Insights on spin polarization through the spin density source function

Understanding how spin information is transmitted from paramagnetic to non-magnetic centers is crucial in advanced materials research and calls for novel interpretive tools. Herein, we show that the spin density at a point may be seen as determined by a local source function for such density, operating at all other points of space. Integration of the local source over Bader''s quantum atoms measures their contribution in determining the spin polarization at any system''s location. Each contribution may be then conveniently decomposed in a magnetic term due to the magnetic natural orbital(s) density and in a reaction or relaxation term due to the remaining orbitals density. A simple test case, ³B₁ water, is chosen to exemplify whether an atom or group of atoms concur or oppose the paramagnetic center in determining a given local spin polarization. Discriminating magnetic from reaction or relaxation contributions to such behaviour strongly enhances chemical insight, though care needs to be paid to the large sensitivity of the latter contributions to the level of the computational approach and to the difficulty of singling out the magnetic orbitals in the case of highly correlated systems. Comparison of source function atomic contributions to the spin density with those reconstructing the electron density at a system''s position, enlightens how the mechanisms which determine the two densities may in general differ and how diverse may be the role played by each system''s atom in determining each of the two densities. These mechanisms reflect the quite diverse portraits of the electron density and electron spin density Laplacians, hence the different local concentration/dilution of the total and (α–β) electron densities throughout the system. Being defined in terms of an observable, the source function for the spin density is also potentially amenable to experimental determination, as customarily performed for its electron density analogue.     

Chemometrics‐enhanced laser‐induced thermal emission detection of PETN and other explosives on various substrates

Infrared emissions (IREs) of samples of pentaerythritol tetranitrate (PETN) deposited as contamination residues on various substrates were measured to generate models for the detection and discrimination of the important nitrate ester from the emissions of the substrates. Mid‐infrared emissions were generated by heating the samples remotely using laser‐induced thermal emission (LITE). Chemometrics multivariate analysis techniques such as principal component analysis (PCA), soft independent modeling by class analogy (SIMCA), partial least squares‐discriminant analysis (PLS‐DA), support vector machines (SVMs), and neural network (NN) were employed to generate the models for the classification and discrimination of PETN IREs from substrate thermal emissions. PCA exhibited less variability for the LITE spectra of PETN/substrates. SIMCA was able to predict only 44.7% of all samples, while SVM proved to be the most effective statistical analysis routine, with a discrimination performance of 95%. PLS‐DA and NN achieved prediction accuracies of 94% and 88%, respectively. High sensitivity and specificity values were achieved for five of the seven substrates investigated. Copyright © 2015 John Wiley & Sons, Ltd.     

A Chemical Strategy for the Relaxivity Enhancement of GdIII Chelates Anchored on Mesoporous Silica Nanoparticles

Functionalised MCM‐41 mesoporous silica nanoparticles were used as carriers of Gd^III complexes for the development of nanosized magnetic resonance imaging contrast agents. Three Gd^III complexes based on the 1,4,7,10‐tetraazacyclododecane scaffold (DOTA; monoamide‐, DOTA‐ and DO3A‐like complexes) with distinct structural and magnetic properties were anchored on the silica nanoparticles functionalised with NH₂ groups. The interaction between Gd^III chelates and surface functional groups markedly influenced the relaxometric properties of the hybrid materials, and were deeply modified passing from ionic ? NH₃⁺ to neutral amides. A complete study of the structural, textural and surface properties together with a full ¹H relaxometric characterisation of these hybrid materials before and after surface modification was carried out. Particularly for the anionic complex 2 attached to MCM‐41, an impressive increase in relaxivity (r_1p) was observed (from 20.3 to 37.8 mM ^?1 s^?1, 86.2 % enhancement at 20 MHz and 310 K), mainly due to a threefold faster water exchange rate after acetylation of the surface ? NH₃⁺ ions. This high r_1p value, coupled with the large molar amount of grafted 2 onto the silica nanoparticles gives rise to a value of relaxivity per particle of 29 500 mM ^?1 s^?1, which possibly allows it to be used in molecular imaging procedures. Smaller changes were observed for the hybrid materials based on neutral 1 and 3 complexes. In fact, whereas 1 shows a weak interaction with the surface and acetylation induced only some decrease of the local rotation, complex 3 appears to be involved in a strong interaction with surface silanols. This results in the displacement of a coordinated water molecule and in a decrease of the accessibility of the solvent to the metal centre, which is unaffected by the modification of ammonium ions to neutral amides.     

