Thermochemistry reaction paths and oxidation kinetics on ketonyl and aldehydic nitrogen oxides, propene and isooctane: a theoretical study

Thermochemical properties for several atmospheric and combustion related species are determined using computational chemical methods coupled with fundamentals of thermodynamics and statistical mechanics. Enthalpies of formation (ΔHf°298) are determined using isodesmic reaction analysis at the CBS-QB3 composite and the B3LYP density functional methods. Entropies (S°298) and heat capacities (Cp°(T)) are determined using geometric parameters and vibration frequencies; internal rotor contributions are included in S and Cp(T) values in place of torsion frequencies. Kinetic parameters are calculated versus pressure and temperature for the chemical activated formation and unimolecular dissociation. Multi-frequency quantum RRK (QRRK) analysis is used for k(E) with Master Equation analysis for fall off. Recommended values for enthalpies of formation of the most stable conformers of nitroacetone, acetonitrite, nitroacetate and acetyl nitrite are -51.6 kcal mol-1, -51.3 kcal mol-1, -45.4 kcal mol-1 and -58.2 kcal mol-1, respectively. The calculated ΔfH º298 for nitroethylene is 7.6 kcal mol-1 and for vinyl nitrite is 7.2 kcal mol-1. The chemically activated R• + NO2 systems associations proceed to RCO• + NO via chemical activation reaction with a fraction to stabilized adducts and lower energy products at atmospheric pressure and temperature. Thermochemical properties of isooctane (2,2,4-trimethyl pentane) and its four carbon radicals from loss of hydrogen atoms, and kinetics of the tertiary isooctane radical reaction with O2 are determined. The computed standard enthalpy of formation of isooctane from this study is -54.40 kcal mol-1. The major products from reaction of the tert-isooctane radical + O2 to form a chemically activated tert-isooctane-peroxy radical are formation of isooctene plus HO2. Next important products are cyclic ethers plus OH radical. This research is the first fundamentally based study of relevant pathways on the potential energy surfaces of tert-isooctane radicals + O2 using high level composite calculation methods. Kinetic modeling for OH addition to propene and subsequent O2 association to the hydroxyl-propyl radical adduct shows that significant forward reaction goes to regenerate OH radicals over the range of temperature and pressure studied. Recycle of OH from the decomposition of the hydroxyl propyl-peroxy radical is up to 78%. Inclusion of activation energy resulting from OH addition to primary carbon (double activation) does not show increase in OH recycle. The introduction of the rate constants presented in this study into existing reaction mechanisms should lead to better kinetic models for olefin oxidation chemistry the atmospheric

Thermochemistry and bond energies of nitro -alkanes, -alkenes, -carbonyls and corresponding nitrites

Density functional and ab initio theory based calculations were performed on a series of nitro -alkanes, -alkenes, carbonyl and corresponding nitrites representing large-scale primary, secondary and tertiary nitro compounds and their radicals resulting from the loss of skeletal hydrogen atoms. Geometries, vibration frequencies and thermochemical properties, ΔfH°298, S °(T) and C°p(T) (10K T 5 5000K) are calculated at the individual B3LYP/6-31G(d,p), B3LYP /6-31+G(2d,2p) and composite CBS-QB3 levels. Potential energy barriers for the internal rotations have been computed at the B3LYP/6-31G(d, p) level of theory and the lower barrier contributions are incorporated into entropy and heat capacity data. The standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Recommended values derived from the most stable conformers of respective nitro- and nitrite isomers include: -30.6 and -28.4 kcal moil for n-propane-, -33.9 and -32.3 kcal mol-1 for iso-propane-, -42.8 and -41.4 kcal mol-1 for tert-butane-nitro compounds and nitrites, respectively. Entropy and heat capacity values are also reported for the lower homologues: nitromethane, nitroethane and corresponding nitrites. C--H bond energies are decreased by ~ 5 kcal moil alpha to the nitro or nitrite groups and increased by ~ 0.5 kcal moil beta to the nitro and nitrite groups. Recommended values for enthalpies of formation of the most stable conformers of nitroacetone, acetonitrite, nitroacetate and acetyl nitrite are -51.6 kcal mol-1, -51.26 kcal mol-1, -45.4 kcal mol-1 and -58.2 kcal mol-1, respectively. The calculated ΔfH°298 for nitroethylene is 7.6 kcal mol-1 and for vinyl nitrite is 7.2 kcal mol-1. The carbonyl and olefin groups retain the major influence on the C-H bond energies. Radicals on carbon adjacent to a nitrite (RC.ONO) group do not exist; they dissociate to the corresponding carbonyl (RC=O + NO) with 38 kcal mol-1 exothermic and no apparent barrier. This results from formation of the strong carbonyl (it bond ~ 80 kcal mol-1) with dissociation of the weak RO--NO bond (-43 kcal mol-1)

Thermochemistry, Reaction Paths, and Kinetics on the <i>tert</i>-Isooctane Radical Reaction with O₂

