Zeolite-based photocatalysts

The compartmentalised intracrystalline void space of zeolites are specially suited to incorporate and organise photoactive guests that can be used as photocatalysts. The rigid micropores allow assembly of multicomponent systems comprising of antenna and relays reminiscent of natural photosynthetic centers. Besides inorganic metal oxide clusters, zeolites as host are particularly attractive to construct organic photocatalysts since the guest becomes significantly stabilized by incorporation. This review gives special emphasis to the commercial and potential application of photocatalysts.     

Evaluation of measurement uncertainty in analytical assays by means of Monte-Carlo simulation

The main limitations of the Guide to the expression of Uncertainty Measurement (GUM) approach for evaluating the measurement uncertainty of analytical assays are presented and explained. The advantages of using Monte-Carlo simulation against the GUM approach are outlined and discussed and the principle of propagation of distributions is explained. The procedure of Monte-Carlo analysis is illustrated by two case studies. A first simple example quoted from the EURACHEM Guide and dealing with the preparation of a calibration standard is used to present the technique with detail in a step-by-step way. In this case the results obtained by both approaches are very similar. A second example deals with the calibration of mass according to a strong non-linear model. In this case, the Monte-Carlo analysis leads to better results.     

Bifurcations of Umbilic Points and Related Principal Cycles

The simplest patterns of qualitative changes on the configurations of lines of principal curvature around umbilic points on surfaces whose immersions into ³ depend smoothly on a real parameter (codimension one umbilic bifurcations) are described in this paper.Global effects, due to umbilic bifurcations, on these configurations such as the appearance and annihilation of periodic principal lines, called also principal cycles, are also studied here.     

A non-cooperative approach to the cost spanning tree problem

We associate to each cost spanning tree problem a non-cooperative game, which is inspired by a real-life problem. We study the Nash equilibria and subgame perfect Nash equilibria of this game. We prove that these equilibria are closely related with situations where agents connect sequentially to the source.Finicial support from the Ministerio de Ciencia y Tecnologia and FEDER, and Xunta de Galicia through grants BEC2002-04102-C02-01 and PGIDIT03PXIC30002PN is gratefully acknowledged.     

On towers of function fields of Artin-Schreier type

In this article we derive strong conditions on the defining equations of asymptotically good Artin-Schreier towers. We will show that at most three kinds of defining equations can give rise to a recursively defined good tower, if we restrict ourselves to prime degrees. ¹A. Garcia and H. Stichtenoth did part of thiswork during their stay at Sabanci University, Istanbul, Turkey (Sept. 2002). ²A. Garcia was partially supported by PRONEX # 662408/1996-3 (CNPq-Brazil).     

Critical Casimir effect near the (3)He-(4)He tricritical point

We present capacitance measurements of the equilibrium thickness of (3)He-(4)He mixture films as a function of temperature and concentration. The films are adsorbed on a Cu substrate situated above bulk liquid mixture. As we scan across the tricritical point, we observe a thickening of the film indicating the presence of a repulsive critical Casimir force.     

Ring‐opening polymerization of ϵ‐caprolactone by iodine charge‐transfer complex

The ring‐opening polymerization of ?‐caprolactone (?‐CL) catalyzed by iodine (I₂) was studied. The formation of a charge‐transfer complex (CTC) among triiodide, I, and ?‐CL was confirmed with ultraviolet–visible spectroscopy. The monomer ?‐CL was polymerized in bulk using I₂ as a catalyst to form the polyester having apparent weight‐average molecular weights of 35,900 and 45,500 at polymerization temperatures of 25 and 70 °C, respectively. The reactivity of both, ?‐CL monomer and ?‐CL:I₂ CTC, was interpreted by means of the potential energy surfaces determined by semiempirical computations (MNDO‐d). The results suggest that the formation of the ?‐CL:I₂ CTC leads to the ring opening of the ?‐CL structure with the lactone protonation and the formation of a highly polarized polymerization precursor (?‐CL)⁺. The band gaps approximated from an extrapolation of the oligomeric polycaprolactone (PCL) structures were computed. With semiempirical quantum chemical calculations, geometries and charge distributions of the protonated polymerization precursor (?‐CL)⁺ were obtained. The calculated band gap (highest occupied molecular orbit/lowest unoccupied molecular orbit differences) agrees with the experiment. The analysis of the oligomeric PCL isosurfaces indicate the existence of a weakly lone pair character of the C?O and C? O bonds suggesting a ?‐CL ring‐opening specificity. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 714–722, 2002     

Conformational studies by dynamic NMR. 84.(1) structure, conformation, and stereodynamics of the atropisomers of N-aryl-tetrahydropyrimidines

The existence of stereolabile atropisomers for a number of N-aryl-tetrahydropyrimidines in solution has been deduced from the observation of the anisochronous NMR signals of prochiral methylene groups. The interconversion barriers for these atropisomers have been measured by line shape analysis of dynamic NMR spectra at various temperatures: a Molecular Mechanics modeling resulted in good agreement with these values. In an appropriate case, distinct NMR signals for the two enantiomeric forms could be observed at ambient temperature in a chiral environment. Evidence was also obtained for an exchange process occurring between two conformers experiencing a very biased equilibrium. Single-crystal X-ray diffraction of one such compound yielded a molecular structure in good agreement with the results obtained by ab initio calculations.