σ-Hole intermolecular interactions between carbon oxides and dihalogens: Ab-initio investigations.

Recently, halogen bonding (XB) has received increased attention as a new type of non-covalent interaction widely present in nature. In this work, quantum chemical calculations at DFT level have been carried out to investigate halogen bonding interactions between COn (n = 1 or 2) and dihalogen molecules XY (X = F, Cl, Br, I and Y = Cl, Br, I). Highly accurate all-electron data, estimated by CCSD(T) calculations, were used to benchmark the different levels of computational methods with the objective of finding the best accuracy/computational cost. Molecular electrostatic potential, interaction energy values, charge transfer, UV spectra, and natural bond orbital (NBO) analysis were determined to better understand the nature of the XB interaction. Density of states (DOS) and projected DOS were also computed. Hence, according to these results, the magnitude of the halogen bonding is affected by the halogen polarizability and electronegativity, where for the more polarizable and less electronegative halogen atoms, the σ-hole is bigger. Furthermore, for the halogen-bonded complexes involving CO and XY, the OC∙∙∙XY interaction is stronger than the CO∙∙∙XY interaction. Thus, the results presented here can establish fundamental characteristics of halogen bonding in media, which would be very helpful for applying this noncovalent interaction for the sustainable capture of carbon oxides.

Plasma rich in growth factors versus Mitomycin C in photorefractive keratectomy.

ABSTRACT: To evaluate the efficacy and safety of plasma rich in growth factors (PRGF) in photorefractive keratectomy (PRK) versus Mitomycin C (MMC).This is a comparative, longitudinal and retrospective case-control study (MMC vs PRGF), in patients with a spherical correction from -0.25 to -8.00 D and cylinder correction from -0.25 to -3.00. The uncorrected distance visual acuity (UDVA), refractive efficacy and safety indices, and changes in endothelial cell density were evaluated. The predictability was assessed with the postoperative manifest spherical equivalent.Forty-four patients (72 eyes) were treated with MMC and twenty-five patients (45 eyes) with PRGF. The final UDVA (LogMar) in MMC was 0.029 ±â€Š0.065 and in PRGF it was 0.028 ±â€Š0.048 (p = 0.383). The efficacy index for MMC was 0.98 ±â€Š0.10 and 1.10 ±â€Š0.46 for patients treated with PRGF (p = 0.062). The safety index for MMC was 1.03 ±â€Š0.11 and 1.12 ±â€Š0.46 (p = 0.158) for PRGF group. The change percentage of endothelial cell density was 0.9 ±â€Š11.6 for MMC and 4.3 ±â€Š13.1 for PRGF (p = 0.593). The predictability for MMC was 92.1% and for the PRGF was 91.9% (p = 0.976). Hyperemia, eye pain and superficial keratitis were observed in 11.1% of the MMC group; no adverse events were observed with the PRGF.The use of PRGF in PRK surgery is as effective as MMC. The PRGF shows a better safety profile than MMC for its intraoperative use in PRK.

Understanding the Photocatalytic Properties of Pt/CeOx /TiO2 : Structural Effects on Electronic and Optical Properties.

Ceria-titania interfaces play a crucial role in different chemical processes but are especially promising for the photocatalytic splitting of water using light in the visible wavelength region when Pt is added to the system. However, the complexity of this hierarchical structure hampers the study of the origin of its outstanding properties. In this article, the structural, electronic and optoelectronic properties of CeO2 /TiO2 systems containing 1D, 2D, and 3D particles of ceria are analyzed by means of density functional calculations. Adsorption sites and vacancy effects have been studied to model Pt adsorption. Density of states calculations and absorption spectra simulations explain the behavior of these systems. Finally, these models are used for the screening of other metals that can be combined with this heterostructure to potentially find more efficient water splitting photocatalysts.

Graphenes as Efficient Metal-Free Fenton Catalysts.

Reduced graphene oxide exhibits high activity as Fenton catalyst with HO(.) radical generation efficiency over 82 % and turnover numbers of 4540 and 15023 for phenol degradation and H2 O2 consumption, respectively. These values compare favorably with those achieved with transition metals, showing the potential of carbocatalysts for the Fenton reaction.

