journal of Physical Chemistry A 10/2013; 118(1). DOI:10.1021/jp408708u · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A complete and consistent set of 60 Benson group additive values (GAVs) for oxygenate molecules and 97 GAVs for oxygenate radicals is provided, which allow to describe their standard enthalpies of formation, entropies and heat capacities. Approximately half of the GAVs for oxygenate molecules and the majority of the GAVs for oxygenate radicals have not been reported before. The values are derived from an extensive and accurate database of thermochemical data obtained by ab initio calculations at the CBS-QB3 level of theory for 202 molecules and 248 radicals. These compounds include saturated and unsaturated, α- and β-branched, mono- and bifunctional oxygenates. Internal rotations were accounted for by using one-dimensional hindered rotor corrections. The accuracy of the database was further improved by adding bond additive corrections to the CBS-QB3 standard enthalpies of formation. Furthermore, 14 corrections for non-nearest-neighbor interactions (NNI) were introduced for molecules and 12 for radicals. The validity of the constructed group additive model was established by comparing the predicted values with both ab initio calculated values and experimental data for oxygenates and oxygenate radicals. The group additive method predicts standard enthalpies of formation, entropies, and heat capacities with chemical accuracy, respectively, within 4 kJ mol(-1) and 4 J mol(-1) K(-1) for both ab initio calculated and experimental values. As an alternative, the hydrogen bond increment (HBI) method developed by Lay et al. (T. H. Lay, J. W. Bozzelli, A. M. Dean, E. R. Ritter, J. Phys. Chem.- 1995, 99, 14514) was used to introduce 77 new HBI structures and to calculate their thermodynamic parameters (Δf H°, S°, Cp °). The GAVs reported in this work can be reliably used for the prediction of thermochemical data for large oxygenate compounds, combining rapid prediction with wide-ranging application.
Chemistry - A European Journal 11/2013; 19(48). DOI:10.1002/chem.201301381 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 2,2,4-Trimethylpentane, also known as isooctane, is used as one of the model fuel species on spark and homogeneous charge compression ignition engines. This study presents thermochemical and kinetic properties in the oxidation of the secondary isooctane radical, which includes the peroxy radical formed from the 3O2 association, the hydroperoxy alkyl radicals formed from the intramolecular hydrogen transfers, and the products formed from reactions of the hydroperoxy alkyl radicals. Geometries, vibration frequencies, internal rotor potentials, and thermochemical properties, ΔfH, S°(T), and C°p(T) (5 K ≤ T ≤ 5000 K) were calculated at the individual B3LYP/6–31G(d,p) and the composite CBS-QB3 calculation method. The standard enthalpies of formation at 298 K were evaluated using isodesmic reaction schemes with several work reactions for each species. Entropy and heat capacities were determined using geometric parameters and frequencies from the B3LYP/6–31G(d,p) calculations for the lowest energy conformer. Internal rotor barriers were determined. Application of group additivity with comparison to calculated values is also illustrated. Transition states and kinetic parameters for intramolecular hydrogen atom transfer and molecular elimination channels were characterized to evaluate reaction paths and kinetics. Kinetic parameters were determined versus pressure and temperature for the chemical activated formation and unimolecular dissociation of the peroxide adduct. Multifrequency quantum Rice–Ramsperger–Kassel analysis was used for k(E) with master equation analysis for falloff. The kinetic analysis shows that the main reaction channels are the formation of isooctene ((CH3)3CCH=C(CH3)2) + HO2•, and the formation of the cyclic: (CH3)2-y(CCH2CHO)-(CH3)2, (CH3)3C-y(CHCO)-(CH3)2, and (CH3)3C-y(CHCHCH2O)-(CH3).
International Journal of Chemical Kinetics 02/2014; 46(2). DOI:10.1002/kin.20825 · 1.52 Impact Factor
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