Short-term changes in delta(13)C and delta(15)N signatures of water discharged from grazed grasslands

The composition of dissolved organic matter (DOM) in a soil is the product of a variety of soil processes. Changes in the composition of DOM in water discharged from soil should, therefore, give an important insight into modifications in these soil processes. We hypothesise that these processes in soils, under different grassland management regimes, would be affected to different extents by the short-term disturbance of a storm event and that evidence of this could be detected in delta(13)C and delta(15)N signatures in drainage and surface runoff waters. During a storm event we collected discharge waters from 1 ha grassland lysimeters, with or without artificial drainage, which received contrasting fertiliser inputs, and delta(13)C and delta(15)N signatures were determined. Changes in (13)C enrichment during the storm event were clearly identifiable, as were differences between plots for (13)C and (15)N, illustrating that this technique has potential to be a useful tool for identifying and investigating short- and long-term changes in soil organic matter dynamics. Copyright 1999 John Wiley & Sons, Ltd.     

Ion-pair recognition by a heteroditopic triazole-containing receptor

A new heteroditopic calix[4]diquinone triazole containing receptor capable of recognising both cations and anions through Lewis base and C-H hydrogen-bonding modes, respectively, of the triazole motif has been prepared. This ion-pair receptor cooperatively binds halide/monovalent-cation combinations in an aqueous mixture, with selectivity trends being established by (1)H?NMR and UV/Vis spectroscopy. Cation binding by the calix[4]diquinone oxygen and triazole nitrogen donors enhances the strength of the halide complexation at the isophthalamide recognition site of the receptor. Conversely, anions bound in the receptor's isophthalamide cavity enhance cation recognition. (1)H?NMR investigations in solution suggest that the receptor's triazole motifs are capable of coordinating simultaneously to both cation and anion guest species. Solid-state X-ray crystallographic structural analysis of a variety of receptor ion-pair adducts further demonstrates the dual cation-anion binding role of the triazole group.     

Hydration of inorganic phosphates in crystal lattices and in aqueous solution. An experimental and theoretical study

B3LYP/6-31G* and 6-311++G** calculations have been carried out in order to study the hydration of phosphates in aqueous media. Optimized geometries and relative stabilities for PO4(-3), HPO(4)-2, H2PO4(-1) have been calculated considering the interaction with one, two, three, four and five discrete water molecules and taking into account the solvent effect by using the self-consistent reaction field theory (Onsager and PCM methods). The role of specific and bulk contributions of solvent effect on the observable properties of phosphate compounds is analysed. Good agreement between theoretical and available experimental results of harmonic vibration frequencies is found. Significant effects on the geometrical and vibrational frequencies are found for those studied phosphate anions. The results presented here provide a first step toward the understanding of the phosphate group as a hydration sensor in lipid bilayers.     

Design of dipicolinic acid ligands for the two-photon sensitized luminescence of europium complexes with optimized cross-sections

The multistep synthesis of an extensive series of push-pull donor-pi-conjugated dipicolinic acid ligands is described. The charge transfer character of the ligand can be tuned by changing the donor group (CH 2R, OR, SR, or NR 2) or the nature of the conjugated backbone (phenyl, phenylethynyl, naphtylethynyl, bis(phenylethynyl), or chalcone). The photophysical properties of related D 3 symmetric europium complexes (absorption and luminescence) were measured. Experiments using two-photon sensitized luminescence of a Eu (III) complex reveal large two-photon absorption (TPA) cross-section values (775 GM at 740 nm) in dichloromethane. Furthermore, some structure-property relationships can be derived from this systematic study, allowing an optimization of TPA properties of lanthanide complexes.     

Specific heat of smoky quartz

Specific heat measurements of smoky quartz are reported between 20 mK and 10 K. The linear temperature dependence observed below 1 K can be attributed to the widely distributed manifold of low-lying energy levels associated with a hole trapped at one oxygen near a substitutional aluminium impurity.     

Neutron Scattering by Copolymers

Abstract

Neutron scattering is a very efficient tool for studying polymers in bulk as well as in solution. This is mainly due to the difference of scattering length between hydrogen and deuterium which allows the possibility of changing the contrast without too serious alteration in the conformation and thermodynamics of the systems.

In order to take full advantage of this possibility, one has to remember the general rules which allow the evaluation of the total scattering function S(q) as a function of the partial scattering functions S _pq (q) which describes the contributions of the interferences between scattering center of species p and q.^1.2 In a first part we shall briefly establish these rules for incompressible systems introducing what has been called by J. Koberstein³ the molecular contrast and the phase contrast.

In the second part of this talk we shall discuss the problem of copolymers. S(o) the intensity scattered at zero angle depends on the heterogeneity in composition of the sample,⁴ for homogeneous composition S(o) = o. This gives the possibility of studying trans-esterification in polyester, for example.

At finite q for a monodisperse system, S(q) is not zero and it will be shown how it depends on the length of the sequences and the architecture of the copolymers. If the number of blocks is large S _pq (q) is practically not affected by the total length of the polymer.⁵

The condition I ^?1(q) = o allows one to write the equation of a q dependent generalized spinodal and therefore to determine the parameters which govern the stability of a multiphase system.

In the last part we shall discuss the problem of the application of this formalism to networks and gels made of different types of units. It will be shown that the scattering is mainly sensitive to the local conformation and that the introduction of translational order between the units does not affect significantly the scattering curves but can explain the occurrence of secondary maxima which have been observed in polyurethane and ionomers.⁶     

Depolarized light scattering from swollen-filled rubber

The H_v small-angle light-scattering patterns from cross-linked swollen rubbers containing glass bead filler particles are shown to arise from the birefringence patterns associated with the inhomogeneous swelling in the vicinity of the particles. Theoretically calculated patterns are obtained by using the stress field calculated by Sternstein and are found to agree with the experimental results.     

Small-angle neutron scattering by two-component particles: Application to block copolymers

General equations for small-angle neutron scattering are derived for block copolymers in dilute solution. The effects of excluded volume and polydispersity are examined. The applicability of the method of variable contrast to such systems is discussed.