Nadia Sebbar

Structures, thermochemical properties (enthalpy, entropy and heat capacity), rotation barriers, and peroxide bond energies of vinyl, allyl, ethynyl and phenyl hydroperoxides

Alkyl hydroperoxides and peroxy radicals are important intermediates in atmospheric chemistry and in low to moderate temperature combustion processes, where they are strongly linked to knock in spark ignition engines and the observed negative temperature coefficient in thermal hydrocarbon oxidation. Enthalpy, ΔH0f298, entropy, S0298, and heat capacities, Cp (T), (300 ⩽ T/K ⩽ 1500), are determined for vinyl, allyl, ethynyl and phenyl hydroperoxides using the density functional B3LYP/6-311G(d, p) calculation method. The molecular structures and vibration frequencies are determined at the B3LYP/6-311G(d,p) level, and frequencies are scaled for zero point energies and for thermal corrections. Enthalpies of formation (ΔH0f298) are determined at the B3LYP/6-311G(d,p) level using three isodesmic working reactions for the hydroperoxides. Entropy (S) and heat capacity (Cp (T), values from vibrational, 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 S and Cp(T) from analysis on the internal rotors are used in place of torsion frequencies. ΔH0f298 for vinyl hydroperoxide, CH2CHOOH, is −9.63 and for allyl hydroperoxide, CH2CHCH2OOH, −13.59 (values in kcal mol−1). Methyl substituted vinyl hydroperoxide values are CH2C(CH3)OOH, −21.80; CH3CHC(CH3)OOH, −30.03 and CH3(CH3)CCHOOH, −30.79. The cis conformation of CH3CHCHOOH, −21.66, is more stable than the trans form, −20.44. Enthalpies for ethynyl hydroperoxides are 42.25 kcal mol−1 for HCCOOH and 30.26 kcal mol−1 for CH3CCOOH. The calculated ΔH0f298 for phenyl hydroperoxide, C6H5OOH, is −2.68 kcal mol−1. The resulting hydroperoxide enthalpies allow determination of the R–OOH, RO–OH, ROO–H bond energies. The vinyl and ethynyl hydroperoxides are found to have weak RO–OH bond energies; they are unstable and their formation in reaction systems can lead to chain branching. Enthalpies of formation were also calculated for a number of unsaturated ethers and alcohols because the values were needed in the working reactions for the hydroperoxides. CH2CHCH2OCH3 (−25.68), cis and trans CH3CHCHOCH3 (−36.24, −34.33 kcal mol−1), CH2C(CH3)OCH3 (−32.55), CH3(CH3)CCHOCH3 (−43.72), CH3(CH3)CCOH (−49.31), CHC–O–CH3 (26.08), CH3–CC–OH and CH3–CC–O–CH3 (9.84, 15.93) (kcal mol−1).

Thermochemistry and kinetics for 2-butanone-1-yl radical (CH2·C(═O)CH2CH3) reactions with O2.

Thermochemistry of reactants, intermediates, transition state structures, and products along with kinetics on the association of CH2·C(═O)CH2CH3 (2-butanone-1-yl) 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 theory (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 a 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 falloff 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.

Thermochemical similarities among three reaction systems:Vinyl O2 - Phenyl O2 - Dibenzofuranyl O2

It is interesting to study and compare the thermochemical parameters in the vinyl + O2, phenyl + O2 and dibenzofuranyl + O2 chemical activation reaction systems as well as the corresponding peroxy radical and hydroperoxide adduct dissociations. The analogy in C–O and C–H bond energies, well depths, bond type environment and reaction paths in these three reaction systems suggest a corresponding similarity in the initial reaction kinetics of these unsaturated R• + O2 reactions. In this study we identify the similarities in thermochemistry, bond energies and reaction kinetics of paths. The similar structures and parameters suggest that calculations on the vinyl system can often serve as surrogate or model for reaction paths and kinetic barriers in the larger phenyl system and the large dibenzofuranyl system for which high-level calculations are difficult. Here the vinyl + O2 and the phenyl + O2 systems can be used as model for the dibenzofuranyl + O2 system.

