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Triacylglycerides (TAGs) and their related fatty acids (FAs) are abundant in biomass (vegetal oils and animal fats). Here, we studied by theoretical calculations the most representative FAs and their chemical reactions under hydrogen atmosphere to produce O-free hydrocarbon molecules. In general, the ΔHr0 and ΔGr0 do not vary due to the size of the molecule. Selected TAGs were also studied by theoretical calculations. Similar as FAs, the overall reaction energies do not depend on the size of the molecules. Possible reaction networks for the hydrogenation and hydrogenolysis processes are proposed for such biomolecules under hydrogen environments. We also studied the profiles of the enthalpy and Gibbs energy of reaction for TAGs at typical operating conditions of temperature or pressure. This self-consistent study is willing to contribute for a better understanding of the reactions of the biomass that take place under hydrogen atmosphere.

We performed a theoretical study of the interaction between Ni-containing compounds and amorphous SiO 2 support, which have been used for preparing supported catalysts. The aim of this work is to understand the effect of the chemical structure of the metal species and its interaction with the support during the preparation of heterogeneous catalysts. The metal–support interactions were revisited for a series of Ni-containing compounds with different ligands adsorbed on SiO 2 . We studied some of most commonly used ligands for preparing Ni/SiO 2 catalysts such as ethylenediamine, citric acid and EDTA. Replacement of the first atom of the ligand bound to Ni by the –OH surface group of the support was modeled using the ONIOM combined quantum mechanics/molecular mechanics (QM/MM) approach. The adsorption geometry and the Gibbs energy are clearly related to the ligand structure. We also noted the stabilization of the adsorbed compounds through cooperative hydrogen bonds with the –OH moiety adjacent surface group present on SiO 2 . We point out the importance of the ligand structure in the metal–support interactions. These results can be useful to understand the metal– and ligand–support interactions during the preparation of supported catalysts.

Nitrous oxide (N2O) is an intermediate compound formed during catalysis occurring in automobile exhaust pipes. In this work, the N2O capture and activation by Pt and Pd atoms in the ground and excited states of many multiplicities are studied. Pt and Pd + N2O reactions are studied at multireference second-order perturbation level of theory using Cs symmetry. The PtN2O (1A′, 5A′, and 5A″) species are spontaneously created from excited states. Only the 5A′ and 5A″ states exhibit N2O activation reaction paths when N2O approaches Pt end-on by the N or O atoms side or side-on yielding NO or N2 as products, respectively. Pt+ cations ground and excited states, capture N2O, although only Pt+ (6A′ and 6A″) states show N2O activation yielding O and N2 as products. In the Pd atom case, PdN2O (1A′ and 5A″) species are also spontaneously created from excited states. The 5A″ state exhibits N2O activation yielding N2 + O as products. Pd+ cations in both ground and excited states capture N2O; however, only the [PdN2O]+ (4A′, 4A″, 6A′, and 6A″) states in side-on approaches and (6A′) in end-on approach activate the N2O and yield the N2 bounded to the metal and O as product. The results obtained in this work are discussed and compared with previous calculations of Rh and Au atoms. The reaction paths show a metal–gas dative covalent bonding character. Lowdin charge population analyses for Pt and Pd active states show a binding done through charge donation and retrodonation between the metals and N2O. © 2013 Wiley Periodicals, Inc.

This work shows that indigo's high stability can be attributed both to the large π conjugation inside the molecule and to intra- and intermolecular hydrogen bonds. The theoretical investigation of indigo's electronic structure has been performed using high-level methods. To understand the interactions in solid state, calculations of the dimer system with both molecules in the same plane was carried out. In the monomer, two intramolecular hydrogen bridges between amino and carbonyl groups occupy positions that would otherwise be the most reactive ones for nucleophilic and electrophilic attacks. In the dimer, amino and carbonyl groups on different monomers form intermolecular multicentred non-linear hydrogen bonds in six-member rings, protecting again the same reactive centres and explaining the limited solubility of indigo. The addition of the free radical OH breaks the central C = C double bond, the conjugation and the hydrogen bridges as a first step. The Gibbs energy calculation favours the addition of ...

The OH abstraction of a hydrogen atom from both the side chain and the ring of toluene has been studied in the range 275-1000 K using quantum chemistry methods. It is found that the best method of calculation is to perform geometry optimization and frequency calculations at the BHandHLYP/6-311++G(d,p) level, followed by CCSD(T) calculations of the optimized structures with the same basis set. Four different reaction paths are considered, corresponding to the side chain and three possible ring hydrogen abstractions, and the branching ratio is determined as a function of temperature. Although negligible at low temperatures, at 1000 K ring-H abstraction is found to contribute 11% to the total abstraction reaction. The calculated rate coefficients agree very well with experimental results. Side chain abstraction is shown to occur through a complex mechanism that includes the reversible formation of a collisionally stabilized reactant complex.

