Mechanism of the initial stages of nitrogen-doped single-walled carbon nanotube growth

We have studied the mechanism of the initial stages of nitrogen-doped single-walled carbon nanotube growth illustrated for the case of a floating catalyst chemical vapor deposition system, which uses carbon monoxide (CO) and ammonia (NH(3)) as precursors and iron as a catalyst. We performed first-principles electronic-structure calculations, fully incorporating the effects of spin polarization and magnetic moments, to investigate the bonding and chemistry of CO, NH(3), and their fragments on a model Fe(55) icosahedral cluster. A possible dissociation path for NH(3) to atomic nitrogen and hydrogen was identified, with a reaction barrier consistent with an experimentally determined value we measured by tandem infrared and mass spectrometry. Both C-C and C-N bond formation reactions were found to be barrierless and exothermic, while a parasitic reaction of HCN formation had a barrier of over 1 eV.     

Spin and temperature dependence of the magnetic hyperfine field of Cd in the rare earth-aluminum Lavesphase compounds RAl2

Perturbed angular correlation spectroscopy has been used to investigate the combined magnetic and electric hyperfine interaction of the probe nucleus ¹¹¹Cd in ferromagnetically ordered rare earth (R)-dialuminides RAl₂ as a function of temperature for the rare earth constituents R=Pr, Nd, Sm, Eu, Tb, Dy, Ho and Er. In compounds with two magnetically non-equivalent Al sites (R=Sm, Tb, Ho, Er), the magnetic hyperfine field was found to be strongly anisotropic. This anisotropy is much greater than the anisotropic dipolar fields, suggesting a contribution of the anisotropic 4f-electron density to magnetic hyperfine field at the closed-shell probe nucleus. The spin dependence of the magnetic hyperfine field reflects a decrease of the effective exchange parameter of the indirect coupling with increasing R atomic number. For the compounds with the R constituents R=Pr, Nd, Tb, Dy and Ho the parameters B₄, B₆ of the interaction of the crystal field interaction have been determined from the temperature dependence of the magnetic hyperfine field. The ¹¹¹Cd PAC spectrum of EuAl₂ at 9 K confirms the antiferromagnetic structure of this compound.     

Simulation models to support the preliminary electoral results program for the Mexican Electoral Institute

On July 1st, 2018, federal elections for president, senators and deputies took place in Mexico. In most states, elections for state governors and representatives took also place in the same polling booths. The Technical Unit for Information Services (UNICOM) of the National Electoral Institute (INE) of Mexico has the responsibility for planning and implementation of the Preliminary Electoral Results Program (PREP) for federal elections. For the 2018 elections UNICOM developed forecasting models for the performance of PREP based on simulation models that were developed using a special purpose simulation software and C++ subroutines for fast simulation of queues. These simulation models were a valuable tool for planning, scheduling and allocation of the main resources that participated in the operational process of the PREP. In this article we report the main features, applications and results obtained by using these simulation models.

     

Vibrational study of A3In2B″O9 (A = Ba,Sr; B″ = U,W) double perovskites

Raman scattering and infrared transmittance techniques were used to investigate the room‐temperature phonon spectra of the polycrystalline Ba₃In₂UO₉, Sr₃In₂UO₉, Ba₃In₂WO₉ and Sr₃In₂WO₉ double perovskites. Although X‐ray diffraction has shown that both Ba₃In₂UO₉ and Ba₃In₂WO₉ crystallize in a disordered cubic (Pm3 m − O_h¹) structure with one formula unit per unit cell (Z = 1), Raman measurements revealed that both Ba₃In₂UO₉ and Ba₃In₂WO₉ compounds should exhibit a 1:1 ordered domains with Fm3 m symmetry, embedded in the Pm3 m matrix. Observation of a ν₁‐like vibration of the InO₆ octahedron reflects the occurrence of a two‐phonon‐like behavior due to the intrinsic occupational disorder at octahedral symmetry sites. Assignment of most of the Raman and infrared phonons is also given. Copyright © 2009 John Wiley & Sons, Ltd.     

A new look at Jarvis' distribution formula

Starting from the well-known transformation law for the Klein functions, we give a proof of a fairly general multiplicative distribution formula for the Siegel functions associated to isogenous complex lattices. This formula has as an immediate consequence the remarkable distribution formula proved by Jarvis in 2000 on the occasion of Rolshausen's thesis on the second K-group of an elliptic curve.     

