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The study uses computational quantum theory to explain thermochemistry of methyl and ethyl nitro, [RNO.sub.2], and nitrite, RONO, organic compounds. The analysis provides data for the structures and enthalpies of the corresponding aci-tatutomers.

Molecular hydrogen plays multiple roles in activationof nitrogen.Among others, it inhibits the overall process of N2-reductioncatalyzed by nitrogenase enzyme. The H2-assisted dehydrogenationand the H-atom transfer reactions (called dihydrogen catalysis, DHC)are suggested as possible mechanisms for the degradation and removalof potential intermediates formed during the reduction of nitrogen.Several iron-organic model reactions associated with the core stereospecificreaction (cis-N2H2+ H2→ N2+ H2+ H2) areexamined using a comprehensive density functional theory and ab initioanalysis of the corresponding potential energy surfaces. A varietyof energetically feasible decomposition pathways are identified forthe DHC-oxidation of iron-bound [NxHy]-species. A liberated diazene intermediate(HNNH) is suggested to interact in situ with two proximalhydridic H-atoms of an activated (hydrided) Fe-catalyst to produceN2and H2with a low or even no activation barrier.The majority of identified pathways are shown to be highly sensitiveto the electronic environment and spin configuration of metallocomplexes.The H2-assisted transport of a single H-atom from a bound[NxHy] moietyto either the proximal or distal (Fe, S or N) active centers of acatalyst provides an alternative degradation (interconversion) mechanismfor the relevant intermediates. The two types of molecular hydrogen-assistedreactions highlighted above, namely, the H2-assisted dehydrogenationand the transport of H-atoms, suggest theoretical interpretationsfor the observed H2-inhibition of N2activationand HD formation (in the presence of D2). The DHC reactionsof various [NxHy] moieties are expected to play significant roles in the industrialhigh-pressure hydrodenitrification and other catalytic processes involvingthe metabolism of molecular hydrogen. [ABSTRACT FROM AUTHOR]

Thermochemical properties of methylthiol-methylhydroperoxide, CH3SCH2OOH and its two terminal radicals resulting from the loss of hydrogen atoms, viz. CH3SCH2OO· and C·H2SCH2OOH, are important to understand the stability, reaction paths, and kinetics of the reactions of dimethyl sulfide and related sulfur-containing hydrocarbons in the atmosphere and the combustion processes. Thermochemical properties Δ Hof, S( T) and Cp( T) and internal rotation potentials for reactants, intermediates, and transition states in the methylthiomethyl (CH3SC·H2) radical + O2 reaction system are analyzed to evaluate reaction paths and kinetics versus temperature and pressure. Isodesmic reaction analysis is used to determine the enthalpies of formation (Δ Hof298) using density functional B3LYP/6-311G(d,p) and complete basis set extrapolation CBS-QB3 computational methods. Entropy and heat capacities Cp( T) are determined using geometric parameters and vibration frequencies obtained at B3LYP/6-311G(d,p) level of calculation. Δ Hfo298 values estimated at the CBS-QB3 level (units: kcal mol−1) are CH3SC·H2 (32.7), CH3SCH2OOH (-29.1), CH3SCH2OO· (5.2), C·H2SCH2OOH (11.5). Quantum Rice-Ramsperger-Kassel analysis is used to calculate energy-dependent rate constants, k( E), and master equation is used to account for collisional stabilization of peroxy adduct and the hydroperoxide isomer. The methylthiomethyl radical associates with O2 to form a methylthiomethyl peroxy radical with a well depth of 27.49 kcal mol−1. The peroxy radical can dissociate back to reactants, isomerize via hydrogen shift to form a hydroperoxide methyl radical C·H2SCH2OOH, decompose via hydrogen transfer, and undergo subsequent low-energy beta scission to form CH3SC(O)H and OH radical. The peroxy radical can also attack the sulfur atom via TS3 to form CH3S(O) + CH2O product set. The C·H2SCH2OOH isomer can decompose via an elimination transition state to CH2S + CH2O + OH, or through a four-member ring transition state to 1,3-Oxathietane + OH products. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

Sulfur-Oxygen containing hydrocarbons are formed in oxidation of sulfides and thiols in the atmosphere, on aerosols and in combustion processes. Understanding their thermochemical properties is important to evaluate their formation and transformation paths. Structures, thermochemical properties, bond energies, and internal rotor potentials of methyl sulfinic acid CHS(O)OH, its methyl ester CHS(O)OCH and radicals corresponding to loss of a hydrogen atom have been studied. Gas phase standard enthalpies of formation and bond energies were calculated using B3LYP/6-311G (2d, p) individual and CBS-QB3 composite methods employing work reactions to further improve accuracy of the . Molecular structures, vibration frequencies, and internal rotor potentials were calculated. Enthalpies of the parent molecules CHS(O)OH and CHS(O)OCH are evaluated as −77.4 and −72.7 kcal mol at the CBSQB3 level; Enthalpies of radicals CHS(O)OH, CHS(O), CHS(O)OCH and CHS(O)OCH (CBS-QB3) are −25.7, −52.3, −22.8, and −26.8 kcal mol, respectively. The CHC(O)O-H bond dissociation energy is of 77.1 kcal mol. Two of the intermediate radicals are unstable and rapidly dissociate. The CHS(O)O. radical obtained from the parent CHS(O)OH dissociates into methyl radical () plus SO with endothermicity (Δ H) of only 16.2 kcal mol. The CHS(O)OCH radical dissociates into CHS=O and CH=O with little or no barrier and an exothermicity of −19.9 kcal mol. DFT and the Complete Basis Set-QB3 enthalpy values are in close agreement; this accord is attributed to use of isodesmic work reactions for the analysis and suggests this combination of B3LYP/work reaction approach is acceptable for larger molecules. Copyright © 2010 John Wiley & Sons, Ltd. Thermochemical properties, bond energies and internal rotor potentials of methyl sulfinic acid CHS(O)OH, its methyl ester CHS(O)OCH and radicals corresponding to loss of a hydrogen atom have been studied. Standard enthalpies of formation and bond energies were calculated using B3LYP/6-311G (2d, p) and CBS-QB3 methods. Enthalpies of CHS(O)OH and CHS(O)OCH are evaluated as −77.4 and −72.7 kcal mol. Enthalpies of radicals CHS(O)OH, CHS(O),CHS(O)OCH and CHS(O)OCH are: −25.7, −52.3, −22.8 and −26.8 kcal mol respectively at CBS-QB3 level. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) based calculations are performed on a series of alkyl nitrites and nitroalkanes representing large-scale primary, secondary, and tertiary nitro compounds and their radicals resulting from the loss of their skeletal hydrogen atoms. Geometries, vibration frequencies, and thermochemical properties [S°(T) and C°p(T) (10 K ⩽ T ⩽ 5000 K)] are calculated at the B3LYP/6-31G(d,p) DFT level. ΔfH°298 values are from B3LYP/6-31G(d,p), B3LYP/6-31+G(2d,2p), and the 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 -30.57 and -28.44 kcal mol-1 for n-propane-, -33.89 and -32.32 kcal mol-1 for iso-propane-, -42.78 and -41.36 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. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 181–199, 2010 [ABSTRACT FROM AUTHOR]

