Synthesis of calix[4]arene-cyclen conjugates

Novel calix[4]arene derivatives constrained in the cone or 1,3-alternate conformations, bearing one or two cyclen (1,4,7,10-tetraazacyclododecane) moieties directly connected to the upper rim, have been synthesized using Buchwald-Hartwig coupling reaction. The complexation ability and hydrolytic activities of selected Zn(II) complexes of these calixarenes were studied. Although the attempts to hydrolyze activated phosphodiester bonds were unsuccessful, the NMR titration experiments revealed binding affinity for chloride, acetate, and benzoate anions in defined stoichiometry.     

Are the known Δ2-cephems inactive as antibiotics because of an unfavourable steric orientation of their 4α-carboxylic group? Synthesis and biology of two Δ2-cephem-4β-carboxylic acids

Two representatives of the yet unknown type of Δ²-cephem-4β-carboxylic acids were prepared. Contrarily to the prediction based on the activity model of Cohen, both acids proved inactive as antibiotics. Possible reasons for this discrepancy are briefly discussed.     

Net signs of molecular graphs: Dependence of molecular structure

The concept of internal connectivity associates to each eigenvector of the molecular graph a graph with signed edges. We show that the respective edge signs can be deduced without knowing the graph eigenvectors. The dependence of the edge signs on molecular structure is elucidated. The sign of an edge whose end vertices are ν_r and ν_s is found to be the result of separate interactions through all paths connecting the vertices ν_r and ν_s. The analysis of the edge signs is particularly simple in the case of acyclic systems. © 1994 John Wiley & Sons, Inc.     

Representing orders on the plane by translating convex figures

Given a finite collection of disjoint, convex figures on the plane, is it possible to assign to each a single direction of motion so that this collection of figures may be separated, through an arbitrary large distance, by translating each figure one at a time, along its assigned direction? We present a computational model for this separability problem based on the theory of ordered sets.     

Constructing greedy linear extensions by interchanging chains

A natural way to prove that a particular linear extension of an ordered set is ‘optimal’ with respect to the ‘jump number’ is to transform this linear extension ‘canonically’ into one that is ‘optimal’. We treat a ‘greedy chain interchange’ transformation which has applications to ordered sets for which each ‘greedy’ linear extension is ‘optimal’.     

Contact Elements on Fibered Manifolds

For every product preserving bundle functor T ^μ on fibered manifolds, we describe the underlying functor of any order (r, s, q), s ≥ r ≤ q. We define the bundle K_k,l^r,s,q YK_{k,l}^{r,s,q} Y of (k, l)-dimensional contact elements of the order (r, s, q) on a fibered manifold Y and we characterize its elements geometrically. Then we study the bundle of general contact elements of type μ. We also determine all natural transformations of K_k,l^r,s,q YK_{k,l}^{r,s,q} Y into itself and of T( K_k,l^r,s,q Y )T\left( {K_{k,l}^{r,s,q} Y} \right) into itself and we find all natural operators lifting projectable vector fields and horizontal one-forms from Y to K_k,l^r,s,q YK_{k,l}^{r,s,q} Y .     

Infinitesimal Hecke algebras

In this Note, we define infinitesimal analogues of the Iwahori–Hecke algebras associated with finite Coxeter groups. These are reductive Lie algebras for which we announce several decomposition results. These decompositions yield irreducibility results for representations of the corresponding (pure) generalized braid groups deduced from Hecke algebra representations through tensor constructions. To cite this article: I. Marin, C. R. Acad. Sci. Paris, Ser. I 337 (2003).     

Electronic transport in Mo/Ni superlattices

An experimental relationship between superconductivity, magnetism and localization is explored in short-wavelength (14Å ? ? ? 40Å) sputtered Mo/Ni superlattices. A crossover to a superconducting state is observed for ? < 9Å consistent with the observed paramagnetic behavior when the Ni strata are four atomic layers thick, or less. All samples show localization effects at helium temperatures and non-superconducting samples develop an unusual resistance plateau below T ? 0.5K.