Effect of capping ligands and TiO2 supporting on the optical properties of a (CdSe)13 cluster.

The influence of different aliphatic and aromatic ligand molecules on the electronic properties of CdSe quantum dots (QDs) has been examined by employing density functional theory (DFT). Optical spectra were simulated with the real-time time-dependent DFT (RT-TDDFT) methodology. The assignment of the first absorption peak features that occur in these spectra was done by taking into account the composition of the frontier molecular orbitals (MOs) of the different systems. While the aliphatic ligands considered-amine, thiol, and phospine oxides-did not show any major influence on the electronic absorption spectra, some of the aromatic ligands do have a noticeable impact on the optoelectronic properties of the QD. Aromatic ligands are mainly aniline-type molecules; additionally, a thiophenol and uracil were employed to saturate the dangling bonds on the Cd atoms. Finally, a more realistic model of a QD-sensitized solar cell consisting of methylamine-capped (CdSe)13 cluster linked to a TiO2 nanoparticle through a mercaptopropionate bridge was considered. The simulations again show that the lowest electronic excitation takes place within the QD subunit, demonstrating the indirect nature of the electron injection mechanism operating in these solar cells.

Thiodiacetate-manganese chemistry with N ligands: unique control of the supramolecular arrangement over the metal coordination mode.

Compounds based on the Mn-tda unit (tda=S(CH(2)COO)(2)(-2) ) and N co-ligands have been analyzed in terms of structural, spectroscopic, magnetic properties and DFT calculations. The precursors [Mn(tda)(H(2)O)](n) (1) and [Mn(tda)(H(2)O)(3)]·H(2)O (2) have been characterized by powder and X-ray diffraction, respectively. Their derivatives with bipyridyl-type ligands have formulas [Mn(tda)(bipy)](n) (3), [{Mn(N-N)}(2)(µ-H(2)O)(µ-tda)(2)](n) (N-N=4,4'-Me(2)bipy (4), 5,5'-Me(2)bipy, (5)) and [Mn(tda){(MeO)(2)bipy}·2H(2)O](n) (6). Depending on the presence/position of substituents at bipy, the supramolecular arrangement can affect the metal coordination type. While all the complexes consist of 1D coordination polymers, only 3 has a copper-acetate core with local trigonal prismatic metal coordination. The presence of substituents in 4-6, together with water co-ligands, reduces the supramolecular interactions and typical octahedral Mn(II) ions are observed. The unicity of 3 is also supported by magnetic studies and by DFT calculations, which confirm that the unusual Mn coordination is a consequence of extended noncovalent interactions (π-π stacking) between bipy ligands. Moreover, 3 is an example of broken paradigm for supramolecular chemistry. In fact, the desired stereochemical properties are achieved by using rigid metal building blocks, whereas in 3 the accumulation of weak noncovalent interactions controls the metal geometry. Other N co-ligands have also been reacted with 1 to give the compounds [Mn(tda)(phen)](2)·6H(2)O (7) (phen=1,10-phenanthroline), [Mn(tda)(terpy)](n) (8) (terpy=2,2':6,2''-terpyridine), [Mn(tda)(pyterpy)](n) (9) (pyterpy=4'-(4-pyridyl)-2,2':6,2''-terpyridine), [Mn(tda)(tpt)(H(2)O)]·2H(2)O (10) and [Mn(tda)(tpt)(H(2)O)](2)·2H(2)O (11) (tpt=2,4,6-tris(2-pyridyl)-1,3,5-triazine). Their identified mono-, bi- or polynuclear structures clearly indicate that hydrogen bonding is variously competitive with π-π stacking.

High catalytic activity of Au/CeOx/TiO2(110) controlled by the nature of the mixed-metal oxide at the nanometer level.

