Alvaro W. Mombrú

Is It Possible to Dope Single‐Walled Carbon Nanotubes and Graphene with Sulfur?

Herein, we investigate sulfur substitutional defects in single-walled carbon nanotubes (SWCNTs) and graphene by using first-principles calculations. The estimated formation energies for the (3,3), (5,5), and (10,0) SWCNTs and graphene lie between 0.9 and 3.8 eV, at sulfur concentrations of 1.7-4 atom %. Thus, from a thermodynamic standpoint, sulfur doping is not difficult. Indeed, these values can be compared with that of 0.7 eV obtained for a nitrogen-doped (5,5) SWCNT. We suggest that it may be possible to introduce sulfur into the SWCNT framework by employing sulfur-containing heterocycles. Our simulations indicate that sulfur doping can modify the electronic structure of the SWCNTs and graphene, depending on the sulfur content. In the case of graphene, sulfur doping can induce different effects: the doped sheet can be a small-band-gap semiconductor, or it can have better metallic properties than the pristine sheet. Thus, S-doped graphene may be a smart choice for constructing nanoelectronic devices, since it is possible to modulate the electronic properties of the sheet by adjusting the amount of sulfur introduced. Different synthetic routes to produce sulfur-doped graphene are discussed.

Mechanical properties of graphene nanoribbons

Herein, we investigate the structural, electronic and mechanical properties of zigzag graphene nanoribbons in the presence of stress by applying density functional theory within the GGA-PBE (generalized gradient approximation-Perdew-Burke-Ernzerhof) approximation. The uniaxial stress is applied along the periodic direction, allowing a unitary deformation in the range of ± 0.02%. The mechanical properties show a linear response within that range while a nonlinear dependence is found for higher strain. The most relevant results indicate that Youngs modulus is considerable higher than those determined for graphene and carbon nanotubes. The geometrical reconstruction of the C-C bonds at the edges hardens the nanostructure. The features of the electronic structure are not sensitive to strain in this linear elastic regime, suggesting the potential for using carbon nanostructures in nano-electronic devices in the near future.

Re(V) complexes with amino acids based on the ‘3+2’ approach

Abstract Five cationic complexes containing the [Re(V)O] 3+ core have been prepared by substitution on the precursor [ReIO 2 (PPh 3 ) 2 ]. Complexes with general formula [ReO(dien-H)(aa)] + have been obtained by reaction of the precursor with 1 equiv. of dien (diethylenetriamine) and one equivalent of an amino acid (glycine, alanine, valine, leucine, proline). Complexes have been characterized by UV–Vis and IR spectrophotometry, elemental analyses, 1 H NMR spectra, HPLC and conductivity measurements. The molecular structures of [ReO(dien-H)(leu)]I·0.5H 2 O and [ReO(dien-H)(pro)]ReO 4 have been determined by means of single-crystal X-ray analyses. Cations consist of a six-coordinated rhenium atom bonded to two trans -oxo groups, one of the [ReO] 3+ core and one from the carboxilate. Four nitrogen atoms make the equatorial plane. Three N belong to the dien ligand and the other one to the amino acid. One of the secondary amine proton of the dien ligand is lost upon coordination. This arrangement corresponds to the so-called ‘3+2’ chelating framework, with the novel NNN/NO atom set. Complexes are not very stable in aqueous solution. However, solutions in MeOH and DMSO are stable enough to be studied by several techniques. The solid state structure seems to be retained in solution.

Magnetism in multivacancy graphene systems

Ab initio calculations using density functional theory (DFT) have been performed in order to study defects in graphene. The structural distortions that can be observed when multi-atom vacancies are created in graphene and the net magnetic moment that can eventually appear are characterized for a variety of vacancy sizes and shapes. We conclude that the configuration arising in the construction of multivacancies in graphene can unambiguously indicate whether a magnetic response of the defected system is to be expected. Making use of the shape of the complementary figure-i.e. the geometric figure of the atomic arrangement that is extracted from graphene when the multivacancy is created-it is possible to construct a set of rules by means of which the optimized structural and magnetic behavior can be predicted. The validity of the rules is determined through DFT calculations.

A new structure in the REBaCuFeO5+δ series: LaBaCuFeO5+δ. Structure and magnetic properties in the La1−xPrxBaCuFeO5+δ system

Abstract The crystal structure of LaBaCuFeO 5+ δ is determined by neutron powder diffraction at 15 K and the structure and magnetic properties of the La 1− x Pr x BaCuFeO 5+ δ system are studied by AC-susceptibility measurements and RT 57 Fe Mossbauer spectroscopy. The best fit using the Rietveld method is obtained for an orthorhombic model in space group I mmm with unit cell parameters a =5.5586(8) A, b =5.5550(9) A and c =7.8155(2) A. The main feature of this model is a disordered structure where the oxygen atom vacancies play an important role. The magnetic susceptibility in the La 1− x Pr x BaCuFeO 5+ δ system is studied and T f , μ eff and Θ are determined. Mossbauer spectra provide information relative to the iron electronic state and environment. They support the hypothesis of the key role that the oxygen atom positions play in the magnetic properties of the system by affecting the superexchange interactions in the network.

