Computational Mechanistic and Stereochemical Studies of Single-Site Polymerisation Catalysts and Reactions

Computational investigations into four metal-centred polymerisation catalysts are presented. The work investigates how and why the catalysts behave as they do, focusing on specific interactions within the catalyst structure itself and also on the transition states involved in the polymerisation reactions. Density functional theory has been used to examine the effect of the metal, the role of the ligand and the interplay between the two. Each study addresses particular mechanistic and structural questions that have been raised during experimental investigations and that are difficult to answer experimentally. Chapter one provides a general overview of computational techniques used in chemical modelling. The specific methods used in this work are presented as well as a brief review of modern trends. Chapter two investigates an unusual pair of metal-hydrogen interactions in a tin bis(triazenide) complex. We have termed this double M-H interaction “bifurcated” and compared other systems in which this interaction is present (and often unidentified). A variety of computational techniques are used to analyse the nature of the interactions both in qualitative and quantitative terms. The third chapter investigates the mechanism of alkyl transfer in a magnesium bis(imino)pyridine complex. A number of mechanistic pathways are explored to explain the original report of non-electrophilic alkylation at a pyridine nitrogen. We consider in particular how the solvent and the role of other species in the reaction mixture may influence the mechanism. Chapter four describes the inversion of configuration occurring in a pseudo-C3- symmetric zirconium tris(phenolate) complex. Variable temperature NMR spectra and simulations complement DFT calculations to explore the mechanism of inversion. We question the long-held assumption that the inversion process is concerted. In chapter five, the polymerisation of rac-lactide by an aluminium salen-type system is investigated in detail through characterisation of the transition state structures. Specifically, we have aimed to explain the different behaviour of two structurally similar catalytic species’ which produce polymer of different tacticity. Application of a variety of additional computational techniques in a number of these studies supplements the density functional calculations. They provide insight into specific interactions in both starting materials and transition states and detailed information about the reaction mechanisms

Specimens as research objects: reconciliation across distributed repositories to enable metadata propagation

Botanical specimens are shared as long-term consultable research objects in a global network of specimen repositories. Multiple specimens are generated from a shared field collection event; generated specimens are then managed individually in separate repositories and independently augmented with research and management metadata which could be propagated to their duplicate peers. Establishing a data-derived network for metadata propagation will enable the reconciliation of closely related specimens which are currently dispersed, unconnected and managed independently. Following a data mining exercise applied to an aggregated dataset of 19,827,998 specimen records from 292 separate specimen repositories, 36% or 7,102,710 specimens are assessed to participate in duplication relationships, allowing the propagation of metadata among the participants in these relationships, totalling: 93,044 type citations, 1,121,865 georeferences, 1,097,168 images and 2,191,179 scientific name determinations. The results enable the creation of networks to identify which repositories could work in collaboration. Some classes of annotation (particularly those regarding scientific name determinations) represent units of scientific work: appropriate management of this data would allow the accumulation of scholarly credit to individual researchers: potential further work in this area is discussed.Comment: 9 pages, 1 table, 3 figure

Modelling the effects of space weather at the Earth’s surface : a UK geoelectric field model

Geomagnetically Induced Currents (GIC), which can flow in technological systems such as power transmission grids, are a consequence of the geoelectric field induced at the surface of the Earth during geomagnetic storms. This poster describes the development of a new 3D 'Thin-Sheet' geoelectric field model which covers the whole of the UK and includes the influence of the surrounding shelf seas. The model can be used to compute the response of the geoelectric field to geomagnetic storms. In conjunction with a power grid model this enables us to estimate GIC flow in power networks. As an example, we consider the major geomagnetic storm of October 2003. It is envisaged that the model will form one component of a near real time GIC warning package which is currently being developed by the British Geological Survey (BGS) in conjunction with Scottish Power Plc. The magnetic field associated with the induced geoelectric field is easily calculated. Thus, the electric field model may also be of interest to those studying the effect of internal (induced) geomagnetic field signals on the total measured geomagnetic field

Disturbance and the Community Composition of Arbuscular Mycorrhizal Fungi in Ontario Tallgrass Prairies

Arbuscular mycorrhizal fungi (Glomeromycota) form beneficial symbiotic relationships with the majority of land plants, especially in grasslands. Tallgrass prairies in Ontario are an endangered plant community currently being restored on former agricultural land. The objectives of my study were to determine if there were differences in the communities of Glomeromycota between disturbed and undisturbed tallgrass prairies in Ontario and if there were any potential indicator taxa for the ends of the disturbance spectrum. A molecular approach using DNA derived from soil samples was used to compare species composition between disturbed and undisturbed tallgrass prairies. A total of 177 operational taxonomic units (OTUs) in nine genera of Glomeromycota were retrieved. Analyses showed a clear pattern of disturbed and undisturbed prairies clustering separately based on species composition of arbuscular mycorrhizal fungi, and distinguished OTUs that were indicators of disturbed (Claroideoglomus and select Glomus spp.) or undisturbed (Ambispora, Diversispora, and Glomus spp.) sites

