153 research outputs found
Investigation of macrocyclisation routes to 1,4,7-triazacyclononanes : efficient syntheses from 1,2-ditosylamides
Two routes to the synthesis of a cyclohexyl-fused 1,4,7-triazacyclononane involving macrocyclisations of tosamides have been investigated. In the first approach, using a classic Richman-Atkins-type cyclisation of a cyclohexyl-substituted 1,4,7-tritosamide with ethylene glycol ditosylate, afforded the cyclohexyl-fused 1,4,7-triazacyclononane in 5.86% overall yield in four steps. The second, more concise, approach involving the macrocyclisation of trans-cyclohexane-1,2-ditosamide with the tritosyl derivative of diethanolamine initially gave poor yields (< 25%). The well-documented problems with efficiencies in macrocyclisations using 1,2-ditosamides led to the use of a wider range of 1,2-ditosamides including ethane-1,2-ditosamide and propane-1,2-ditosamide. These extended studies led to the development of an efficient macrocyclisation protocol using lithium hydride. This new method afforded 1,4,7-tritosyl-1,4,7-triazacyclononanes in good yield (57-90%) from 1,2-ditosamides in a single step. These efficient methods were then applied to the preparation of a chiral cyclohexyl-fused 1,4,7-tritosyl-1,4,7-triazacyclononane (65-70%). This key chiral intermediate was then converted into a copper(II) complex following detosylation and N-methylation. The resulting chiral copper(II) complex catalysed the aziridination of styrene but it did so in a racemic fashion
Phase Separation of Rigid-Rod Suspensions in Shear Flow
We analyze the behavior of a suspension of rigid rod-like particles in shear
flow using a modified version of the Doi model, and construct diagrams for
phase coexistence under conditions of constant imposed stress and constant
imposed strain rate, among paranematic, flow-aligning nematic, and log-rolling
nematic states. We calculate the effective constitutive relations that would be
measured through the regime of phase separation into shear bands. We calculate
phase coexistence by examining the stability of interfacial steady states and
find a wide range of possible ``phase'' behaviors.Comment: 23 pages 19 figures, revised version to be published in Physical
Review
Interdisciplinary and multi-institutional higher learning : reflecting on a South African case study investigating complex and dynamic environmental challenges
Complex social-ecological problems need sustained
interdisciplinary engagements across multiple disciplines, yet
academic offerings continue to reflect disciplinary silos. To
address this, a five-year program, within a developing country
context, was conceived to follow an interdisciplinary research
mode using a team of students and supervisors from various
institutions across the disciplines of ecology, hydrology and
economics. By using a flexible student training model, regional/
site specific knowledge was developed while simultaneously
developing a shared vision and a model to combine information
from each student project. Graduates felt enabled by the
program that actively encouraged interdisciplinary interactions
and engagements while simultaneously furthering disciplinary
development. Cross disciplinary communication, was
achieved through multiple engagement opportunities and
common research outputs, all facilitated by an external
boundary organization. While lengthy time frames are required
for such collaborative interdisciplinary programs, researchers,
higher learning institutions and funding agencies should not
avoid this type of program and investment.The Water Research Commission of South
Africahttp://www.journals.elsevier.com/current-opinion-in-environmental-sustainability2017-04-30hb2016Economic
Percolating granular superconductors
We investigate diamagnetic fluctuations in percolating granular
superconductors. Granular superconductors are known to have a rich phase
diagram including normal, superconducting and spin glass phases. Focusing on
the normal-superconducting and the normal-spin glass transition at low
temperatures, we study he diamagnetic susceptibility and the mean
square fluctuations of the total magnetic moment of large
clusters. Our work is based on a random Josephson network model that we analyze
with the powerful methods of renormalized field theory. We investigate the
structural properties of the Feynman diagrams contributing to the
renormalization of and . This allows us to determine
the critical behavior of and to arbitrary order in
perturbation theory.Comment: 18 pages, 2 figure
Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET
The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR
Relationship of edge localized mode burst times with divertor flux loop signal phase in JET
A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM
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