Search

A bachelor's degree program in chemistry typically starts with basic concepts in mathematics, physics, and chemistry. Those principles are very important because they pave the ground for future advanced studies and, therefore, one needs to present them in a clear but rigorous way. One example is provided by the mathematical notion of an iterative process which acquires a significant relevance in chemistry. The popular Newton-Raphson algorithm has been selected to exemplify how iterative methods work as it is highly effective and suitable for undergraduate students. Additionally, this work presents a straightforward and interactive pedagogical tool, created with the widespread Microsoft Office Excel, which can be applied in a classroom to solve nonlinear equations numerically. Four pervasive topics in undergraduate programs are given to illustrate the importance of iterative processes, namely the calculation of pH of a weak acid solution, the molar volume of a real gas, the molecular geometry delivered by VSEPR theory, and the conformational analysis of 1,2-dichloroethane molecule.

The mechanisms of radiationless decay involved in the photodissociation of formaldehyde into H2 and CO have been investigated using complete active space self-consistent field (CASSCF) calculations and direct dynamics variational multiconfiguration Gaussian (DD-vMCG) quantum dynamics in the S1, T1, and S0 states. A commonly accepted scheme involves Fermi Golden Rule internal conversion from S1 followed by dissociation of vibrationally hot H2CO in S0. We recently proposed a novel mechanism [M. Araujo et al., J. Phys. Chem. A 112, 7489 (2008)] whereby internal conversion and dissociation take place in concert through a seam of conical intersection between S1 and S0 after the system has passed through an S1 transition barrier. The relevance of this mechanism depends on the efficiency of tunneling in S1. At lower energy, an alternative scheme to internal conversion involves intersystem crossing via T1 to regenerate the reactant before the S0 barrier to dissociation. We propose here a previously unidentified mechanism leading directly to H2 and CO products via T1. This channel opens at medium energies, near or above the T1 barrier to dissociation and still lower than the S1 barrier, thus making T1 a possible shortcut to molecular dissociation. [ABSTRACT FROM AUTHOR]

Copyright of Educação, Sociedade & Culturas is the property of Centro de Investigacao e Intervencao Educativa (CIIE) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The inner space of carbon nanotubes has already been proven to provide a type of confinement, which can dramatically alter the energetics of chemical reactions when compared to the gas phase. Moreover, BN doping can be used to fine-tune electronic properties, which might influence the enthalpy and activation energy of chemical reactions that take place inside their inner space. The energy profile of the prototype Menshutkin S N 2 reaction between ammonia and chloromethane has been analyzed in a variety of carbon-based materials at the DFT (density functional theory) level. Pristine zigzag (9,0) and (12,0) single-walled carbon nanotubes and graphene sheets have been doped with boron and nitrogen at different stoichiometries, namely, BN, BNC, BNC 2 , and BNC 4 , that resulted in remarkable variations of their catalytic effects. Graphene has revealed to be the support material, which depends less on doping in terms of enthalpy and energy barrier of the reaction. However, when graphene is rolled up into tubular forms, the influence of doping becomes increasingly stronger as the nanotube radius decreases. In the case of BNC 4 doping of (9,0) nanotubes, the activation energy drops 10 kcal/mol when compared to the pristine case, and the reaction became even exothermic by more than 15 kcal/mol.

A novel multilayer film containing a cationic phosphonium-derivatized Ni(salen)-type complex and poly(sodium-4-styrenesulfonate (NaPSS) was assembled onto quartz, mica, and metal surfaces using the layer-by-layer (LbL) technique. Spectroscopic (UV-vis) and gravimetric (QCM) responses for the multilayer films show regular stepwise growth and the signature of strong electrostatic interactions between the component layers. The gravimetric responses indicate the presence of substantial additional (net neutral) material in the PSS layers, which XPS shows is not polyelectrolyte or salt, so charge compensation is intrinsic; we deduce the presence of space-filling solvent. Direct electrostatic interaction of the two-component layers is enhanced by a secondary noncovalent interaction between the delocalized pi-systems of the two components. Permeability of the film to the redox probe [Fe(CN)(6)](3-/4-) was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Qualitatively similar results were obtained in the absence and presence of a precursor PSS/PAH multilayer, but with a general shift in kinetic and diffusional processes to longer time scales (lower frequencies) in the presence of the precursor layer and with increasing numbers of PSS/[Ni(salen)] bilayers. Quantitatively, the EIS data were interpreted using a capillary membrane model (CMM) to yield values of coverage, apparent charge transfer resistance, double-layer capacitance, pore size, and diffusion coefficient. The coverage values were consistent with a model in which there are no preferential growth sites and the surface charge density is independent of the number of bilayers.

Prot-2S is a bioinformatics web application devised to analyse the protein chain secondary structures (2S) (http:/ /www.requimte.pt:8080/Prot-2S/). The tool is built on the RCSB Protein Data Bank PDB and DSSP application/files and includes calculation/graphical display of amino acid propensities in 2S motifs based on any user amino acid classification/code (for any particular protein chain list). The interface can calculate the 2S composition, display the 2S subsequences and search for DSSP non-standard residues and for pairs/triplets/quadruplets (amino acid patterns in 2S motifs). This work presents some Prot-2S applications showing its usefulness in protein research and as an e-learning tool as well.