Thermo­chemical properties of <i>tert</i>-isooctane hydro­peroxide and its radicals are determined by computational chemistry. Enthalpies are determined using isodesmic reactions with B3LYP density function and CBS QB3 methods. Application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities are determined using geometric parameters and frequencies from the B3LYP/6-31G­(d,p) calculations for the lowest energy conformer. Internal rotor potentials are determined for the <i>tert</i>-isooctane hydro­peroxide and its radicals in order to identify isomer energies. Recommended values derived from the most stable conformers of <i>tert</i>-isooctane hydro­peroxide of are −77.85 ± 0.44 kcal mol^–1. Isooctane is a highly branched molecule, and its structure has a significant effect on its thermo­chemistry and reaction barriers. Intra­molecular interactions are shown to have a significant effect on the enthalpy of the isooctane parent and its radicals on peroxy/peroxide systems, the R• + O₂ well depths and unimolecular reaction barriers. Bond dissociation energies and well depths, for <i>tert</i>-isooctane hydroperoxide → R• + O₂ are 33.5 kcal mol^–1 compared to values of ∼38 to 40 kcal mol^–1 for the smaller <i>tert</i>-butyl-O₂ → R• + O₂. Transition states and kinetic parameters for intra­molecular hydrogen atom transfer and molecular elimination channels are characterized to evaluate reaction paths and kinetics. Kinetic parameters are determined versus pressure and temperature for the chemically activated formation and unimolecular dissociation of the peroxide adducts. Multi­frequency quantum RRK (QRRK) analysis is used for <i>k</i>(<i>E</i>) with master equation analysis for falloff. The major reaction paths at 1000 K are formation of isooctane plus HO₂ followed by cyclic ether plus OH. Stabilization of the <i>tert</i>-isooctane hydro­peroxy radical becomes important at lower temperatures

Thermochemical Properties for Isooctane and Carbon Radicals: Computational Study

Thermochemical properties for isooctane, its internal rotation conformers, and radicals with corresponding bond energies are determined by use of computational chemistry. Enthalpies of formation are determined using isodesmic reactions with B3LYP density function theory and composite CBS-QB3 methods. Application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities are determined using geometric parameters, internal rotor potentials, and frequencies from B3LYP/6-31G­(d,p) calculations for the lowest energy conformer. Internal rotor potentials are determined for the isooctane parent and for the primary, secondary, and tertiary radicals in order to identify isomer energies. Intramolecular interactions are shown to have a significant effect on the enthalpy of formation of the isooctane parent and its radicals. The computed standard enthalpy of formation for the lowest energy conformers of isooctane from this study is −54.40 ± 1.60 kcal mol^–1, which is 0.8 kcal mol^–1 lower than the evaluated experimental value −53.54 ± 0.36 kcal mol^–1. The standard enthalpy of formation for the primary radical for a methyl on the quaternary carbon is −5.00 ± 1.69 kcal mol^–1, for the primary radical on the tertiary carbon is −5.18 ± 1.69 kcal mol^–1, for the secondary isooctane radical is −9.03 ± 1.84 kcal mol^–1, and for the tertiary isooctane radical is −12.30 ± 2.02 kcal mol^–1. Bond energy values for the isooctane radicals are 100.64 ± 1.73, 100.46 ± 1.73, 96.41 ± 1.88 and 93.14 ± 2.05 kcal mol^–1 for C3•CCCC2, C3CCCC2•, C3CC•CC2, and C3CCC•C2, respectively. Entropy and heat capacity values are reported for the lowest energy homologues

Structures, Internal Rotor Potentials, and Thermochemical Properties for a Series of Nitrocarbonyls, Nitroolefins, Corresponding Nitrites, and Their Carbon Centered Radicals

Structures, enthalpy (Δ_f<i>H</i>°₂₉₈), entropy (<i>S</i>°(<i>T</i>)), and heat capacity (<i>C</i>_<i>p</i>(<i>T</i>)) are determined for a series of nitrocarbonyls, nitroolefins, corresponding nitrites, and their carbon centered radicals using the density functional B3LYP and composite CBS-QB3 calculations. Enthalpies of formation (Δ_f<i>H</i>°₂₉₈) are determined at the B3LYP/6-31G(d,p), B3LYP/6-31+G(2d,2p), and composite CBS-QB3 levels using several work reactions for each species. Entropy (<i>S</i>) and heat capacity (<i>C</i>_<i>p</i>(<i>T</i>)) values from vibration, translational, and external rotational contributions are calculated using the rigid-rotor-harmonic-oscillator approximation based on the vibration frequencies and structures obtained from the density functional studies. Contribution to Δ_f<i>H</i>(<i>T</i>), <i>S</i>, and <i>C</i>_<i>p</i>(<i>T</i>) from the analysis on the internal rotors is included. Recommended values for enthalpies of formation of the most stable conformers of nitroacetone <i>cc(o)cno2</i>, acetonitrite <i>cc(o)ono</i>, nitroacetate <i>cc(o)no2</i>, and acetyl nitrite <i>cc(o)ono</i> are −51.6 kcal mol^–1, −51.3 kcal mol^–1, −45.4 kcal mol^–1, and −58.2 kcal mol^–1, respectively. The calculated Δ_f<i>H</i>°₂₉₈ for nitroethylene <i>ccno2</i> is 7.6 kcal mol^–1 and for vinyl nitrite <i>ccono</i> is 7.2 kcal mol^–1. We also found an unusual phenomena: an intramolecular transfer reaction (isomerization) with a low barrier (3.6 kcal mol^–1) in the acetyl nitrite. The NO of the nitrite (R-ONO) in CH₃C(O′)ONO moves to the CO′ oxygen in a motion of a stretching frequency and then a shift to the carbonyl oxygen (marked as O′ for illustration purposes)