Linking genetics with biology in disease research: an interview with Nick Hastie

Professor Nick Hastie is Director of the MRC Human Genetics Unit in Edinburgh, a centre originally famous for early studies of chromosome biology. He is also Director of the newly formed Institute of Genetics and Molecular Medicine, which includes the Human Genetics Unit. In addition to overseeing the work on cancer and developmental genetics in his own lab, he is involved in a number of large-scale genetic studies aimed at uncovering genetic risk factors for various human diseases

Committing to Class-Size Reduction and Fining the Resources to Implement It: A Case Study of Resource Reallocation

This article discusses how a medium-sized school district in Wisconsin was able to reallocate resources to reduce class sizes in grades K-5 without spending more money or increasing its tax rate. Previous research on resource reallocation found that the bulk of reallocated resources were those supporting categorical program services. This district was able to use a different strategy. As a growing district, its marginal costs of adding an extra class of students were much less than its average expenditures per pupil, which were reimbursed by the state school finance formula. As the district grew, therefore, it acquired excess revenues. Also, by implementing full-day kindergarten, the district acquired more excess revenues because this added (0.5 pupil) X (Number of kindergartners) to its current enrollment, and the cost of educating these students was less than the amount they received from the state funding formula. It then used these revenues to reduce class sizes to between 15 and 20 in all Kindergarten through grade 3 classrooms and to between 15 and 22 for grades 4-5

Comparing Agriculture‐Related Characteristics of Flash and Normal Drought Reveals Heterogeneous Crop Response

Despite rapid progress in the burgeoning field of flash drought research, few studies directly compare the differences in characteristics between flash drought (commonly understood as quick, rapid-onset drought) and drought traditionally defined as slow-moving (henceforth normal drought), particularly over agricultural regions where drought effects may be economically the most disastrous. In this study, flash and normal drought events are identified using reanalysis of soil moisture in the data-rich agricultural region of the California Central Valley for investigation of characteristics related to agriculture. In particular, we investigate the relative duration of pixels in drought events, the correlation of drought intensity with vegetation condition, the impact of aridity on vegetation response and drought, and the differences in the different characteristics between rainfed and irrigated agriculture. Overall, we found considerable differences between flash and normal drought, particularly in their spatial distributions and behavior in relation to aridity. Flash droughts even indicate a counterintuitive improvement in vegetation condition in the northern, more humid regions, likely due to the release of growth limiting factors (e.g., below-optimum temperature and radiation) associated with drought. Results also indicate improvements in vegetation conditions during normal drought for irrigated land over rainfed, highlighting the importance of irrigation as a drought protection strategy in agriculture

The evolution of inverted magnetic fields through the inner heliosphere

Local inversions are often observed in the heliospheric magnetic field (HMF), but their origins and evolution are not yet fully understood.Parker Solar Probe has recently observed rapid, Alfvénic, HMF inversions in the inner heliosphere, known as ‘switchbacks’, which have been interpreted as the possible remnants of coronal jets. It has also been suggested that inverted HMF may be produced by near-Sun interchange reconnection; a key process in mechanisms proposed for slow solar wind release. These cases suggest that the source of inverted HMF is near the Sun, and it follows that these inversions would gradually decay and straighten as they propagate out through the heliosphere. Alternatively, HMF inversions could form during solar wind transit, through phenomena such velocity shears, draping over ejecta, or waves and turbulence. Such processes are expected to lead to a qualitatively radial evolution of inverted HMF structures. Using Helios measurements spanning 0.3–1 AU, we examine the occurrence rate of inverted HMF, as well as other magnetic field morphologies, as a function of radial distance r, and find that it continually increases. This trend may be explained by inverted HMF observed between 0.3–1 AU being primarily driven by one or more of the above in-transit processes, rather than created at the Sun. We make suggestions as to the relative importance of these different processes based on the evolution of the magnetic field properties associated with inverted HMF. We also explore alternative explanations outside of our suggested driving processes which may lead to the observed trend

Modeling potential population-level impacts of localized oil spills on Puget Sound Pacific herring stocks

Localized oil spills that contaminate nearshore spawning areas pose a threat to developing Pacific herring embryos. Fish embryos are particularly susceptible to the developmental toxicity of oil derived PAHs, which can be acutely lethal or, at lower exposure doses, lead to permanent changes in heart structure, craniofacial morphology, and metabolic processes that cause delayed mortality, or more subtle delayed impacts that may be associated with premature subadult mortality. A variety of localized oil spill scenarios simulating direct and delayed mortality to young of the year were used to examine the potential response of Puget Sound stocks. The healthy stocks remaining in Puget Sound could withstand short term impacts, but the growing number of depressed and unhealthy stocks are vulnerable to an increased risk of localized extinction. Model output predicted that for short term, low level impacts the stock abundance did not exceed the natural variability observed in the population demographic data. This indicates a limited ability to observe in the field any predictions made by the model. High mortality in a single year or impacts across multiple year classes may cause stock abundance changes that exceed natural variability. Despite this, the model does indicate the magnitude of impact on the intrinsic growth rate that could reduce productivity of affected stocks. Additional toxic endpoints and effects thresholds are currently being investigated, including sublethal impacts on cardiac function, immune function, and lipid metabolism that may all lead to delayed mortality. Characterizing these adverse outcome pathways may alter predicted impacts of oil spills on herring stocks