806 research outputs found
Determination of an organic crystal structure with the aid of topochemical and related considerations: correlation of the molecular and crystal structures of α-benzylidene-γ-butyrolactone and 2- benzylidenecyclopentanone with their solid state photoreactivity
The crystal structure of α-benzylidene-γ-butyrolactone 2, can be determined with the aid of atom-atom pairwise energy evaluation procedures, because its (previously reported) solid state photoreactivity coupled with topochemical principles, greatly restricts the number of possible orientations of the molecule in the unit cell. Crystals of lactone 2 are monoclinic with space group P2/n and with Z = 4, a = 11.014(2), b = 5.959(1), c = 14.286(5), β = 108.05(2). Refinement on 846 non-zero reflections led to an R (reliability) of 0.046. In contrast, the isoelectronic ketone 2-benzylidenecyclopentanone (3) is photostable, and crystallizes in the same space group with Z = 4, a = 7.466(4), b = 6.821(4), c = 19.005(1), β = 94.14(1). The structure of 3 was solved by direct methods and refined on 1037 non-zero reflections to an R of 0.036. The difference between the two structures can be rationalized in terms of intramolecular conformation and weak C-HO hydrogen bonding. Differences in the solid state photoreactivities of the two compounds can be related to the extent of orbital overlap between 'potentially reactive' double bonds on nearest neighbour molecules that are related by inversion. Compound 2 reacts in the solid state topochemically but not topotactically showing directional preference, while 3, which has reduced orbital overlap, is photostable
Analyses of shocked quartz at the global K-P boundary indicate an origin from a single, high-angle, oblique impact at Chicxulub
Accepted versio
Load spreading in ultra‑thin high-strength steel-fibre-reinforced concrete pavements
Ultra-Thin Continuously Reinforced Concrete Pavement (UTCRCP) consists of a 50 mm thin High-
Strength Steel-Fibre-Reinforced Concrete (HS-SFRC) overlay placed on existing pavements as
rehabilitation or used as part of new pavements. Difficulties have been experienced with the
construction of UTCRCP. Additionally, the thin HS-SFRC has superior fatigue properties, but
poor load-spreading ability compared to conventional concrete pavements due to its reduced
thickness. This results in high deflections when the pavement is loaded. The substructure
of UTCRCP plays an important role in its performance. Cement-stabilised granular materials
can be used to ensure gradual load spreading with depth, but its behaviour under flexible
concrete layers is not yet well understood. In this study the effect of increasing the HS-SFRC
layer thickness and the effect of incorporating cement-stabilised base layers were investigated
using linear elastic finite element modelling. From stress levels calculated, it was found that C1
and C2 materials perform well underneath a 50 mm HS-SFRC layer subjected to standard axle
loads of 80 kN, while C3 and C4 would deteriorate faster. Stabilised layers placed below a thin,
flexible concrete layer may however crack, resulting in increased damage to supporting layers. It
is recommended that the response of UTCRCP should be investigated using advanced material
models for the cement-stabilised base and other substructure layers.http://www.journals.co.za/ej/ejour_civileng.htmlam2022Civil Engineerin
Heterogeneity in lunar anorthosite meteorites: Implications for the lunar magma ocean model
The lunar magma ocean model is a well-established theory of the early evolution of the Moon. By this model, the Moon was initially largely molten and the anorthositic crust that now covers much of the lunar surface directly crystallized from this enormous magma source. We are undertaking a study of the geochemical characteristics of anorthosites from lunar meteorites to test this model. Rare earth and other element abundances have been measured in situ in relict anorthosite clasts from two feldspathic lunar meteorites: Dhofar 908 and Dhofar 081. The rare earth elements were present in abundances of approximately 0.1 to approximately 10× chondritic (CI) abundance. Every plagioclase exhibited a positive Eu-anomaly, with Eu abundances of up to approximately 20×CI. Calculations of the melt in equilibrium with anorthite show that it apparently crystallized from a magma that was unfractionated with respect to rare earth elements and ranged in abundance from 8 to 80×CI. Comparisons of our data with other lunar meteorites and Apollo samples suggest that there is notable heterogeneity in the trace element abundances of lunar anorthosites, suggesting these samples did not all crystallize from a common magma source. Compositional and isotopic data from other authors also suggest that lunar anorthosites are chemically heterogeneous and have a wide range of ages. These observations may support other models of crust formation on the Moon or suggest that there are complexities in the lunar magma ocean scenario to allow for multiple generations of anorthosite formation
Minimal Assumptions for Optimal Serology Classification: Theory and Implications for Multidimensional Settings and Impure Training Data
Minimizing error in prevalence estimates and diagnostic classifiers remains a
challenging task in serology. In theory, these problems can be reduced to
modeling class-conditional probability densities (PDFs) of measurement
outcomes, which control all downstream analyses. However, this task quickly
succumbs to the curse of dimensionality, even for assay outputs with only a few
dimensions (e.g. target antigens). To address this problem, we propose a
technique that uses empirical training data to classify samples and estimate
prevalence in arbitrary dimension without direct access to the conditional
PDFs. We motivate this method via a lemma that relates relative conditional
probabilities to minimum-error classification boundaries. This leads us to
formulate an optimization problem that: (i) embeds the data in a parameterized,
curved space; (ii) classifies samples based on their position relative to a
coordinate axis; and (iii) subsequently optimizes the space by minimizing the
empirical classification error of pure training data, for which the classes are
known. Interestingly, the solution to this problem requires use of a
homotopy-type method to stabilize the optimization. We then extend the analysis
to the case of impure training data, for which the classes are unknown. We find
that two impure datasets suffice for both prevalence estimation and
classification, provided they satisfy a linear independence property. Lastly,
we discuss how our analysis unifies discriminative and generative learning
techniques in a common framework based on ideas from set and measure theory.
