Theoretical Study on the Selective 1,2-Reduction of α,ß-Unsaturated Ketones by Hydrogen Transfer Reaction on MgO(100).

Catalytic reduction of α,ß-unsaturated ketones with MgO has been found to improve selectivity to the desired unsaturated alcohol product. Using density functional calculations, we have studied the competitive hydrogenation of C=O and C=C bonds on Mg(100) employing two α,ß-unsaturated ketones: mesityl oxide (MO) and 2-cyclohexenone (CH), with isopropanol (IPA) as hydrogen source. For both ketones, MgO promotes the formation of a six-membered cyclic transition state for selective C=O reductions via a Meerwein-Ponndorf-Verley mechanism. Similar concerted mechanism is also possible for the C=C hydrogenation following an Eley-Rideal mechanism, in which the IPA interacts directly with the adsorbed ketone. The activation barriers are smaller for C=O reduction because this bond is activated on MgO(100). The selective C=O reduction is more favorable for CH than for MO due to the tendency of CH to be perpendicularly oriented. The desorption energies of the unsaturated alcohol are lower than the subsequent C=C reductions.

Theoretical study of glycine amino acid adsorption on graphene oxide.

The non-dissociative and dissociative adsorptions of zwitterionic Gly on graphene oxide (GO) was studied in the framework of DFT using a cluster model approach. In this work, the interaction with an epoxy group of GO basal plane was mainly considered. As a comparison, the non-dissociative and dissociative adsorptions of neutral Gly were also taken into account. The non-dissociative adsorption modes for zwitterionic and neutral Gly conformers show binding energies of 12.2 and 14.4 kcal mol-1, respectively. These molecules are thought to remain over the GO surface due to attractive noncovalent interactions. Two dissociative adsorption modes, for Z-Gly and N-Gly, show smaller binding energies of 7.2 and 8.4 kcal mol-1, where the deprotonated species links strongly through a C-O or C-N covalent bond to the GO surface. The results obtained in the present theoretical approach to the glycine/graphene oxide system support the fact that glycine can be attached to epoxy groups of graphene oxide basal planes in addition to the anchoring on edge oxidation groups. In summary, we conclude that glycine can be used as a reducing agent as well as a functionalizer of GO sheets.

Hydrogenated polycyclic aromatic hydrocarbons (HnPAHs) as catalysts for hydrogenation reactions in the interstellar medium: a quantum chemical model.

The sticking of H atoms onto dust grains and large hydrocarbon molecules has received considerable attention because it is thought to govern the formation of H2 and other H-containing molecules in the interstellar medium. Using the density functional theory (DFT) approximation, we have investigated the capacity of neutral hydrogenated polycyclic aromatic hydrocarbons (HnPAH) to catalyze simple hydrogenation reactions by acting as a source of atomic hydrogen. In particular, the interaction of OH and CO with H1-anthracene (singly hydrogenated) and H14-anthracene (fully hydrogenated) to form H2O and HCO was modeled following the Eley-Rideal mechanism. In this process, a hydrogen atom is abstracted from the HnPAH molecule forming the corresponding hydrogenated compound. The results were compared to the most known case of the HnPAH-catalyzed formation of H2. It was observed that whereas H2 is formed by overcoming activation barriers of approximately 0.02 and 0.10 eV with H1-anthracene and H14-anthracene, respectively, H2O is produced in a barrierless fashion with both hydrocarbon molecules. The production of HCO was found to be a highly unfavorable process (with activation barriers of 0.73 eV and 3.13 eV for H1- and H14-anthracene, respectively). Complementary calculations performed using the rest of the Hn-anthracene molecules (from 2 to 13 extra H atoms) showed that in all the cases the reaction with OH is barrierless as well. This efficient mechanism could therefore be a possible route for water formation in the cold interstellar medium.

Quantum chemical study on surface complex structures of phosphate on gibbsite.

Quantum mechanics calculations based on the density functional theory (DFT) were used to identify phosphate surface complexes on gibbsite at low and high pH. The different phosphate species were represented using the Al6(OH)18(H2O)6 cluster model considering four different geometries: monodentate mononuclear (Pmm), monodentate binuclear (Pmb), bidentate mononuclear (Pbm) and bidentate binuclear (Pbb). The corresponding adsorption reactions were modelled via ligand exchange between phosphate species and surface functional groups (hydroxyls and protonated hydroxyls at high and low pH, respectively). The theoretical results indicate that phosphate surface complexes are thermodynamically more favored at acid pH, in agreement with experimental evidences. The first step in these reactions, i.e., the generation of required aluminum vacant sites, was predicted to be particularly favorable when singly coordinated aquo groups are released. Stretching and bending vibrational frequencies associated with the different surface structures were calculated at both pH conditions. The corresponding values at low pH were found to be shifted to higher frequencies with respect to those ones at high pH. ATR-FTIR studies were also carried out. The resulting spectra are dominated by a strong band within the 800-840 cm(-1) interval due to P-OH stretching modes. The corresponding peak appearing around 820 cm(-1) at high pH is shifted to lower frequencies with respect to the position at low pH, a tendency well predicted by DFT calculations.

An ethanol-solvated centrosymmetric dimer of bismuth(III) and thiosaccharinate resulting from `semicoordination' contacts.

In the title compound, bis(µ-1,1-dioxo-1,2-benzothiazole-3-thiolato)-κ(3)N,S:S;κ(3)S:N,S-bis[(1,1-dioxo-1,2-benzothiazole-3-thiolato-κ(2)N,S)(ethanol-κO)bismuth(III)] ethanol hemisolvate, [Bi2(C7H4NO2S2)6(C2H5OH)2]·0.5C2H5OH, three independent thiosaccharinate (tsac) anions chelate the metal centre through the endocyclic N and exocyclic S atoms. The complex also presnts two `semicoordination' contacts, one from a pendant ethanol solvent molecule and a second one from an S atom of a centrosymmetrically related molecule. This latter interaction complements two π-π interactions between tsac rings to form a dimeric entity which is the elemental unit that builds up the crystal structure. These dinuclear units are connected to each other via a second type of π-π interaction, generating chains along [111]. Two ethanol molecules, one of them of full occupancy at a general position and semicoordinated to the central cation, and a second one depleted and disordered around a symmetry centre, stabilize the structure. The complex was studied theoretically and the vibrational assignations were confirmed by employing theoretical density functional theory (DFT) methods.