Thermochemistry and Kinetics for 2-Butanone-3yl Radical (CH3C(=O)CH•CH3) Reactions with O2

Abstract Thermochemistry and chemical activation kinetics for the reaction of the secondary radical of 2-butanone, 2-butanone-3yl, with 3O2 are reported. Thermochemical and kinetic parameters are determined for reactants, transition states structures and intermediates. Standard enthalpies and kinetic parameters are evaluated using ab initio (G3MP2B3 and G3), density functional (B3LYP/6-311g(d,p)) calculations and group additivity (GA). The C–H bond energies are determined for the three carbons of the 2-butanone, showing that the C–H bond energy (BE) on the secondary carbon is low at 90.5 kcal mol−1. The CH3C(=O)CH•CH3 radical + O2 association results in chemically-activated peroxy radical with 26 kcal mol−1 excess of energy. The chemically activated adduct can dissociate to butanone-oxy radical + O, react back to butanone-yl + O2, form cyclic ethers or lactones, eliminate HO2 to form an olefinic ketone, or undergo rearrangement via intramolecular abstraction of hydrogen to form hydroperoxide and/or OH radicals. The hydroperoxide-alkyl radical intermediates can undergo further reactions forming cyclic ethers (lactones) and OH radicals. Quantum RRK analysis is used to calculate k(E) and master equation analysis is used for evaluation of pressure fall-off in these chemical activated reaction systems.

Thermodynamic properties (S298, Cp(T), internal rotations and group additivity parameters) in vinyl and phenyl hydroperoxides

Thermodynamic properties, S298° and Cp,298(T) (300 ⩽ T/K ⩽ 1500)) and internal rotational barriers for trans-CH3CHCHOOH, cis-CH3CHCHOOH, (CH3)2CCHOOH and CH3CHC(CH3)OOH are calculated using density functional calculations at the B3LYP/6-311G(d,p) levels. Entropies (S298° in cal mol−1 K−1) and heat capacities Cp298(T) in cal mol−1 K−1) were calculated using the B3LYP/6-311G(d,p) determined frequencies and geometries. Thermodynamic properties for the oxygenated carbon group O/Cd/O are determined with existing literature values of group parameters and data on trans-CH3CHCHOOH, cis-CH3CHCHOOH, (CH3)2CCHOOH, CH3CHC(CH3)OOH, CH2C(CH3)OOH. The O/Cb/O group was estimated by using the data calculated on C6H5OOH. The moments of inertia and vibrational frequencies as well as the structure parameters have been reported in an earlier study.

Thermochemistry and reaction paths in the oxidation reaction of benzoyl radical: C6H5C•(═O).

Alkyl substituted aromatics are present in fuels and in the environment because they are major intermediates in the oxidation or combustion of gasoline, jet, and other engine fuels. The major reaction pathways for oxidation of this class of molecules is through loss of a benzyl hydrogen atom on the alkyl group via abstraction reactions. One of the major intermediates in the combustion and atmospheric oxidation of the benzyl radicals is benzaldehyde, which rapidly loses the weakly bound aldehydic hydrogen to form a resonance stabilized benzoyl radical (C6H5C(•)═O). A detailed study of the thermochemistry of intermediates and the oxidation reaction paths of the benzoyl radical with dioxygen is presented in this study. Structures and enthalpies of formation for important stable species, intermediate radicals, and transition state structures resulting from the benzoyl radical +O2 association reaction are reported along with reaction paths and barriers. Enthalpies, ΔfH298(0), are calculated using ab initio (G3MP2B3) and density functional (DFT at B3LYP/6-311G(d,p)) calculations, group additivity (GA), and literature data. Bond energies on the benzoyl and benzoyl-peroxy systems are also reported and compared to hydrocarbon systems. The reaction of benzoyl with O2 has a number of low energy reaction channels that are not currently considered in either atmospheric chemistry or combustion models. The reaction paths include exothermic, chain branching reactions to a number of unsaturated oxygenated hydrocarbon intermediates along with formation of CO2. The initial reaction of the C6H5C(•)═O radical with O2 forms a chemically activated benzoyl peroxy radical with 37 kcal mol(-1) internal energy; this is significantly more energy than the 21 kcal mol(-1) involved in the benzyl or allyl + O2 systems. This deeper well results in a number of chemical activation reaction paths, leading to highly exothermic reactions to phenoxy radical + CO2 products.