Throughout, we validate our methods in the context of synthetic data and a
research-use SARS-CoV-2 enzyme-linked immunosorbent (ELISA) assay
Effect of paste content on the properties of high-strength concrete pavements
Ultra-thin continuously reinforced concrete pavement (UTCRCP) is an innovative pavement type
that has the potential to fulfil South Africa’s pavement repair strategy requirements. Premature
failure in UTCRCP is linked to the formation of cracks that allow water ingress into supporting
layers. Environmental conditions, as well as concrete properties, determine the concrete
cracking tendency. Concrete properties are a function of the mixture proportions, and unlike
environmental conditions, mixture proportions are controllable. The effect of mixture proportions
on the properties of high-strength concrete (HSC) used in UTCRCP should be investigated.
The objective of this research was to study the influence of paste content on the properties
of HSC used in UTCRCP. Two sets of concrete were tested. The paste content of the first
set was varied from 23% to 37% by mass, using multivariable analysis in conjunction with
superplasticiser (SP) dosage. The paste content of the second set was varied from 25% to 60%
by mass, only varying SP dosage to control the workability.
The multivariable analysis revealed that, within the parameter range tested, paste content
influenced early-age properties, but not long-term properties. Through variation of the paste
content over a wider range during the second set it was found that paste content does influence
both the early-age and long-term properties of HSC. From the results it could be seen that
increasing the paste content of HSC generally has a detrimental effect. The paste content of HSC
used in UTCRCP should be minimised, while maintaining a reasonable workability.http://www.journals.co.za/ej/ejour_civileng.htmlam201
Making sense of a diagnosis of incurable cancer: The importance of communication
Purpose: Patients diagnosed with incurable cancer may experience existential distressand difficulty in re-appraising their lives because of uncertainty about the future. Objectives: This study sought to understand how patients living with incurable cancer made sense of their diagnosis, how they prepared for the future and what support they wanted from their health professionals.  Subjects:  27 patients were recruited from the oncology and palliative care service at three metropolitan hospitals. Methods: A qualitative research approach was used. Semi-structured face-to-face interviews were conducted. Interviews were audio-taped and transcribed verbatim. Data was analyzed using the constant-comparative method.  Results: Participants did not express a need to make sense of their diagnosis nor always ascribe to a particular religious belief; rather, many relied on a personal spirituality or philosophy to bring meaning to their experience. Importance was placed on their doctor keeping up with technology, being honest, and being confident and positive. Conclusion: Participants in this study had incurable cancer but making sense of their current situation was not a conscious priority. For these patients, uncertainty was a positive, as certainty for them indicates death is approaching. What these interviews suggest, from the patient’s perspective, is that there is an implied contract between doctor and patient during this period which involves the doctor managing the flow of difficult information so that the patient can maintain normality for as long as possible. Understanding this helps to explain the difficulty of having advance care planning conversations within this setting, despite the many opportunities that a longer disease trajectory would seem to offer.
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SEM-EDS analyses of small craters in stardust aluminium foils: implications for the Wild-2 dust distribution
Implications for the Wild-2 dust distribution of the statistical results obtained by SEM-EDS from nearly 300 impact craters on aluminium foils of the Stardust sample tray assembly
Rapid Quantification of Molecular Diversity for Selective Database Acquisition
There is an increasing need to expand the structural diversity of the molecules investigated in lead-discovery programs. One way in which this can be achieved is by acquiring external datasets that will enhance an existing database. This paper describes a rapid procedure for the selection of external datasets using a measure of structural diversity that is calculated from sums of pairwise intermolecular structural similarities
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