Molecular rearrangement of an Aza-Scorpiand macrocycle induced by pH: A computational study

Rearrangements and their control are a hot topic in supramolecular chemistry due to the possibilities that these phenomena open in the design of synthetic receptors and molecular machines. Macrocycle aza-scorpiands constitute an interesting system that can reorganize their spatial structure depending on pH variations or the presence of metal cations. In this study, the relative stabilities of these conformations were predicted computationally by semi-empirical and density functional theory approximations, and the reorganization from closed to open conformations was simulated by using the Monte Carlo multiple minimum method

Visualisation of Avogadro’s Number

Con motivo de la redefinición de la constante de Avogadro, se presenta una breve historia y algunas reflexiones didácticas sobre su magnitud. Se revisan algunas analogías y se sugieren otras para ayudar a visualizar el alcance de su magnitud y se evalúa su utilidad. Estas analogías se enmarcan en el contexto docente de los cursos primero y segundo de las titulaciones en diversas disciplinas científicas y técnicas. Su efectividad se discute por primera vez sobre la base de un cuestionario realizado por los estudiantes correspondientes. Los resultados sugieren que los modelos más útiles, siguiendo la opinión de los estudiantes, son aquellos relacionados con ítems más sustanciales, por ejemplo, neuronas, individuos, planetas, por encima de analogías sobre construcciones geométricas. Podemos concluir, en contraste el pensamiento actual, que las descripciones visuales no siempre son las más ventajosas.On the occasion of the redefinition of the Avogadro constant, a brief history and some didactic reflections on its magnitude are presented. Some analogies are reviewed and others are suggested to help visualize the extent of its magnitude, and their usefulness is assessed. These analogies are set in the teaching context of the first and second courses of the degrees in several scientific and technic disciplines. Their effectiveness is discussed for the first time on the basis of a questionnaire filled by the corresponding students. The suggestions for educating and learning are that the most helpful models, following the opinion of the students, are those related to more substantial items, for example, neurons, individuals, planets, above analogies on geometric constructions. Challenging current thought, pictorial descriptions are not all the times so advantageousCiencias Experimentale

True prediction of lowest observed adverse effect levels

Summary A database of structurally heterogeneous chemical structures with their experimental values of Lowest Observed Adverse Effect Levels (LOAELs) was modeled using graph theoretical descriptors. Variable selection for multiple linear regression (MLR) and linear discriminant analysis (LDA) was accomplished by the Internal Test Set (ITS) method in order to achieve true predicted LOAEL values. The results obtained can be considered good if we take in count the structural diversity of the training set

Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity

Two oxoiron(IV) isomers (R2a and R2b) of general formula [FeIV(O)(RPyNMe3)(CH3CN)]2+ are obtained by reaction of their iron(II) precursor with NBu4IO4. The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between R2a and R2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that R2a reacts one order of magnitude faster than R2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in R2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand

Internal Test Sets Studies in a Group of Antimalarials

Topological indices have been applied to build QSAR models for a set of 20 an-timalarial cyclic peroxy cetals. In order to evaluate the reliability of the proposed linearmodels leave-n-out and Internal Test Sets (ITS) approaches have been considered. The pro-posed procedure resulted in a robust and consensued prediction equation and here it isshown why it is superior to the employed standard cross-validation algorithms involvingmultilinear regression models

Communications on Quantum Similarity (2): A Geometric Discussion on Holographic Electron Density Theorem and Confined Quantum Similarity Measures

Abstract: The so-called holographic electron density theorem (HEDT) is analyzed from an algebraic perspective, and a brief analytical point of view is also given. The connection of the HEDT with quantum similarity measures (QSM) over electronic density functions (DF) is studied using GTO functions, atomic ASA DF, and promolecular ASA DF. Restricted integration of QSM over a box of finite side length is discussed for all this DF. This work emphasizes the geometric aspects of HEDT, but for the sake of completeness, some analytical insight based on a general Taylor series expansion is also given at the end

Select-divide-and-conquer method for large-scale configuration interaction

A select-divide-and-conquer variational method to approximate configuration interaction (CI) is presented. Given an orthonormal set made up of occupied orbitals (Hartree-Fock or similar) and suitable correlation orbitals (natural or localized orbitals), a large N-electron target space S is split into subspaces S0,S1,S2,...,SR. S0, of dimension d0, contains all configurations K with attributes (energy contributions, etc.) above thresholds T0={T0egy, T0etc.}; the CI coefficients in S0 remain always free to vary. S1 accommodates KS with attributes above T1≤T0. An eigenproblem of dimension d0+d1 for S0+S 1 is solved first, after which the last d1 rows and columns are contracted into a single row and column, thus freezing the last d1 CI coefficients hereinafter. The process is repeated with successive Sj(j≥2) chosen so that corresponding CI matrices fit random access memory (RAM). Davidson's eigensolver is used R times. The final energy eigenvalue (lowest or excited one) is always above the corresponding exact eigenvalue in S. Threshold values {Tj;j=0, 1, 2,...,R} regulate accuracy; for large-dimensional S, high accuracy requires S 0+S1 to be solved outside RAM. From there on, however, usually a few Davidson iterations in RAM are needed for each step, so that Hamiltonian matrix-element evaluation becomes rate determining. One μhartree accuracy is achieved for an eigenproblem of order 24 × 106, involving 1.2 × 1012 nonzero matrix elements, and 8.4×109 Slater determinant