About the SDS inclusion in PDMS/TEOS ORMOSIL: a vibrational spectroscopy and confocal Raman scattering study

Organically modified silicates (ORMOSILs) are attractive materials due to their vast applicability and easy synthesis. The doping of these materials with sodium dodecyl sulfate (SDS) is interesting in the search for good protonic conductors. The inclusion of different concentrations of SDS in ORMOSIL membranes is investigated in the present work using Raman and infrared spectroscopy, confocal Raman microscopy and confocal imaging microscopy. The spectroscopic measurements allow us to assign the vibrational modes to the chemical groups of the structures of SDS and ORMOSIL. Furthermore, these measurements show that these materials are composites, as no interactions are observed between the SDS and the ORMOSIL matrix. The confocal Raman and confocal imaging techniques are useful to study qualitatively the SDS insertion on the surface of ORMOSIL. It was observed that the SDS sizes are very nonuniform. Copyright © 2011 John Wiley & Sons, Ltd.     

Conformational polymorphism of the antidiabetic drug chlorpropamide

In this paper, the main features of Raman spectroscopy, one of the first choice methods in the study of polymorphism in pharmaceuticals, are presented taking chlorpropamide as a case of study. The antidiabetic drug chlorpropamide (1‐[4‐chlorobenzenesulphonyl]‐3‐propyl urea), which belongs to the sulfonylurea class, is known to exhibit, at least, six polymorphic phases. These forms are characterized not only by variations in their molecular packing but also in their molecular conformation. In this study, the polymorphism of chlorpropamide is discussed on the basis of Raman scattering measurements and quantum mechanical calculations. The main spectroscopic features that fingerprint the crystalline forms are correlated with the corresponding crystalline structures. Using a theoretical approach on the energy dependence of the conformers, simulated molecular torsion angles are plotted versus the formation energy, which provides a satisfactory agreement between the torsion angles at the energy minima and the experimental values observed in the different solid forms of chlorpropamide. Copyright © 2011 John Wiley & Sons, Ltd.     

Normal modes of redox-active tyrosine: conformation dependence and comparison to experiment

Redox-active tyrosine residues play important roles in long-distance electron reactions in enzymes such as prostaglandin H synthase, ribonucleotide reductase, and photosystem II (PSII). Spectroscopic characterization of tyrosyl radicals in these systems provides a powerful experimental probe into the role of the enzyme in mediation of long-range electron transfer processes. Interpretation of such data, however, relies critically on first establishing a spectroscopic fingerprint of isotopically labeled tyrosinate and tyrosyl radicals in nonenzymatic environments. In this report, FT-IR results obtained from tyrosinate, tyrosyl radical (produced by ultraviolet photolysis of polycrystalline tyrosinate), and their isotopologues at 77 K are presented. Assignment of peaks and isotope shifts is aided by density-functional B3LYP/6-311++G(3df,2p)//B3LYP/6-31++G(d,p) calculations of tyrosine and tyrosyl radical in several different charge and protonation states. In addition, characterization of the potential energy surfaces of tyrosinate and tyrosyl radical as a function of the backbone and ring torsion angles provides detailed insight into the sensitivity of the vibrational frequencies to conformational changes. These results provide a detailed spectroscopic interpretation, which will elucidate the structures of redox-active tyrosine residues in complex protein environments. Specific application of these data is made to enzymatic systems.     

Using phonon resonances as a route to all-angle negative refraction in the far-infrared region: the case of crystal quartz

We consider how all-angle negative refraction may be induced in anisotropic crystals by making use of the phonon response. We investigate the example of crystal quartz at far-infrared wavelengths. Reflection and transmission measurements confirm the expected behavior, and show relatively high transmission efficiency at frequencies at which negative refraction occurs.     

Organotin compounds : X. Organotin hydride addition to methyl cyclohexene-1-carboxylate and methyl indene-3-carboxylate

Free radical hydrostannation of methyl cyclohexene-1-carboxylate (I) and methyl indene-3-carboxylate (III) with trialkyltin hydrides, R₃SnH (R = Me, n-Bu, Ph) gives the energetically unfavourable cis products, 2-trialkylstannyl cyclohexanecarboxylate (II) and 2-trialkylstannyl indane-1-carboxylate (IV) in high yields, via a trans addition of the tin hydrides. The hydride abstractions by the intermediate trialkylstannylcyclohexanyl (VIII) and trialkylstannylindanyl (IX) intermediate radicals take place stereospecifically, and exclusively from the less hindered ring side. The structures of the isomers II and IV were established by (a) their transformation into the corresponding chlorodialkylstannyl derivatives V and VI, which were shown spectroscopically to have cis stereochemistries by intramolecular complexation of the ester group, and (b) their NMR data. Full ¹H, ¹³C, and ¹¹⁹Sn NMR data are given.