Comprehensive Characterization of the Self-Folding Cavitand Dynamics

The conformational equilibria and guest exchange process of a resorcin[4]arene derived self-folding cavitand receptor have been characterized in detail by molecular dynamics simulations (MD) and ¹H EXSY NMR experiments. A multi-timescale strategy for exploring the fluxional behaviour of this system has been constructed, exploiting conventional MD and accelerated MD (aMD) techniques. The use of aMD allows the reconstruction of the folding/unfolding process of the receptor by sampling high-energy barrier processes unattainable by conventional MD simulations. We obtained MD trajectories sampling events occurring at different timescales from ns to s: 1) rearrangement of the directional hydrogen bond seam stabilizing the receptor, 2) folding/unfolding of the structure transiting partially open intermediates, and 3) guest departure from different folding stages. Most remarkably, reweighing of the biased aMD simulations provided kinetic barriers that are in very good agreement with those determined experimentally by ¹H NMR. These results constitute the first comprehensive characterization of the complex dynamic features of cavitand receptors. Our approach emerges as a valuable rational design tool for synthetic host-guest systems     

Estimation of the uncertainty of mass measurements from in-house calibrated analytical balances

The uncertainty evaluation of mass measurements when using “in-house” calibrated analytical balances is revisited according to the Guide to the expression of Uncertainty Measurement (GUM). The calibration of analytical balances is discussed according to the guidelines of several bodies such as ASTM, UKAS and DKD/PTB. The remainder components of uncertainty can be estimated from the balance data sheet specifications.     

Fractal calibration in size-exclusion chromatography I. An introduction

The elution behaviour of different polymer-solvent systems in three types of organic columns for SEC has been compared and interpreted. The experimental data show that the classical universal calibration is not accomplished. Deviations from a unique curve are observed due to the binary and ternary interactions between the components of the system (solvent, polymer and gel) which results on secondary mechanisms accompanying the main pure or "ideal" SEC separation mechanism. Both, enthalpic and entropic effects are interpreted in terms of the swelling and crosslinking degrees of the gel packings, and are also related with the fractal characteristics of their surfaces, such as the fractal dimension and the available pore size. Moreover, a relationship between the fractal dimension of the pore surface and the chromatographic distribution coefficient is proposed.     

The role of amide ligands in the stabilization of Pd(II) tricoordinated complexes: is the Pd–NR2 bond order single or higher?

The existence of tricoordinated Pd(II) complexes has been a matter of controversy for a long time. The recent X-ray characterization of a family of Pd complexes [PdArXL] allowed to certify the existence of true tricoordinated Pd(II) species. The unique role played by the amido ligand (X = NR₂), among a family of X ligands, was noticed in a previous computational work. Here, the influence of the R substituents at the amide and the nature of the Pd–N_amido bond are theoretically analyzed. The relative stability of d ⁸ tricoordinated [PdLAr(NR₂)] complexes versus d ⁸ tetracoordinated derivatives as a function of the R substituents is studied by analyzing the two most common ways to fill the vacant coordination site in a tricoordinated complex: solvent coordination (with tetrahydrofuran as solvent), or dimerization giving [(μ-NR₂)₂Pd₂L₂Ar₂]) complexes. The nature of the Pd–N bonding interaction is analyzed using several theoretical schemes as molecular orbitals, QTAIM, ELF and NBO. Each of these schemes suggests that the order of the Pd–N bond in this family of complexes is higher than one. An asymmetric π interaction between the nitrogen lone pair and the LUMO over the tricoordinated Pd center is proposed as an important source of additional stabilization of tricoordinated species provided by amido ligands.     

Lead Sensor Using Gold Nanostructured Screen‐Printed Carbon Electrodes as Transducers

Gold nanostructured screen‐printed carbon electrodes are demonstrated to be suitable transducers for the determination of lead using square‐wave voltammetry. Reproducible gold nanostructures have been obtained by direct electrochemical deposition. A calibration plot from 2.5 to 250 μg/L was obtained in acidic solutions of Pb(II) with a reproducibility of 4% (n=10). The detection limit was 0.09 μg/L of lead. The method is then applied to perform a blood lead analysis by adjusting square‐wave parameters in capillary or venous blood with a minimum sample pretreatment and excellent accuracy and reproducibility.     

Synthesis,characterisation and crystal structure of a cobalt(II)-hexamethylenetetramine coordination polymer

A novel one-dimensional zigzag coordination polymer, dinitrodiaqua-bis(hexamethylenetetramine)cobalt(II) was synthesised and characterised, and the structure was determined by single-crystal X-ray diffraction. The compound has a chain structure with each cobalt atom covalently bonded to two nitrate ions, two water molecules and two HMTA molecules, giving a slightly distorted octahedral geometry about the cobalt atom. Each HMTA ligand uses two of its N atoms to bond to two cobalt atoms giving an approximately bent Co–HMTA–Co configuration. Each chain is hydrogen bonded through OH···N and OH···O interactions with neighbouring chains leading to an overall polymer structure. Thermal studies show significant mass loss corresponding to the loss of the coordinated water molecules and the decomposition of both the nitrate ions and the HMTA.     

The Wacker Process: Inner‐ or Outer‐Sphere Nucleophilic Addition? New Insights from Ab Initio Molecular Dynamics

The Wacker process consists of the oxidation of ethylene catalyzed by a Pd^II complex. The reaction mechanism has been largely debated in the literature; two modes for the nucleophilic addition of water to a Pd‐coordinated alkene have been proposed: syn‐inner‐ and anti‐outer‐sphere mechanisms. These reaction steps have been theoretically evaluated by means of ab initio molecular dynamics combined with metadynamics by placing the [Pd(C₂H₄)Cl₂(H₂O)] complex in a box of water molecules, thereby resembling experimental conditions at low [Cl^?]. The nucleophilic addition has also been evaluated for the [Pd(C₂H₄)Cl₃]^? complex, thus revealing that the water by chloride ligand substitution trans to ethene is kinetically favored over the generally assumed cis species in water. Hence, the resulting trans species can only directly undertake the outer‐sphere nucleophilic addition, whereas the inner‐sphere mechanism is hindered since the attacking water is located trans to ethene. In addition, all the simulations from the [Pd(C₂H₄)Cl₂(H₂O)] species (either cis or trans) support an outer‐sphere mechanism with a free‐energy barrier compatible with that obtained experimentally, whereas that for the inner‐sphere mechanism is significantly higher. Moreover, additional processes for a global understanding of the Wacker process in solution have also been identified, such as ligand substitutions, proton transfers that involve the aquo ligand, and the importance of the trans effect of the ethylene in the nucleophilic addition attack.     

Special issue on “Advances on model-based clustering and classification”

Advances in Data Analysis and Classification -     

Radical of submodules and symmetric algebra

A prime ideal of the symmetric algebra of an R-module N is associated to every prime submodule M ?N and this assignment is then used to obtain a new characterization of the radical of a submodule. Several applications of these results are also included.     

A review of robust clustering methods

Deviations from theoretical assumptions together with the presence of certain amount of outlying observations are common in many practical statistical applications. This is also the case when applying Cluster Analysis methods, where those troubles could lead to unsatisfactory clustering results. Robust Clustering methods are aimed at avoiding these unsatisfactory results. Moreover, there exist certain connections between robust procedures and Cluster Analysis that make Robust Clustering an appealing unifying framework. A review of different robust clustering approaches in the literature is presented. Special attention is paid to methods based on trimming which try to discard most outlying data when carrying out the clustering process.