Bimodal association of a bis-1,2,3-dithiazolyl radical

The N-ethyl pyrazine-bridged bis-1,2,3-dithiazolyl radical (R(1) = Et) associates at room temperature as a C-C bonded sigma-dimer which, on heating, converts to a laterally S-S sigma-bonded structure.     

Electronic and magnetic interactions in pi-stacked bisthiadiazinyl radicals

The preparation of two bisthiadiazinyls (7, R1 = Me, Et; R2 = Cl, R3 = Ph), the first examples of a new class of resonance-stabilized heterocyclic thiazyl radical, are reported. Both radicals have been characterized in solution by EPR spectroscopy and cyclic voltammetry, which confirm highly delocalized spin distributions and low electrochemical cell potentials, features which augur well for the use of these materials as building blocks for neutral radical conductors. In the solid state, the radicals are undimerized, crystallizing in slipped pi-stack arrays which ensure the availability of electrons as potential charge carriers. However, despite these favorable electrochemical and structural properties, both materials exhibit low conductivities, with sigma(300K) < 10-7 S cm-1, a result which can be rationalized in terms of their EHT band electronic structures, which indicate that intermolecular interactions lateral to the pi-stacks are limited. The materials are thus very 1-D with low bandwidths, so that a Mott insulating state prevails. When R1 = Me, the intermolecular overlap along the pi-stacks is weak and the material is essentially paramagnetic. When R1 = Et, intermolecular pi-overlap is greater and variable-temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, the behavior of which has been modeled in terms of a molecular-field modified 1-D Heisenberg chain of S = 1/2 centers. Broken-symmetry DFT methods have been used to estimate the magnitude of individual exchange interactions within both structures.     

A dynamic study of earthworm feeding ecology -using stable isotopes

Changes in the specific diet of earthworms with time in relation to landuse changes and two different climates were studied by analysing (13)C and (15)N natural abundance in soils and animals. Soil samples from three depths (0-10, 10-20 and 20-30 cm) and earthworms were collected from two sites: Santiago (Northwest Spain) and North Wyke (Southwest England) both consisting of replicated long-term grasslands and recently converted to maize plots. Earthworms were hand-sorted in the field at the peak of the maize growth and after harvesting at both sites. In the Spanish plots, nine and eight earthworm species, all belonging to the three ecological categories (epigeic, anecic and endogeic), were found under maize and permanent pasture, whereas at the English site five and seven different species were, respectively, identified. At both sites (13)C isotopic values of the earthworm tissues reflected changes in diet from C(3) to C(4) with epigeic and epi/anecic worms in the maize plots showing one delta unit difference in relation to the ones found in the grassland plots. Anecic worms seemed to be less responsive to landuse changes. The higher (13)C values of the Spanish soils were also reflected in the earthworm tissues when compared with the English samples. (15)N values showed no clear relationship with the cropping treatments but were clearly related to the ecological grouping, with endogeic worms reaching the highest values whereas for the epigeic and epi/anecic species the lowest values were obtained. This finding was also previously recorded by other authors1 and suggests that, in the future, stable isotope techniques could also be a useful tool in taxonomic studies. Copyright 1999 John Wiley & Sons, Ltd.     

Secondary ion yields produced by keV atomic and polyatomic ion impacts on a self-assembled monolayer surface

A suite of keV polyatomic or 'cluster' projectiles was used to bombard unoxidized and oxidized self-assembled monolayer surfaces. Negative secondary ion yields, collected at the limit of single ion impacts, were measured and compared for both molecular and fragment ions. In contrast to targets that are orders of magnitude thicker than the penetration range of the primary ions, secondary ion yields from polyatomic projectile impacts on self-assembled monolayers show little to no enhancement when compared with monatomic projectiles at the same velocity. This unusual trend is most likely due to the structural arrangement and bonding characteristics of the monolayer molecules with the Au(111). Copyright 1999 John Wiley & Sons, Ltd.     

Multiple ionization of argon in coincidence with projectile ions in 60–120 MeV Si q+-Ar collisions

A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer processes in collisions of 60–120 MeV Si ^q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization, electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated. The data reported on the present collision system result from a direct measurement in the considered impact energy for the first time. The total ionization cross-sections for the recoil ions are shown to scale as q ^1.7/E _p ^0.5 , where E _p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j ² upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing cross-sections σ _qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow our data for different ionization processes is reviewed and discussed.     

Resonance stabilized bisdiselenazolyls as neutral radical conductors

An efficient and versatile synthetic route to resonance stabilized bisselenathiazolyl and bisdiselenazolyl radicals 3 and 4 is described. Structural analysis of 3 and 4 confirm that lattice and pi-delocalization energies are sufficient to offset solid-state dimerization of the radicals and that the two selenium-containing radicals are isostructural with the all-sulfur based system 1. Variable temperature conductivity measurements indicate that sequential replacement of sulfur by selenium leads to a progressive increase in conductivity and reduction in thermal activation energy.     

New method to measure packing densities of self-assembled thiolipid monolayers

For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.     

SOP and PMD monitoring on deployed aerial optical fibres

State of polarization (SOP) and polarization mode dispersion (PMD) are stochastic in nature due to changes in the properties of the optical fibres and its positions because of both intrinsic and extrinsic perturbations. With 184 PMD values obtained by use of the FTB-5700 single-ended dispersion analyzer, the predicted theoretical Gaussian fit was obtained with a mean of 0.47 ps and standard deviation of 0.08 ps. This small standard deviation was justification for its accuracy in measuring PMD of aerial optical fibres. A comparison is also made on the accuracy of the GINTY and the FTB-5700 on measurement of PMD of aerial optical fibres with results showing that the latter is more accurate compared to the former. The time scale over which to compensate PMD in aerial optical fibres was determined and it is slightly higher than 400 s; the decorrelation time obtained for SOPs on a particular windy and hot day. This is because the changes of the PMD vector are known to be slower than SOP changes.     

Overcoming the boundary effects in surface spline interpolation

Surface spline interpolation when the domain is all of R^d isknown to converge much faster to the data function f than inthe case when the domain is the unit ball. This difference isunderstood to be due to boundary effects which, as will be shown,also affect the size of the surface spline's coefficients. Wepropose a modified form of surface spline interpolation which,to a great extent, overcomes these boundary effects. This modifiedsurface spline interpolant uses only the values of f at thegiven interpolation points.