Maya blue is a marvelous pigment with extraordinary properties. It was invented by the Maya around VII–VIII century and used by many Mesoamerican peoples in prehispanic times. It is made by encapsulating natural indigo into an inorganic clay matrix of palygorskite. The palygorskite–indigo mixture becomes acid-resistant when a moderate thermal treatment is applied. The chemical reasons of the unusual stability of the pigment and the exact mechanism of interaction between the indigo and the clay are not well understood. We present a Raman study of different preparations of Maya blue and other mixtures of indigo with other inorganic materials. We found that the unheated mixture of indigo with palygorskite presents the same Raman spectrum as Maya blue, indicating that the differences with respect to the indigo spectrum are not due to the interaction produced during the thermal treatment, which makes the mixture acid-resistant. Moreover, indigo mixed with other clays, like sepiolite or montmorillonite, presents a Raman spectrum very similar to that of Maya blue. Some chemical mechanisms that could explain these spectra, and the suitability of Raman spectroscopy for identifying Maya blue are discussed. Copyright © 2006 John Wiley & Sons, Ltd.

In this work, the reaction of the atomic radicals H, O( 3 P), F and Cl with xylenes has been studied using ab initio theoretical methods. The stability of the isomeric adducts is analyzed and rules for the selectivity of addition of the different radicals are discussed. Ipso addition is found to be clearly favored for fluorine. In all cases, positions that are either ortho or ipso to a methyl group yield the most stable adducts, while carbon atoms that are in para positions with respect to the methyl groups seem to be the least susceptible of attack. These preferences contrast with normal addition rules for electrophilic reactants. Estimates of the enthalpy changes at 298 K for the reaction C 6 H 4 ( CH 3 ) 2 + X → C 6 H 4 ( CH 3 ) 2 X are determined for the first time. Values for Δ H r are compared with estimates obtained using Benson's additivity rules. They correlate very well with the atomic radius, the ionization potential, hardness and polarizability of the atoms.

In recent work we have found that, in the case of the OH addition to toluene, ipso addition may be important. Moreover, quite different values were calculated for several of the EPR coupling constants of the different adduct isomers, suggesting that electron paramagnetic resonance measurements could provide decisive information on the selectivity of the OH radical addition reaction to toluene. In this work, the reaction of the atomic radicals, H, O(3P), F and Cl with toluene has been studied using ab initio theoretical methods to compare the stability of the isomeric adducts and to predict their expected hyperfine coupling constants. Ipso addition is found to be favored only for fluorine, although the ipso and ortho isomers have very similar energies for oxygen and chlorine. Ipso adducts are expected to contribute nonnegligibly in all cases.

High-level theoretical methods based on density functional and Hartree−Fock−Moller−Plesset theories have been employed to study the selectivity of the OH radical attack on toluene and to predict expected hyperfine coupling constants of the isomeric addition adducts. In recent work, we have found that the adduct with OH• added to the methyl-substituted carbon atom (ipso addition) of the ring may be especially stable both in toluene and in the xylenes. However, since this kind of adduct does not form a phenol, it is not directly identified in the products. The calculations indicate that quite different values should be observed for the EPR signals of the methyl hydrogen atoms of the four isomers, suggesting that electron paramagnetic resonance measurements could provide decisive information on the selectivity of the OH• addition to toluene. The calculated values of the hyperfine coupling constants are compared with the experimental values reported by Jolibois et al. for thymine.

Two different theoretical approaches are used to study the OH radical attack on toluene: the Moller−Plesset perturbation theory and the B3LYP density functional method. The critical points of the potential energy surface for the OH addition to toluene are determined, and rate−equilibrium relationships are discussed. A stable structure corresponding to a prereactive complex which is formed when the OH radical is at about 2.5 A from toluene is obtained. The existence of this loosely bound system is necessary to explain the experimentally observed negative activation energy. The geometry of transition states and products are determined for addition at different positions in the ring, including the ipso position, which has not been considered in previous works. Energy results at the MP4 and coupled cluster levels calculated at the optimized MP2 and B3LYP geometries confirm that the ipso adduct is more stable than the ortho adduct by about 0.5 kcal/mol. Several routes are proposed for the subsequent reactions...

Unrestricted Hartree-Fock calculations have been carried out to study the benzylperoxyl radical, an intermediate atmospheric contaminant. For this purpose, quantum mechanical MP2//UHF ab initio calculations were performed by using 6-31G∗∗ and 6-31 + G ∗∗ basis sets. Because no previous experimental nor theoretical data were available for benzylperoxyl, the results were compared with those of the smaller unsaturated peroxyl radicals, such as allylperoxyl. It is found that the calculated absolute minimal energy corresponds to a trans conformation in which the OO bond is sticking away from the benzene ring. Benzylperoxyl is found also to exhibit an unusually OO small bond order, which reflects the weakness of the bond, i.e. its ability to react with NO forming NO2 and subsequently ozone. A peculiar MO diagram is obtained, with the unpaired MO deeply located in the “doubly” occupied MO space. This effect is also observed in allylperoxyl. Structural parameters, charges, spin densities and dipole moments are also reported.