Cu(II) complexation with His-Gly and His-Ala. X-ray structure of [Cu(his-gly)2(H2O)2].6H2O

Abstract Two novel Cu(II)–dipeptide complexes containing l -histidyl– l -glycine and l -histidyl– l -alanine as ligands were prepared and characterised by infrared and electronic spectroscopies. EPR measurements were also made in solution and in the solid state. Cyclic voltammetric studies were carried out on aqueous solution. The crystalline structure of the [Cu( l -histidyl– l -glycine) 2 (H 2 O) 2 ]·6H 2 O was determined by X-ray diffractometry. It crystallises in the space group C 2. The Cu(II) is in a tetragonally distorted octahedral coordination, where the basal plane is occupied by N-atoms belonging to the histidine portion. The apical positions are occupied by two molecules of water.

Influence of processing conditions on the crystal structure and magnetic behavior of La0.7Ca0.3MnO3±δ samples

Abstract In this work we report crystallographic structure variations and the related modifications on the magnetic behavior of La 0.7 Ca 0.3 MnO 3± δ introduced by heat-treatments in different synthesis atmospheric conditions. We have prepared polycrystalline ceramic samples using a modified polymeric precursors method, which produces highly homogeneous specimens. The use of argon atmosphere enlarges the crystalline c -axis as detected by Rietveld refinements. As a consequence, an improvement in the magnetic transition temperature T C of the samples was observed. Our results also indicate that different heat-treatment conditions change the magnetic interactions between the ferromagnetic (F) and antiferromagnetic (AF) structures of these systems. Our conclusions rely on the use of AC magnetic susceptibility measurements as the experimental tool for measuring these variations.

Use of bibliometric information to assist research policy making. A comparison of publication and citation profiles of Full and Associate Professors at a School of Chemistry in Uruguay

SummaryPublication and citation profiles of Full and Associate Professors at the School of Chemistry of the Universidad de la República in Uruguay were investigated. The groups do not exhibit markedly different age averages. However, the average time since they started publishing, as well as other characteristics of their publication records, like productivity or citations, set them apart. From the point of view of both the number of papers per author and per year of activity, on one side, and of the number of citations per year of activity, on the other, the group of Full Professors has statistically significant larger averages than the Associate Professors. The impact of self-citations, multi-authorship and internationalization of the publications were analyzed within the two groups and shown to have no excessive or predictable influence on those parameters, except in the case of few (≤ 2) or many (>8) authors. It is suggested in this paper that these two indicators, number of papers per author per production year and number of citations per production year, combined in a plot allowing a bidimensional ranking of the individuals in the groups, may be used profitably as one of the components in the development of a policy toward promotion of Associate Professors. The analysis showed also that the quotient of citations received to number of papers published, even when derived from actual citation data of the scientists without involving the impact factors of the journals in which they publish, are not good parameters to use for that purpose, essentially because there is a reduction in the information content of the indicator with respect to those described before.

Sm(III) complexation with α-amino acids: X-ray crystal structure of [Sm2(Hala)4(H2O)8](ClO4)4(Cl)2

Abstract Sm(III) coordination compounds are currently used as radiotherapeutic agents. Moreover, it is known that tumour cells show enhanced intake of α-amino acids (H n aa). With this in mind, the study of samarium complexes with these ligands has obvious interest. In this work, potentiometric studies of Sm(III) in the presence of glycine, alanine, proline, tryptophane, valine, glutamic acid and cysteine have been carried out. Experiments were performed in aqueous solution ([Sm 3+ ]=5.0–60.0 mM) at 37°C and 0.15 M NaClO 4 . Mono and dinuclear species were detected in the pH range 1.5–5.3. For higher pH values, Sm(OH) 3 competes with the formation of Sm complexes. In addition, Sm(III)–H n aa complexes were isolated and characterized at solid state. The structure of [Sm 2 (Hala) 4 (H 2 O) 8 ](ClO 4 ) 4 (Cl) 2 is reported. In this complex, two Sm centres are joined by four μ-COO bridges. Four molecules of water complete the positions of each eight-coordinate Sm atom.

Synthesis, structural characterization, and pro-apoptotic activity of 1-indanone thiosemicarbazone platinum(II) and palladium(II) complexes: potential as antileukemic agents.

In the search for alternative chemotherapeutic strategies against leukemia, various 1‐indanone thiosemicarbazones, as well as eight novel platinum(II) and palladium(II) complexes, with the formula [MCl2(HL)] and [M(HL)(L)]Cl, derived from two 1‐indanone thiosemicarbazones were synthesized and tested for antiproliferative activity against the human leukemia U937 cell line. The crystal structure of [Pt(HL1)(L1)]Cl.2MeOH, where L1=1‐indanone thiosemicarbazone, was solved by X‐ray diffraction. Free thiosemicarbazone ligands showed no antiproliferative effect, but the corresponding platinum(II) and palladium(II) complexes inhibited cell proliferation and induced apoptosis. Platinum(II) complexes also displayed selective apoptotic activity in U937 cells but not in peripheral blood monocytes or the human hepatocellular carcinoma HepG2 cell line used to screen for potential hepatotoxicity. Present findings show that, in U937 cells, 1‐indanone thiosemicarbazones coordinated to palladium(II) were more cytotoxic than those complexed with platinum(II), although the latter were found to be more selective for leukemic cells suggesting that they are promising compounds with potential therapeutic application against hematological malignancies.