Magnetic properties of lanthanum orthoferrite fine powders prepared by different chemical routes

Fine powders of lanthanum iron oxide, LaFeO₃, have been prepared by solid state reaction as well as sol-gel synthesis and nebulized spray pyrolysis. Structures, morphologies and magnetic susceptibility measurements of these powders have been examined. The powders prepared by all the three low-temperature routes contain nearly spherical particles with an average diameter of 40 nm. These samples show a lower Neel temperature than the powder prepared by solid state reaction besides showing much lower magnetic susceptibility at low temperatures. Dedicated to Professor C N R Rao on his 70th birthday     

Parallel algorithm for the computation of characteristic polynomials of chemical graphs

A parallel algorithm is developed for the first time based on Frame's method to compute the characteristic polynomials of chemical graphs. This algorithm can handle all types of graphs: ordinary, weighted, directed, and signed. Our algorithm takes only linear time in the CRCW PRAM model with O(n³) processors whereas the sequential algorithm takes O(n⁴) time. Especially when the number of vertices of the graph is large this method will be more efficient than the recently developed vectorized Frame and Givens–Householder methods.     

Ab initio Computational Modeling of Glyphosate Analogs: Conformational Perspective

A historical perspective on the application of conformational analysis to structure-based ligand design approach is presented. The application of isodensity molecular electrostatic potential surfaces with the conformational energy surfaces (CES) have allowed us to reach pertinent conclusions for aiding synthetic and biochemical studies. Here we illustrate such an application on the modeling of the potent analogs of an important, environmentally stringent herbicidal compound glyphosate by constructing conformational energy surfaces. The systems were modeled by substituting F, Cl, and NH— OH moiety to the position of pharmacophoric nitrogen center in glyphosate structure. All the calculations were thoroughly performed with ab initio MO theory at Hartree–Fock method using 3-21G(d) basis functions. On the basis of the results, we identified the bioactive conformations for N-fluoro-glyphosate, N-chloro-glyphosate, and N-hydroxyamino-glyphosate as (−38^∘, 77^∘), (−61^∘, 111^∘), and (−167^∘, −169^∘), respectively. Geometry optimization of certain selected conformations of these compounds using hybrid DFT method with 6–31+G(d) basis functions provides nearly equal values of φ and ψ. Moreover, the results indicate that the global minimum structures of N-fluoro and N-chloro analogs of glyphosate show cyclic conformation whereas the N-hydroxyamino-glyphosate global minimum structure shows spyrocyclic and zig-zag conformation. Also, the predicted bioactive conformation of N-hydroxyamino analog optimally overlaps with glyphosate backbone in EPSPS complex with 0.1 Å RMSD value. However, the other two compounds slightly deviate from the backbone of glyphosate with RMSD of 0.92 Å for N-fluoro-glyphosate and 0.83 Å for N-chloro-glyphosate. The linear N-hydroxyamino-glyphosate exhibits relatively more number of intermolecular hydrogen bond interactions as compared to the other two analogs. Further, comparison of CES of previously studied glyphosate analogs such as N-hydroxy-glyphosate (2.2 μM) and N-amino-glyphosate (0.61 μM) with the present systems reveals the order of activity as: N-hydroxyamino-glyphosate > N-fluoro-glyphosate > N-chloro-glyphosate based on CES flexibility. Also, the calculated heats of formation of N-fluoro-glyphosate, N-chloro-glyphosate, and N-hydroxyamino-glyphosate are −288, −209, and −288 kcal/mol, respectively, which clearly indicate that the N-hydroxyamino and N-fluoro analogs of glyphosate are thermodynamically more stable than N-amino-glyphosate (−278 kcal/mol).     

Ab initio and DFT modeling of stereoselective deamination of aziridines by nitrosyl chloride

The stereochemical course of the deamination of cis‐2,3‐dimethylaziridine by nitrosyl chloride was investigated at the QCISD/6‐31G(d) level. Calculations reveal that the reaction takes place in two steps. In the first step, the reactants form a pre‐reactive complex, followed by a spiro‐type bicyclic transition state, which on dissociative cycloelimination gives the N‐nitrosoaziridine intermediate. In the second step, this intermediate undergoes cycloreversion through a slightly asynchronous concerted transition state to form an alkene with the same stereochemistry, which is in total agreement with experiment. In the whole reaction, the denitrosation step is found to be rate‐determining. For comparison, geometry optimizations and energies were also obtained at the B3LYP/6‐31G(d) level. It was found that the B3LYP energy results differed significantly from the QCISD ones. To analyze the reason for this difference, B3LYP calculations were repeated by varying the contribution of exact exchange in the Becke functional. With respect to the QCISD results, it has been shown that the functional with 0% exact exchange yields the best activation barriers, whereas the functional with 30% exact exchange is the most suitable one to carry out the complexation and reaction energy calculations. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ring cleavage of aziridines by difluoroamine: mechanistic insights from ab initio and DFT study

cis-2,3-Dimethylaziridine reacts with difluoroamine to give the corresponding alkene and nitrogen with retention of configuration. We have carried out a DFT study of this reaction to clarify the reaction mechanism by considering a multistep reaction pathway with possible intermediacy of several three- and four-membered cyclic intermediates and transition states (TSs). The energetics of this reaction shows that the reaction takes place in four steps including a three-membered azamine intermediate. Both the energetics and the stereochemical outcome of this reaction rule out the formation of a four-membered diazetine intermediate during the reaction. Although the first N-N bond formation step is rate determining, the final step, asynchronous concerted cleavage of the azamine intermediate, explains the stereochemistry of this reaction. The asynchronous nature of the final step makes the reaction Woodward-Hoffmann allowed, as reported by Yamabe and Minato (J. Phys. Chem. A 2001, 105, 7281). Computations at HF and MP2 levels confirm the same trends in energetics. Single point energy computations at B3LYP, MP2, and QCISD levels with the 6-311++G(d,p) and cc-pVTZ basis sets show that the larger basis sets predict higher free energies of activation and less negative free energies of reaction. Intrinsic reaction coordinate (IRC) analyses reveal the asynchronous nature of the first and the last steps of the reaction. The deamination of trans-2,3-dimethylaziridine was shown to follow a course of reaction similar to that of the cis isomer.     