Activated Random Walkers: Facts, Conjectures and Challenges

We study a particle system with hopping (random walk) dynamics on the integer lattice ? ^d . The particles can exist in two states, active or inactive (sleeping); only the former can hop. The dynamics conserves the number of particles; there is no limit on the number of particles at a given site. Isolated active particles fall asleep at rate λ>0, and then remain asleep until joined by another particle at the same site. The state in which all particles are inactive is absorbing. Whether activity continues at long times depends on the relation between the particle density ζ and the sleeping rate λ. We discuss the general case, and then, for the one-dimensional totally asymmetric case, study the phase transition between an active phase (for sufficiently large particle densities and/or small λ) and an absorbing one. We also present arguments regarding the asymptotic mean hopping velocity in the active phase, the rate of fixation in the absorbing phase, and survival of the infinite system at criticality. Using mean-field theory and Monte Carlo simulation, we locate the phase boundary. The phase transition appears to be continuous in both the symmetric and asymmetric versions of the process, but the critical behavior is very different. The former case is characterized by simple integer or rational values for critical exponents (β=1, for example), and the phase diagram is in accord with the prediction of mean-field theory. We present evidence that the symmetric version belongs to the universality class of conserved stochastic sandpiles, also known as conserved directed percolation. Simulations also reveal an interesting transient phenomenon of damped oscillations in the activity density.     

Simultaneous Differential Pulse Voltammetric Determination of Sulfamethoxazole and Trimethoprim on a Boron‐Doped Diamond Electrode

The performance of hydrogen‐ (HT) and oxygen‐terminated (OT) boron‐doped diamond (BDD) electrodes (electrochemically pretreated) on the simultaneous differential pulse voltammetric determination of sulfamethoxazole and trimethoprim in pharmaceutical products is presented. Under the optimum analytical experimental conditions, the HT‐BDD electrode presented two well‐defined oxidation peaks at 920 and 1100 mV vs. Ag/AgCl for sulfamethoxazole and trimethoprim, respectively. On the other hand, when the OT‐BDD electrode was used, the sulfamethoxazole oxidation current peak was decreased twenty fold. The calculated LOD values for sulfamethoxazole and trimethoprim using the HT‐BDD electrode were 3.65 μg L^?1 and 3.92 μg L^?1, respectively. The results obtained in the simultaneous determination of sulfamethoxazole and trimethoprim in three different commercial formulations were similar to those obtained using a standard HPLC method at 95% confidence level.     

Influence of thermoplastic starch plasticized with biodiesel glycerol on thermal properties of pp blends

Plastics have been used in short-life products, which have presented harmful consequences for the nature, because of the low degradation rate reached by the most common polyolefins. This work evaluates the mechanical and thermal properties of pure iPP, plasticized starch (TPS) with biodiesel (TPS_Bio) or commercial (TPS_Com) glycerols, and their blends (iPP/TPS_Plas). The addition of TPS_Plas caused an increasing on the cristallinity of iPP, mainly for the compositions 90/10 and 80/20, probably due to morphological alterations such as crosslinking, that may have modified the molecular arrangement of the iPP in the presence of glycerol.     

Stereoselective synthesis of novel 4,5-epoxy-1,2-oxazin-6-ones and alpha,beta-epoxy-gamma-amino acids from beta-lithiated oxazolinyloxiranes and nitrones

[reaction: see text] A stereoselective synthesis of 9,10-epoxy-1,6-dioxa-4,7-diazaspiro[4,5]decanes has been developed on the basis of the addition of beta-lithiated oxazolinyloxiranes to nitrones. Conversion of these spirocyclic derivatives into 4,5-epoxy-1,2-oxazin-6-ones and successively into alpha,beta-epoxy-gamma-amino acids, alpha-hydroxy-gamma-amino acids, and gamma-butyrolactams is described.     

Asymptotic stability of Riemann solutions in BGK approximations to certain multidimensional systems of conservation laws

We prove the asymptotic stability of nonplanar two-states Riemann solutions in BGK approximations of a class of multidimensional systems of conservation laws. The latter consists of systems whose flux-functions in different directions share a common complete system of Riemann invariants, the level surfaces of which are hyperplanes. The asymptotic stability to which the main result refers is in the sense of the convergence as t→∞ in of the space of directions ζ=x/t. That is, the solution z(t,x,ξ) of the perturbed Cauchy problem for the corresponding BGK system satisfies as t→∞, in , where R(ζ) is the self-similar entropy solution of the two-states nonplanar Riemann problem for the system of conservation laws.     

GKB-FP: an algorithm for large-scale discrete ill-posed problems

We describe an algorithm for large-scale discrete ill-posed problems, called GKB-FP, which combines the Golub-Kahan bidiagonalization algorithm with Tikhonov regularization in the generated Krylov subspace, with the regularization parameter for the projected problem being chosen by the fixed-point method by Bazán (Inverse Probl. 24(3), 2008). The fixed-point method selects as regularization parameter a fixed-point of the function ‖r _λ ‖₂/‖f _λ ‖₂, where f _λ is the regularized solution and r _λ is the corresponding residual. GKB-FP determines the sought fixed-point by computing a finite sequence of fixed-points of functions ||r_l^(k)||₂/||f_l^(k)||₂\|r_{\lambda}^{(k)}\|_{2}/\|f_{\lambda}^{(k)}\|_{2}, where f_l^(k)f_{\lambda}^{(k)} approximates f _λ in a k-dimensional Krylov subspace and r_l^(k)r_{\lambda}^{(k)} is the corresponding residual. Based on this and provided the sought fixed-point is reached, we prove that the regularized solutions f_l^(k)f_{\lambda}^{(k)} remain unchanged and therefore completely insensitive to the number of iterations. This and the performance of the method when applied to well-known test problems are illustrated numerically.