Ketene Formation Through Interaction Reactions During P2O3/P2O5/CH3C(=O)OH Pyrolysis

ABSTRACT Ketene is formed by thermal elimination of H2O from acetic acid at temperatures above 923 K. In industry, this process is catalyzed by P2O3/P2O5, which is introduced via organic phosphorous compounds. To optimize the ketene formation, a detailed chemical kinetic mechanism for the P2O3/P2O5-catalyzed formation of ketene (CH2=C=O) in the 923–1123 K temperature range has been developed. With the help of Quantum Chemistry methods DFT (at B3LYP/6-311g(d,p)) and ab-initio (G3MP2B3) calculations, a model describing the high temperature CH3C(=O)OH/P2O3/P2O5 system has been developed and combined with the modified mechanism from Mackie and Doolan (1984). The addition of P2O3/P2O5 to acetic acid results in a markedly accelerated ketene formation. The conversion of acetic acid increases with increasing temperature and with higher content of P2O5. The mechanism allows the design of temperature profiles for the ketene synthesis with optimal conversion of acetic acid.

Thermochemical Properties for Hydrogenated and Oxy-Hydrogenated Aluminum Species

The availability of accurate thermochemical data such as heats of formation, heat capacities, and entropies for AlxHy, AlxOy, and AlxOyHz species in the gas phase, is essential in many applications. In this study, we predict geometries and enthalpy of formation, , of a large set of aluminum species (reactants, intermediates, radicals, and products) describing the AlxOyHz system. Three calculation methods with proven accuracy, Density Functional theory (DFT) at (B3LYP/6‐311G(d,p)), G3MP2, and CBS-QB3, are used for each species. The ab initio and Density Functional calculations are combined with isodesmic reaction analysis, whenever possible, in order to improve the accuracy of the enthalpy values. Estimation and analysis of bond energies in AlHx and AlHxOy are reported. Entropies, , and heat capacities, were calculated using DFT calculation results.

Prediction of Thermodynamic and Kinetic Parameters for Interfacial Reactions of the SIO2 System by Quantum Chemistry Methods

With the help of Quantum Chemistry methods, this study analyzed gas/solid chemical reactions, as well as weak interaction reactions of the SiO2 system with the surface. Requirements are for the prior calculation of thermodynamic properties of the partner reactions and the determination of all transition states products, which occur during the complicated reaction processes and between the precursors and the surface. A molecule model or molecule cluster will represent the surface of the solid particles. The final target for this study was to derive and develop new reaction paths for the development of a mechanism to describe the formation and growth of SiO2 in gas phase synthesis from SiH4 through flame synthesis. After validation of the accuracy of the Density Functional Theory (DFT), enthalpies, , of a series of stable molecules, radicals, and transition state structures are calculated using ab initio and (DFT) calculations. The ab initio and Density Functional calculations were combined with isodesmic reaction analysis, whenever possible, in order to improve the accuracy of the enthalpy values. Entropies, , and heat capacities, Cpf298 (T) were calculated using Density Functional (DFT) calculation results. Kinetic parameters for new reaction paths were estimated as well for 500 K to 1800 K temperature range and 0.001 to 300 atm pressure range.

A thermochemical study on the primary oxidation of sulfur

ABSTRACT Several chemical reactions related to the oxidation and combustion of sulfur are investigated using a number of computational chemistry methods with the objective of determining appropriate methods for use in developing an elementary reaction mechanism for oxidation of sulfur. Calculations are focused on thermochemical properties and reaction energetics for reactive species and transition state structures for reactions in the oxidation/combustion of sulfur. Reactions involving several intermediates resulting from the reactions of S2 with oxygen were investigated with the density functional theory B3LYP (with several basis sets) and BB1K/GTLarge. The composite ab-initio methods G2, G3, G3MP2, G3B3, G3MP2B3 and CBS-QB3 were also used. Enthalpies of a series of sulfur compounds and transition state structures are calculated using the ab-initio and DFT calculations. The calculations were combined with isodesmic reaction analysis, whenever possible, in order to cancel error and improve the accuracy of the calculations. Results show that all B3LYP DFT calculations including the 6–311++G(3df,2p) basis set show poor outcome in estimating the enthalpy of reactions involving S2. The six composite methods have all shown consistency with each other and their calculated reaction energies/bond energies are in good agreement with the available literature. Kinetic parameters for calculation of the kinetic parameters on SO3 dissociation to SO2 and O using the canonical transition state theory are reported.