Origin of asymmetry of the EFG in cubic metal alloys

Unlike the earlier works, which assume the impurity-induced electron density δn to be spherically symmetric, we have taken δn to have the more realistic cubic symmetry. The corresponding valence EFG q^v shows the interesting property of non-axial symmetry along the [110] direction. Contrary to earlier assumptions, this reveals that the size EFG is not the only source for the asymmetry of the EFG in cubic metal alloys.     

Effect of crystal potential on electric field gradient calculations due to impurities in cubic metals

The electric field gradients caused by interstitial and substitutional point defects in f.c.c. metals have been studied. The perturbation on the host electron distribution has been calculated self-consistently using density functional formalism. The effect of strain caused by size difference between host and impurity atoms is treated in the point-ion model. The influence of the host potential on impurity perturbation is incorporated within the spherical solid approximation. The theory is applied to positive muon and mono-vacancy in Al and Cu hosts. The theoretical results are in good agreement with experiment and illustrate the importance of both strain and conduction electron contribution to the electric field gradient.     

Distribution of electric field gradients around impurities in aluminum

The electric field gradients caused by a vacancy, Mg, Ga, In, Si, Ge and Sn impurities at several near-neighbor sites in Al hosts have been calculated. The theory takes into account the contribution from the conduction electron screening cloud and the lattice strain caused by the impurities. The perturbed core as well as conduction electron densities around the impurities are calculated selfconsistently using the density functional theory. Assuming that the charge distribution around the impurity is spherically symmetric, an exact expression, valid at all distances, is derived for the conduction electron contribution to the electric field gradient. While this result is substantially different from those using the conventional asymptotic or pre-asymptotic expressions, it is found to be entirely inadequate in explaining the observed asymmetry and magnitude of the electric field gradient distribution in cubic metal alloys. The contribution due to the lattice strain is calculated using the point-ion model and a new analytic form for the elastic strain tensor. The combined strain and charge screening effect provides a satisfactory agreement between calculated and experimental electric field gradients. The difficulties standing in the way of an overall quantitative understanding of the electric field gradient in cubic metal alloys are discussed. The subsequent stages of improvement in both theory and experiment that can result in a better understanding of the problem are pointed out.     

Inter-comparison and validation of computational fluid dynamics codes in two-stage meandering channel flows

This paper presents a study in the inter-comparison and validation of three-dimensional computational fluid dynamics codes which are currently used in river engineering. Finite volume codes PHOENICS, FLUENT and SSIIM; and finite element code TELEMAC3D are considered in this study. The work has been carried out by competent hydraulic modellers who are users of the codes and not involved in their development. This paper is therefore written from the perspective of independent practitioners of the techniques. In all codes, the flow calculations are performed by solving the three-dimensional continuity and Reynolds-averaged Navier–Stokes equations with the k–ε turbulence model. The application of each code was carried out independently and this led to slightly different, but nonetheless valid, models. This is particularly seen in the different boundary conditions which have been applied and which arise in part from differences in the modelling approaches and methodology adopted by the different research groups and in part from the different assumptions and formulations implemented in the different codes. Similar finite volume meshes are used in the simulations with PHOENICS, FLUENT and SSIIM while in TELEMAC3D, a triangular finite element mesh is used. The ASME Journal of Fluids Engineering editorial policy is taken as a minimum framework for the control of numerical accuracy. In all cases, grid convergence is demonstrated and conventional criteria, such as Y⁺, are satisfied. A rigorous inter-comparison of the codes is performed using large-scale experimental data from the UK Flood Channel Facility for a two-stage meandering channel. This example data set shows complex hydraulic behaviour without the additional complications found in natural rivers. Standardised methods are used to compare each model with the available experimental data. Results are shown for the streamwise and transverse velocities, secondary flow, turbulent kinetic energy, bed shear stress and free surface elevation. They demonstrate that the models produce similar results overall, although there are some differences in the predicted flow field and greater differences in turbulent kinetic energy and bed shear stress. This study is seen as an essential first step in the inter-comparison of some of the computational fluid dynamics codes used in the field of river engineering.     

Ring opening of boriranes vis‐à‐vis aziridines: An ab initio and DFT probe on the mechanisms

Borirane undergoes ring opening reaction with NOCl and HNF₂ yielding the corresponding alkenes. Ab initio and density functional investigations of this reaction with cis‐ and trans‐2,3‐dimethylboriranes reveal that these reactions take place in a single step through the formation of a prereactive complex and a transition state giving the alkene with the same stereochemistry. Calculations clearly show that the concerted cleavage of C? B bonds leads to retention of stereochemistry. Further, it shows that HNF₂ cleaves boriranes more efficiently than does NOCl. Intrinsic reaction coordinate analyses and bond order analysis describe the nature of the transition state very well and fix the reaction mechanism. Solvent effect calculations through PCM model, with acetonitrile and CCl₄ as solvents, do not alter the gas phase results significantly. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007