Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Two-photon absorption and two-photon circular dichroism of hexahelicene derivatives: a study of the effect of the nature of intramolecular charge transfer

Abstract: Herein we report on the theoretical–experimental analysis of the one- and two-photon absorption and circular dichroism spectra of two intrinsically chiral aromatic molecules – hexahelicene derivatives – with helical chirality and intramolecular charge transfer (ICT). The primary outcomes of our investigation demonstrate that the TPA cross-section and the amplitude of the TPCD signal of this type of helicenes are strongly affected by the strength of the ICT and the nature of the extension of the electronic delocalization, i.e. beyond (EXO-ICT) or within (ENDO-ICT) the helicene core. These results were corroborated through the comparative theoretical analysis of the corresponding contributions of the magnetic dipole transition moment and the electric quadrupole transition moment to the TPA rotatory strength on a series of five similar helicene derivatives with different molecular electron delocalization disposition. Two-photon absorption (TPA) and two-photon circular dichroism (TPCD) spectra were obtained using the double L-scan technique over a broad spectral range (400–900 nm) using 90 fs pulses at a low repetition rate (2–50 Hz) produced by an amplified femtosecond system. The theoretical simulations were performed using modern analytical response theory within the Time-Dependent Density Functional Theory (TD-DFT) approach using B3LYP and CAM-B3LYP, and the aug-cc-pVDZ and 6-311++G(d,p) basis sets.

Microstructure development of WCoB–TiC based hard materials

Abstract: The WCoB is a complex boride that has a very high hardness (45 GPa). In previous works, a very fine coating with a high percentage of this compound was obtained as a product of surface reaction of pre-sintered samples of WC–Co and WC–Co–TiC with TiB2 or other compounds rich in boron, at 1100 °C in vacuum. The aim of this work was to promote the formation of the WCoB as a second phase in a bulk by liquid phase reactive sintering of equimolar mixtures of WC and TiB2 with Co varying between 10–47 wt% (20–66 mol%), in order to evaluate its influence on the sintering and formation of WCoB. Sintering was performed in the temperature range: 1400 °C. The sintered samples were evaluated using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and EPMA analysis. It was found that the alloy showing the best properties was that with 18 wt% of cobalt which corresponded to a nominal equimolar mixture of WC, TiB2 and Co. The sintered microstructure consisted primarily of WCoB, TiC and some minor quantities of CoB and W2CoB2. A relation between initial Co contents, volume fraction of the different phases formed and mechanical properties is presented.

Protein quality of germinated Phaseolus vulgaris

Abstract: Advantages of seed germination consist of an increase in the bioavailability of proteins as well as the change in the antinutritional factors which limit their utilization. Throughout this work, the effects of germination and cooking after germination on the protein in black and white beans (Phaseolus vulgaris) were evaluated. Antinutritional factors that affect the utilization of such protein were also assessed. The amounts of protein, available lysine, tannins, PER, and protein digestibility in vitro and apparent, of beans germinated and germinated-cooked were quantified. The germination significantly (p≤0.05) increased the content of proteins, inactived trypsin inhibitors, and raised the available lysine. Germination and cooking completely inactivated the trypsin inhibitors, which became lysine less available, decreased tannins, and increased protein digestibility and PER value. Differences between black and white beans were observed and attributed to variations in structure, composition, and varieties, among other factors. Cooking complements the effect of germination by improving the protein quality of P. vulgaris and increasing its bioavailability for the human consumption.

Loop calculus for lattice gauge theories

Abstract: Hamiltonian calculations are performed using a loop-labeled basis where the full set of identities for the SU(/ital N/) gauge models has been incorporated. The loops are classified as clusterlike structures and the eigenvalue problem leads to a linear set of finite-difference equations easily amenable to numerical treatment. Encouraging results are reported for SU(2) at spatial dimension 2.