The most relevant chemical reactions that take place in the atmosphere involve free radicals and volatile organic compounds (usually termed VOC). In the troposphere, the main sink of volatile organic compounds (VOC) is oxidation, initiated typically by reaction with hydroxyl (OH) free radicals. Many of these processes give rise to the formation of new radicals, which ultimately cause higher OH radical levels and thus higher rates of reactions of the other VOC present.

OH radical reactions with benzene and toluene have been studied in the 200-600 K temperature range via the CBS-QB3 quantum chemistry method and conventional transition-state theory. Our study takes into account all possible hydrogen abstraction and OH-addition channels, including ipso addition. Reaction rates have been obtained under pseudo-first-order conditions, with aromatic concentrations in large excess compared to OH concentrations, which is the case in the reported experiments as well as in the atmosphere. The reported results are in excellent agreement with the experimental data and reproduce the discontinuity in the Arrhenius plots in the 300 KT400 K temperature range. They support the suggestion that the observed nonexponential OH decay is caused by the existence of competing addition and abstraction channels and by the decomposition of thermalized OH-aromatic adducts back to reactants. We also find that the low-temperature onset of the nonexponential decay depends on the concentration of the aromatic compounds and that the lower the concentration, the lower the temperature onset. Under atmospheric conditions, nonexponential decay was found to occur in the 275-325 K range, which corresponds to temperatures of importance in tropospheric chemistry. Branching ratios for the different reaction channels are reported. We find that for Tor = 400 K the reaction occurs exclusively by H abstraction. At 298 K, ipso addition contributes 13.0% to the overall OH + toluene reaction, while the major products correspond to ortho addition, which represents 43% of all possible channels.

International audience; All electron ab initio calculations for the interaction of H2O with Cl-2 and Br-2 are reported for the ground state and the lowest triplet and singlet Pi excited states as a function of both the X-X and O-X bond lengths (X = Cl or Br). For the ground state and lowest triplet state, the calculations are performed with the coupled cluster singles, doubles, and perturbative triple excitation level of correlation using an augmented triple-zeta basis set. For the (1)Pi state the multireference average quadratic coupled cluster technique was employed. For several points on the potential, the calculations were repeated with the augmented quadruple-zeta basis set. The ground-state well depths were found to be 917 and 1183 cm(-1) for Cl-2 and Br-2, respectively, with the triple-zeta basis set, and they increased to 982 and 1273 cm(-1) for the quadruple-zeta basis set. At the geometry of the ground-state minimum, the lowest energy state corresponding to the unperturbed (1)Pi states of the halogens increases in energy by 637 and 733 cm(-1), respectively, relative to the ground-state dissociation limit of the H2O-X-2 complex. Adding the attractive ground-state interaction energy to that of the repulsive excited state predicts a blue-shift, relative to that of the free halogen molecules, of approximate to 1600 cm(-1) for H2O-Cl-2 and approximate to 2000 cm(-1) for H2O-Br-2. These vertical blue-shifts for the dimers are greater than the shift of the band maximum upon solvation of either halogen in liquid water

The initial reaction in the atmospheric oxidation of xylenes is the attack by OH radicals, which occurs mainly by addition of OH to the aromatic cycle. In this work, this reaction has been modeled for o-, m- and p- xylenes by means of ab-initio molecular orbital calculations, using unrestricted MP2 method. The xylene + OH reaction is studied in detail with the 6-31G* basis, by optimizing the geometry of the possible products and of the corresponding transition states. OH-xylene adducts formed by addition of the OH radical to the carbon atom attached to a methyl radical are found to be very stable, and the energy barriers for their formation are comparable to the ones for the addition at the other positions. As far as we know, except for o- xylene, these adducts have not been considered in previous experimental work. Results of B3LYP calculations at the MP2 optimized geometries are also reported. Trends with the two methods agree well.

This paper is an extension of the techniques developed by us [A. Vivier-Bunge, V. H. Uc, and Y. G. Smeyers, J. Chem. Phys. 109, 2279 (1998)] for standard propanal. In that paper the potential energy surface for the simultaneous methyl and asymmetric aldehydic torsions was calculated at RHF/MP2 level using the 6-311(3df,p) basis set for propanal. The fit of the energy values to symmetry-adapted functional forms was carried out by using the 28 energy values which retain the C(3) dynamical symmetry of the methyl group in the optimization procedure. With this potential, as well as with the kinetic parameters and the electric dipole moment variations, the FIR frequencies and intensities for the methyl and aldehyde torsions of seven symmetric isotopomers of propanal were determined theoretically using two-dimensional calculations. The calculated spectra of propanal and three of its isotopomers were compared with the available experimental data. It is found that the calculations for the cis conformer satisfactorily reproduce the aldehyde and methyl torsion spectra and furnish also methyl torsionally excited progressions for the aldehyde torsion modes. The methyl torsion frequencies agree especially well whenever the methyl group is nondeuterated. The small deviations encountered for the deuterated compound are probably due to some mass effect, such as the zero-point vibrational energy correction, which is not taken into account in the present calculations. Finally, the influence of the deuteration on the intensities is discussed. Copyright 2000 Academic Press.