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Lignin is the most complex component of biomass, and development of a detailed chemical kinetic model for biomass pyrolysis mainly relies on the understanding of the lignin decomposition kinetics. para -Coumaryl alcohol ( p -CMA, HOPh-CH═CH-CH 2 OH), the focus of our analysis, is the simplest of the lignin monomers (monolignols) containing a typical side-chain double bond and both alkyl- and phenolic-type OH-groups. In parts I and II of our work (Asatryan, R. J. Phys. Chem. A 2019, 123, 2570-2585; Hudzik, J. M. J. Phys. Chem. A 2020, current issue), we created a detailed potential energy surface (PES) and performed a kinetic analysis of chemically activated, unimolecular, and bimolecular reactions pathways for p -CMA + OH. Reaction pathways analyzed include dissociation, intramolecular abstraction, group transfer, and elimination processes. The α- and β-carbon addition reactions generate 1,3- ( RA1 ) and 1,2-diol ( RB1 ) adduct radicals, respectively. Well depths are approximately 29 and 41 kcal/mol below the p -CMA + OH entrance level. Kinetic analysis aides in determining the major pathways for our conventional and fractional pyrolysis experiments. The current paper focuses on the H-abstraction reactions via H, OH, and CH 3 light ("pool") radicals from p -CMA. The thermochemical properties of all stable, radical, and transition-state species were determined using the ωB97XD density functional theory (DFT) and higher-level CBS-QB3 composite methods. Barrier heights from the prereaction complexes, for OH-radical abstractions, to the transition states for the propanoid side chain are compared to the model H-abstraction reactions of allyl alcohol (AA) with OH and p -CMA with H and CH 3 radicals. The lowest-energy, most stable, p -CMA radical formed is at the C9 allylic position ( p -CMA-C9j) with exothermicity of 26.63, 41.32, and 27.34 kcal/mol for H, OH, and CH 3 , respectively. For OH-radical abstraction at this position, our findings are consistent with corresponding data on AA + OH at 37.44 kcal/mol and similar to that of RB1 . A similar stable radical with an exothermicity of 34.95 kcal/mol occurs for the phenol hydroxyl group, generating the p -CMA-O4j radical. H-abstraction pathways are considered in relation to other major pathways previously considered for p -CMA + OH reactions including H-atom shifts, dehydration, and β-scission reactions. Derived rate coefficients for substituted phenols can be utilized in detailed kinetic models for lignin/biomass pyrolysis.

This paper provides a first-principles theoretical investigation of the polytopal rearrangements and fluxional behavior of five-coordinate d 7 -transition metal complexes. Our work is primarily based on a potential energy surface analysis of the iron tetracarbonyl hydride radical HFe˙(CO) 4 . We demonstrate the existence of distorted coordination geometries in this prototypical system and, for the first time, introduce three general rearrangement mechanisms, which account for the non-ideal coordination. The first of these mechanisms constitutes a modified version of the Berry pseudorotation via a square-based pyramidal C 4v transition state that connects two chemically identical edge-bridged tetrahedral stereoisomers of C 2v symmetry. It differs from the classical Berry mechanism, which involves two regular D 3h equilibrium structures and a C 4v transition state. The second mechanism is related to the famous "tetrahedral jump" hypothesis, postulated by Muetterties for a number of d 6 HML 4 and H 2 ML 4 complexes. Here, our study suggests two fluxional rearrangement pathways via distinct types of C 2v transition states. Both pathways of this mechanism can be described as a single-ligand migration to a vacant position of an "octahedron", thus interchanging (switching) the apical and basal ligands of the initial quasi-square pyramidal isomer, which is considered as an idealized octahedron with a vacancy. Accordingly, we call this mechanism "octahedral switch". The third mechanism follows a butterfly-type isomerization featuring a key-angle deformation, and we thus call it "butterfly isomerization". It connects the quasi-square pyramidal and edge-bridged tetrahedral isomers of HFe˙(CO) 4 through a distorted edge-bridged tetrahedral transition state of C s symmetry. Our paper discusses the overall features of the isomers and rearrangement mechanisms as well as their implications. We rationalize the existence of each stationary point through an electronic structure analysis and argue their relevance for isolobal analogues of HFe˙(CO) 4 .