Experimental calibration and field investigation of the oxygen isotopic fractionation between biogenic aragonite and water

Marine molluscs have long been recognised as potential records of palaeoclimate change using the patterns and differences in the stable isotopic composition of the carbonate shells. The aim of this study is to improve the robustness of this approach for aragonitic molluscs by completing the first experimental calibration of the fractionation between water and biogenic aragonite. Fractionation factors were calibrated by growing specimens of the freshwater mollusc Lymnaea peregra under controlled conditions of water temperature and isotopic composition. Fifteen populations of L. peregra were maintained at constant temperature and isotopic conditions for five months (at five different temperatures and using three different water compositions). Water samples and temperature measurements were taken regularly throughout the experiment. The temperature dependence of the fractionation factor, between 8 and 24 degrees C, is given by: 1000 ln alpha=16.74x(1000T(-1))-26.39 (T in Kelvin) and the relationship between temperature (T), delta(18)O(carb) and delta(18)O(wat) is given by: T=21.36-4.83xdelta(+ degrees )O(carb)-delta(+ degrees )O(wat) (T is in degrees C, delta(18)O(carb) is with respect to Vienna Pee Dee Belemnite (PDB), the International Atomic Energy Agency (IAEA) replacement standard for PDB, and delta(18)O(wat) is with respect to Vienna standard mean ocean water (VSMOW)) The outcome of the controlled experiment is compared with previous studies on synthetic, and biogenic, calcite and aragonite from field and laboratory investigations. These comparisons suggest that although a vital offset exists between the fractionation of isotopes in synthetic and biogenic aragonite for molluscs in general, there is no vital effect that is specific either to freshwater, or to individual, genera. Therefore, the calibrated relationship may be used for any freshwater or marine mollusc to derive palaeotemperatures providing the isotopic composition of the environmental water can be reliably constrained. Copyright 1999 John Wiley & Sons, Ltd.     

Experimental and theoretical study of 2,6-difluorophenylnitrene, its radical cation, and their rearrangement products in argon matrices.

2,6-Difluorophenylnitrene was reinvestigated both experimentally, in Ar matrices at 10 K, and computationally, by DFT and CASSCF/CASPT2 calculations. Almost-pure samples of both neutral rearrangement products (the bicyclic azirine and the cyclic ketenimine) of a phenylnitrene were prepared and characterized for the first time. These samples were then subjected to X-irradiation in the presence of CH2Cl2 as an electron scavenger, which led to ionization of the neutral intermediates. Thereby, it was shown that only the phenylnitrene and the cyclic ketenimine yield stable radical cations, whereas the bicyclic azirine decays to both of these compounds on ionization. The cyclic ketenimine yields a novel aromatic azatropylium-type radical cation. The electronic structure of the title compound is discussed in detail, and its relation to those of the iso-pi-electronic benzyl radical and phenylcarbene is traced.     

Band structure and atomic sum rules for X-ray dichroism

Corrections to the atomic orbital sum rule for circular magnetic X-ray dichroism in solids are derived using orthonormal LMTOs as a single-particle basis for electron band states. Received 4 September 2000     

Sensitivity Analysis on Indium Phosphide Liquid Encapsulated Czochralski Growth

The optimization of the InP liquid encapsulated Czochralski systems is usually difficult, time consuming and very expensive. Here, the relative importance of the different growth parameters (e.g., pull rate, system temperatures and geometry) on the growth interface deflection, the temperature gradients within the melt and the crystal and the dislocation density has been investigated through a sensitivity analysis. The sensitivity coefficients have been calculated by means of a mathematical model, previously validated, based on the thermoelastic theory for the dislocation formation and on the thermal capillary theory for the temperature field within the system. The crucible temperature profile has been selected as the more important parameter to control the crystal quality during the growth.     

Vibrational spectroscopy of structural defects in oligothiophenes

Vibrational spectra of oligothiophenes with structural defects are calculated within the density-functional-theory methodology. The effects of the defective αβ linkages on the infrared (IR) and Raman spectra are characterized from calculations of all isomers up to the hexamer. The signatures of αβ linked monomers can be found in IR spectra from broken symmetry arguments which result in absorptions localized in the defective region. The positions of the absorption peaks in the Raman spectra seem to be unaffected by the presence of such defects; however, strong reductions in the intensities are observed because of the shortening of the conjugation length.     

Germyl mesolytic dissociations in the allylgermane and penta- 2,4-dienylgermane radical anions. A theoretical study

In two stable structures have a trigonal bipyramidal arrangement around Ge, with the extra electron in equatorial (tbp eq) or axial (tbp ax) position. In only tbp ax is found, while a second structure with a tetrahedral germyl group has the extra electron on the conjugated π system. C−Ge bond cleavage yields allyl/ pentadienyl radicals plus germide. Both dissociation reactions require 4–6 kcal mol⁻¹, less than the analogous C and Si systems (ca. 30 and 14 kcal mol⁻¹, respectively). Fragmentation is dramatically activated with respect to homolysis in the corresponding neutrals. The wavefunction is dominated by one single configuration at all distances, in contrast to homolytic cleavage, in which two configurations are important. C−Ge bond dissociation is at variance also with heterolysis, due to spin recoupling of one of the C−Ge bond electrons with the originally unpaired electron. Contribution to the Fernando Bernardi Memorial Issue.     

Effect of hexafluorobenzene on the photophysics of pyrene

Absorption and fluorescence spectroscopy studies reveal the formation of a weak complex between pyrene and C(6)F(6) even in very dilute systems. The complex affects the photophysics of pyrene and reveals a combination of static and dynamic-quenching phenomena in both polar and nonpolar solvents. The results are supported by computational studies that shed light on the structure of the complex and the interactions involved and suggest that ground and excited-state interactions are of comparable magnitude; the association is believed to be driven by quadrupolar interactions. Understanding these interactions in solution is important for applications that aim at controlling the regio- or stereoselectivity of organic reactions.     

Estimation of tensile force in tie-rods using a frequency-based identification method

A technique is developed to identify in-situ the tensile force in tie-rods which are used in ancient monumental masonry buildings to eliminate the lateral load exercised by the vaults and arcs. The technique is based on a frequency-based identification method that allows to minimize the measurement error and that is of simple execution. In particular, the first natural frequencies of the tie-rods are experimentally identified by measuring the frequency response functions (FRFs) with instrumented hammer excitation; four to six natural frequencies can be easily identified with a simple test. Then, a numerical model, based on the Rayleigh-Ritz method, is developed for the axially loaded tie-rod by using the Timoshenko beam theory retaining shear deformation and rotary inertia. Non-uniform section of the rod is considered since this is often the case for hand-made tie-rods in old buildings. The part of the tie-rod inserted inside the masonry wall is also modeled and a simple support is assumed at the extremities inside the walls. The constraints given to the part of the tie-rod inserted inside the masonry structure are assumed to be elastic foundations. The tensile force and the stiffness of the foundation are the unknowns. In some cases, the length of the rod inside the masonry wall can be also assumed as unknown. The numerical model is used to calculate the natural frequencies for a given set of unknowns. Then, a weighted difference between the calculated and identified natural frequencies is calculated and this difference is minimized in order to identify the unknowns, and in particular the tensile force. An estimation of the error in the identification of the force is given. The technique has been tested on five tie-rods at the ground floor of the famous castle of Fontanellato, Italy.