Mixed-metal oxides play a very important role in many areas of chemistry, physics, materials science, and geochemistry. Recently, there has been a strong interest in understanding phenomena associated with the deposition of oxide nanoparticles on the surface of a second (host) oxide. Here, scanning tunneling microscopy, photoemission, and density-functional calculations are used to study the behavior of ceria nanoparticles deposited on a TiO(2)(110) surface. The titania substrate imposes nontypical coordination modes on the ceria nanoparticles. In the CeO(x)/TiO(2)(110) systems, the Ce cations adopt an structural geometry and an oxidation state (+3) that are quite different from those seen in bulk ceria or for ceria nanoparticles deposited on metal substrates. The increase in the stability of the Ce(3+) oxidation state leads to an enhancement in the chemical and catalytic activity of the ceria nanoparticles. The codeposition of ceria and gold nanoparticles on a TiO(2)(110) substrate generates catalysts with an extremely high activity for the production of hydrogen through the water-gas shift reaction (H(2)O + CO --> H(2) + CO(2)) or for the oxidation of carbon monoxide (2CO + O(2) --> 2CO(2)). The enhanced stability of the Ce(3+) state is an example of structural promotion in catalysis described here on the atomic level. The exploration of mixed-metal oxides at the nanometer level may open avenues for optimizing catalysts through stabilization of unconventional surface structures with special chemical activity.

Synthesis, solid-state structure, and bonding analysis of the beryllocenes [Be(C5Me4H)2], [Be(C5Me5)2], and [Be(C5Me5)(C5Me4H)].

The beryllocenes [Be(C(5)Me(4)H)(2)] (1), [Be(C(5)Me(5))(2)] (2), and [Be(C(5)Me(5))(C(5)Me(4)H)] (3) have been prepared from BeCl(2) and the appropriate KCp' reagent in toluene/diethyl ether solvent mixtures. The synthesis of 1 is facile (20 degrees C, overnight), but generation of decamethylberyllocene 2 demands high temperatures (ca. 115 degrees C) and extended reaction times (3-4 days). The mixed-ring beryllocene 3 is obtained when the known [(eta(5)-C(5)Me(5))BeCl] is allowed to react with K[C(5)Me(4)H], once more under somewhat forcing conditions (115 degrees C, 36 h). The structures of the three metallocenes have been determined by low-temperature X-ray studies. Both 1 and 3 present eta5/eta1 geometries of the slip-sandwich type, whereas 2 exhibits an almost regular, ferrocene-like, sandwich structure. In the mixed-ring compound 3, C(5)Me(5) is centrally bound to beryllium and the eta(1)-C(5)Me(4)H ring bonds to the metal through the unique CH carbon atom. This is also the binding mode of the eta(1)-ring of 1. To analyze the nature of the bonding in these molecules, theoretical calculations at different levels of theory have been performed on compounds 2 and 3, and a comparison with the bonding in [Be(C(5)H(5))(2)] has been made. As for the latter molecule, energy differences between the eta5/eta5 and the eta5/eta1 structures of 2 are very small, being of the order of a few kcal mol(-1). Constrained space orbital variations (CSOV) calculations show that the covalent character in the bonding is larger for [Be(C(5)Me(5))(2)] than for [Be(C(5)H(5))(2)] due to larger charge delocalization and to increased polarizability of the C(5)Me(5) fragment.

Synthesis, Structural Characterization, and MO Calculations of Vanadium Imido Complexes Containing Bidentate Phosphine Coligands.

Treatment of complex V(N-2,6-(i)Pr(2)C(6)H(3))Cl(3) with 1,2-dimethoxyethane (dme) gives in quasi-quantitative yield the adduct V(N-2,6-(i)Pr(2)C(6)H(3))Cl(3)(dme) (1). Interaction of 1 with bidentate phosphines gives V(N-2,6-(i)Pr(2)C(6)H(3))Cl(3)(P-P) (P-P = depe, 2a; dppe, 2b) compounds. An X-ray analysis (monoclinic, space group P2(1)/c, a = 14.7387(14) Å, b = 10.6738(10) Å, c = 16.999(3) Å, beta = 90.954(2) degrees, Z = 4, R = 0.0544), carried out on complex 2a, shows a mer arrangement of the chloride ligands and a nonsymmetrical coordination of the diphosphine ligand. One of the phosphorus atoms occupies the trans position with respect to the organoimido ligand. MO calculations on the models V(NR)Cl(3)(H(2)PCH(2)CH(2)PH(2)) (R = H, C(6)H(5)) of complex 2a were performed. The mer isomer, which is more stable than the fac isomer, shows good agreement with the experimental data.