Alicia H. Jubert

A molecular-orbital picture of thiophene hydrodesulfurization: Part 1. Catalyst activation process

Abstract Hydrogen adsorption, generation of anion vacancies, and the change in Mo oxidation state are studied in this work as an approach to understanding the activation process of the well-known HDS catalyst MoS 2 . Semi-empirical molecular-orbital calculations are performed, and cluster modeling is used to represent the solid. Results agree quite well with some previously published data, and they also allow us to make some interesting suggestions. Moreover, two important points considered in doing the calculations (sulfur d orbitals and dangling bonds) are discussed in the text with numerical examples.

A molecular orbital picture of thiophene hydrodesulfurization. Part 2. Thiophene adsorption

Abstract The second step of the hydrodesulfurization process on MoS2, thiophene adsorption, is studied in this work by means of semiempirical molecular orbital calculations. Several interesting conclusions can be drawn from the results concerning the adsorption process. Moreover, some suggestions can be made for the thiophene desulfurization step comparing the results obtained with different experimental and theoretical models.

UV-visible spectroscopy in the interpretation of the tautomeric equilibrium of N,N′(bis-3,5-di-bromo-salicyliden)-1,2-diaminobenzene and the redox activity of its Co(II) complex. A quantum chemical approach

Abstract By means of quantum chemical calculations, the origin of the redox capability of N,N′(bis-3,5-di-bromo-salicyliden)-1,2-diaminobenzene Co (II) complexes, of major interest in the catalytic reduction of naphtas, is analyzed. The geometry, electronic structure and UV-vis spectra of both the complex and the ligand are studied. AM1 and INDO/1 hamiltonians are used for the conformational analysis, whereas the UV-visible spectra are accurately reproduced by means of INDO/S-CI calculations, justifying the color of the ligand and offering an explanation to the catalytic redox activity of the Co(II) complex, based on the electronic transitions associated to the spectroscopic features and the electronic characteristics of the redox states.

Conformational and electronic (AIM/NBO) study of unsubstituted A-type dimeric proanthocyanidin

The conformational space of the unsubstituted A-type dimeric proanthocyanidin was scanned using molecular dynamics at a semiempirical level, and complemented with functional density calculations. The lowest energy conformers were obtained. Electronic distributions were analysed at a higher calculation level, thus improving the basis set. A topological study based on Bader’s theory (AIM: atoms in molecules) and natural bond orbital (NBO) framework was performed. Furthermore, molecular electrostatic potential maps (MEPs) were obtained and analysed. NMR chemical shifts were calculated at ab initio level and further compared with previous experimental values; coupling constants were also calculated. The stereochemistry of the molecule is thoroughly discussed, revealing the key role that hyperconjugative interactions play in defining experimental trends. These results show the versatility of geminal spin–spin coupling 2J(C-1′,O) as a probe for stereochemical studies of proanthocyanidins.

Theoretical study of Z isomers of A-type dimeric proanthocyanidins substituted with R=H, OH and OCH3: stability and reactivity properties

The stereochemistry of A-type dimeric proanthocyanidins was studied, focusing on the factors that determine it, and the changes that occur with R = OCH3, R′ = H, and R = OH, R′ = H as substituents, starting with the study of the conformational space of each species. Using molecular dynamics at a semiempirical level, and complementing with functional density calculations, two conformers of lowest energy were characterized for R = H, eight conformers for R = OH, and three conformers for R = OCH3. Electronic distributions were analyzed at a higher calculation level, thus improving the basis set. Intramolecular interactions were examined and characterized by the theory of atoms in molecules (AIM). Detailed natural bond orbitals (NBO) analysis allowed the description of subtle stereoelectronic aspects of fundamental importance for understanding the stabilization and antioxidant function of these structures. The study was enriched by a deep analysis of maps of molecular electrostatic potential (MEP). The coordinated analysis of MEP, together with the NBO and AIM results, allowed us to rationalize novel distribution aspects of the potential created in the space around a molecule.

H2on MoS2: a molecular orbital study

Abstract Hydrogen adsorption on molybdenite was examined in the cluster approximation based on the extended Huckel method. Molybdenum aulfide is a typical catalyst for the hydrodesulfurization process and the hydrogen adsorbed plays a fundamental role in its activation. Our results agree with the experimental ones: dissociative chemisorption, two forms of adsorbed hydrogen, small crystallites which ensure a high concentration of edge sites, and a justification for catalyst activity in the absence of hydrogen. We give a qualitative molecular-orbital explanation.

Topological Insights into the Nature of the Halogen−Carbon Bonds in Dimethylhalonium Ylides and Their Cations

In this study the nature of the bonding in a series of dimethylhalonium ylides (fluoronium, chloronium, bromonium and iodonium) was analyzed by means of topological methodologies (AIM and ELF analysis), to document the changes in the nature of the C-X bonds (X = F, Cl, Br, I) upon the series. For the sake of comparison the same study was performed on the corresponding dimethylhalonium cations (XC 2H 6 (+)) and the XCH 3 series. The wave functions used for the topological analysis were obtained at B3LYP level using extended triple-zeta basis sets. The formation of the cationic XC 2H 6 (+) structures can be interpreted to arise from the interaction between the XCH 3 and CH 3 (+) moieties. The resultant structures can be explained in terms of the superposition of two electrostatically interacting and two dative mesomeric structures. The halogen-carbon bonds have all the characteristics of the charge-shift (CS) bonds. The analysis of the C-X bond in the XC 2H 5 series shows a progressive reinforcing of the CH 3X-CH 2 bond, from FC 2H 5 that can be considered as formed from two fragments, FCH 3 and CH 2, to IC 2H 5, in which the CH 3I-CH 2 bond has all the features of a multiple bond involving atoms bearing lone pairs. Particularly interesting is BrC 2H 5, in which a special type of bond (hybrid covalent-dative double bond) has been characterized. The energetic stability of the XC 2H 5 structures with respect to the dissociation into the XCH 2 + CH 3 and XCH 3 + CH 2 ground-state fragments was studied in detail.

Density functional calculations of Ni5 and Ni6 clusters

Abstract Density functional calculations on the electronic structure and magnetic properties of Ni5 and Ni6 clusters are presented in this work. The geometry and spin state of clusters are optimized for several starting symmetries. Moreover, those calculations are followed by a vibrational analysis to discriminate between real minima and saddle-points on the potential energy surface of clusters. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies of stable Ni5 and Ni6 clusters are reported.

A theoretical study of the relative stability of the isomeric forms of N2O3

The electronic structure, geometrical parameters and relative stability of the isomeric forms of N2O3 are analysed by means of ab initio calculations. Total energies of the different isomers are given. The energy difference between the most stable conformers of the symmetric N2O3 is 4.31 Kcal mol−1 as provided by 6–31G basis set. The height of the rotational barrier determined by the ab initio technique is 7.12 kcal mol−1.

A conformational study of 3,3,6,6-tetramethyl-1,2,4,5-tetroxane using molecular dynamics and density functional theory

Abstract A theoretical study of the conformational space of the cyclic diperoxide 3,3,6,6-tetramethyl-1,2,4,5-tetroxane is presented in this work. Molecular dynamics simulations are performed to scan the conformational space of the molecule. Density functional calculations are carried out on the stable conformers to get a better description of molecular geometries and total energy differences. The conformers are properly characterized by means of a vibrational analysis after geometry optimization. Further, a natural bond orbital analysis is used to shed light into the relative stability of the different stable conformers.