Effects of surface site distribution and dielectric discontinuity on the charging behavior of nanoparticles: a grand canonical Monte Carlo study

The surface site distribution and the dielectric discontinuity effects on the charging process of a spherical nanoparticle (NP) have been investigated. It is well known that electrostatic repulsion between charges on neighbouring sites tends to decrease the effective charge of a NP. The situation is more complicated close to a dielectric breakdown, since here a charged site is not only interacting with its neighbours but also with its own image charge and the image charges of all its neighbours. Coexistence of opposite charges, titration sites positions, and pH dependence are systematically studied using a grand canonical Monte Carlo method. A Tanford and Kirkwood approach has been applied to describe the interaction potentials between explicit discrete ampholytic charging sites. Homogeneous, heterogeneous and patch site distributions were considered to reproduce the titration site distribution at the solid/solution interface of natural NPs. Results show that the charging process is controlled by the balance between Coulomb interactions and the reaction field through the solid-liquid interface. They also show that the site distribution plays a crucial role in the charging process. In patch distributions, charges accumulate at the perimeter of each patch due to finite size effects. When homogeneous and heterogeneous distributions are compared, three different charging regimes are obtained. In homogeneous and heterogeneous (with quite low polydispersity indexes) distributions, the effects of the NP dielectric constant on Coulomb interactions are counterbalanced by the reaction field and in this case, the dielectric breakdown has no significant effect on the charging process. This is not the case in patch distributions, where the dielectric breakdown plays a crucial role in the charging process.     

The first unequivocally ferromagnetically coupled squarato complex: origin of the ferromagnetism in an interlocked 3D Fe(II) system

An unprecedented interlocked 3D Fe(II) system, [Fe(squarate)(bpy)(2H2O)] x 3H2O, 1 (bpy = 4,4'-bipyridine), the first unequivocally ferromagnetically coupled squarato complex to be reported, has been synthesised and magnetically characterized.     

A direct synthetic approach to novel quadrupolar [14] azolophanes

A convergent ‘3+1’ synthesis allowed a simple entrance to the first examples of [1₄] metaazolophanes 1 and [1₄](meta-ortho)₂azolophanes 2 built up from heterocyclic betaine subunits, illustrating a prototype of phanes constructed by both highly π-excessive and highly π-deficient heteroaromatic moieties linked in a 1,3-alternating fashion.     

Addition of diethylzinc to dicobalt hexacarbonyl complexes of alpha,beta-acetylenic aldehydes with virtually complete enantioselectivity. A formal synthesis of (+)-incrustoporin

[reaction: see text] The addition of diethylzinc to dicobalt hexacarbonyl complexes of acetylenes mediated by (R)-2-piperidino-1,1,2-triphenylethanol takes place with very high enantioselectivity (96-99% ee) to afford the S enantiomers of dicobalt hexacarbonyl complexes of 1-alkynyl-1-propanols. The utility of this process is exemplified by the development of a short, highly enantioselective (99% ee) synthesis of unnatural incrustoporin.     

Solid phase synthesis of cyclopropenes

Cyclopropene derivatives of high purity were prepared by reaction of acetylenes with a rhodium carbenoid bound to a polystyrene resin. This solid phase method avoids the formation of undesired byproducts obtained in the corresponding solution reaction and an eventual extension to combinatorial synthesis of cyclopropene derivatives could be achieved.     

The application of modulated differential scanning calorimetry to the glass transition

The modulated differential scanning calorimetry (MDSC) technique superimposes upon the conventional DSC heating rate a sinusoidally varying modulation. The result of this modulation of the heating rate is a periodically varying heat flow, which can be analysed in various ways. In particular, MDSC yields two components (reversing and non reversing) of the heat flow, and a phase angle. These each show a characteristic behaviour in the glass transition region, but their interpretation has hitherto been unclear. The present work clarifies this situation by a theoretical analysis of the technique of MDSC, which introduces a kinetic response of the glass in the transition region. This analysis is able to describe all the usual features observed by MDSC in the glass transition region. In addition, the model is also able to predict the effects of the modulation variables, and some of these are discussed briefly.Financial support has been provided by the DGICYT (Project no.PB93/1241). J.M.H. wishes to acknowledge financial assistance for a sabbatical period from the Generalitat de Catalunya.     

Case-based reasoning (CBR) for multicomponent analysis using sensor arrays: application to water quality evaluation

This paper reports by the first time the use of a simple, convenient and improved method of artificial intelligence (case-based reasoning, CBR) for multicomponent analysis, involving the data treatment afforded by electrochemical sensor arrays. The suitability of this method has been shown in aqueous solutions containing mixtures of ions of different nature and concentration, being applied in this case to a rapid recognition and evaluation of mineral water samples chosen as an illustrative example. The convenience of such a procedure opens the way to dispose of a versatile tool applicable for obtaining analytical information about ions present in a wide range of other multicomponent systems, and also to ascribe properties inherent to the examined samples.     

Synthesis of polyhydroxycyclohexanes and relatives from (-)-quinic acid

An efficient and versatile strategy for the synthesis of polyhydroxycyclohexanes and related compounds 3-6 is reported. The successful synthesis of these analogues has been achieved from a common intermediate, quinic acid derived lactone 2, rapidly accessible from cheap and commercially available (-)-quinic acid (1) as a chiral template. A practical route involving stereocontrolled epoxide formation and hydrolysis has been developed for the synthesis of 2,3-trans analogues 3 and 4. The preparation of the 2,3-cis analogues 5 and 6 has been realized by diasteroselective oxidation of a 5,6-double bond.     

A case for enantioselective allylic alkylation catalyzed by palladium nanoparticles

Palladium nanoparticles (4 nm, fcc) were prepared through decomposition of [Pd2(dba)3] by H2 in the presence of a chiral xylofuranoside diphosphite. These particles catalyze the allylic alkylation of rac-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate leading to an almost total conversion of the (R) enantiomer and almost no reaction with the (S). This gives rise to 97% ee for the alkylation product and a kinetic resolution of the substrate recovered with ca. 90% ee. This behavior was compared to that of a molecular catalyst at various dilutions, and the differences between the two systems are discussed. This is the first colloidal system shown to display such a high enantioselectivity besides the well-known Pt/cinchonidine system.