274 research outputs found
Towards a fuller understanding of selected molecular compounds
No full textThis work is separated into two distinct sections. The first section deals with the analysis by variable temperature single crystal X-ray diffraction of crystalline molecular materials with novel physical properties. The second section details the electron density analysis of selected crystalline compounds using high resolution, low temperature single crystal X-ray diffraction. Chapter 1 provides introduction to X-ray diffraction outlining the theory this work is based on. Chapter 2 introduces the molecular material studies based on materials containing the bis(ethylenedithio)-tetrathiaftilvalene molecule. The aim of these studies was to achieve a greater understanding of the materials and the changes they undergo when exposed to variations in temperature whilst in the crystalline state. These structural changes observed are very minor but have dramatic effects on the electrical properties of the materials studied. It became apparent that the only way to form a full understanding of these materials was to be able to 'see' the electronic configuration of the materials, not just their gross structure. The field of electron density analysis by X- ray diffraction is now a well documented area. Chapter 4 provides an introduction to the field of electron density analysis. Diffraction experiments were conducted on systems of different complexity to investigate this field and these results are reported in Chapter 5. No electron density analyses have been conducted on the molecular materials studied in Chapter 3 as yet, due to the lack of crystals of suitable quality. It was also noted that to fully categorise the nature of the physical changes occurring in these structures that a diffractometer capable of analysing these samples throughout the temperature range of their physical phenomena was needed. An outline for the development of such a diffractometer is detailed in Chapter 6, 'future works'
The structure of a pentachromium(II) extended metal atom chain at 3 K: Cotton's conjecture proven
No full textWe provide definitive experimental proof that the archetypal string-like compound [Cr5(tpda)4(NCS)2] has alternating long and short Cr–Cr separations in the solid state, as conjectured by F. A. Cotton, rather than essentially equally spaced Cr atoms, as initially claimed (H2tpda = N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine). Single-crystal X-ray data collected from 292 to 3 K revealed that the misinterpretation is caused by pseudo-merohedral twinning and that bond length alternation is enhanced at low temperature
Minimizing polymorphic risk through cooperative computational and experimental exploration
No full textWe combine state-of-the-art computational crystal structure prediction (CSP) techniques with a wide range of experimental crystallization methods to understand and explore crystal structure in pharmaceuticals and minimize the risk of unanticipated late-appearing polymorphs. Initially, we demonstrate the power of CSP to rationalize the difficulty in obtaining polymorphs of the well-known pharmaceutical isoniazid and show that CSP provides the structure of the recently obtained, but unsolved, Form III of this drug despite there being only a single resolved form for almost 70 years. More dramatically, our blind CSP study predicts a significant risk of polymorphism for the related iproniazid. Employing a wide variety of experimental techniques, including high-pressure experiments, we experimentally obtained the first three known non-solvated crystal forms of iproniazid, all of which were successfully predicted in the CSP procedure. We demonstrate the power of CSP methods and free energy calculations to rationalize the observed elusiveness of the third form of iproniazid, the success of high-pressure experiments in obtaining it, and the ability of our synergistic computational-experimental approach to “de-risk” solid form landscapes
Dynamic dimer copper coordination redox shuttles
No full textA new generation of hybrid solar cells using low-cost, Earth-abundant materials can transform the energy sector. The majority of progress toward this goal has come from improving light-absorbing materials. Charge transfer issues remain a barrier to widespread adoption of this solar technology, and this is the challenge that this article addresses. We developed copper complexes using tetradentate ligands, which form tetrahedral dimeric structures with Cu(I) and planar monomeric structures with Cu(II). Despite the fact that their structures are significantly diverse, their reorganization energy is remarkably low. The Cu(I) dimer’s 2-electron oxidation process yields two Cu(II) monomers, thus stabilizing both oxidation states. Because Cu(I) monomers are highly energetically unfavored, recombination processes in dye-sensitized solar cells become largely inhibited. The dynamic dimer system represents a new generation of efficient redox mediators for molecular devices
Environmentally induced transgenerational changes in seed longevity: Maternal and genetic influence
No full textBackground and Aims: Seed longevity, a fundamental plant trait for ex situ conservation and persistence in the soil of many species, varies across populations and generations that experience different climates. This study investigates the extent to which differences in seed longevity are due to genetic differences and/or modified by adaptive responses to environmental changes. Methods: Seeds of two wild populations of Silene vulgaris from alpine (wA) and lowland (wL) locations and seeds originating from their cultivation in a lowland common garden for two generations (cA1, cL1, cA2 and cL2) were exposed to controlled ageing at 45 °C, 60 % relative humidity and regularly sampled for germination and relative mRNA quantification (SvHSP17.4 and SvNRPD12). Key Results: The parental plant growth environment affected the longevity of seeds with high plasticity. Seeds of wL were significantly longer lived than those of wA. However, when alpine plants were grown in the common garden, longevity doubled for the first generation of seeds produced (cA1). Conversely, longevity was similar in all lowland seed lots and did not increase in the second generation of seeds produced from alpine plants grown in the common garden (cA2). Analysis of parental effects on mRNA seed provisioning indicated that the accumulation of gene transcripts involved in tolerance to heat stress was highest in wL, cL1 and cL2, followed by cA1, cA2 and wA. Conclusions: Seed longevity has a genetic basis, but may show strong adaptive responses, which are associated with differential accumulation of mRNA via parental effects. Adaptive adjustments of seed longevity due to transgenerational plasticity may play a fundamental role in the survival and persistence of the species in the face of future environmental challenges. The results suggest that regeneration location may have important implications for the conservation of alpine plants held in seed banks. © 2014 The Author 2014. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: [email protected]
Conductivity in Thin Films of Transition Metal Coordination Complexes
No full textTwo coordination complexes have been made by combining
the dithiolene
complexes [M(mnt)2]2– (mnt = maleonitriledithiolate;
M = Ni2+ or Cu2+) as anion, with the copper(II)
coordination complex [Cu(Stetra)] (Stetra = 6,6′-bis(4,5-dihydrothiazol-2-yl)-2,2′-bipyri-dine)
as cation. The variation of the metal centers leads to a dramatic
change in the conductivity of the materials, with the M = Cu2+ variant (Cu–Cu) displaying semiconductor behavior with a
conductivity of approximately 2.5 × 10–8 S
cm–1, while the M = Ni2+ variant (Ni–Cu)
displayed no observable conductivity. Computational studies found
Cu–Cu enables a minimization of reorganization energy losses
and, as a result, a lower barrier to the charge transfer process,
resulting in the reported higher conductivity
Extending Highthroughput Technologies: The Automation of Mechanism Discovery Investigations into the mode and action of the thio-Michael Reaction
No full textThis project concerns the investigation of the thio-Michael reaction (see scheme). The approach has employed a process development methodology to chemical discovery rather than more traditional research methods. Stage One involved the investigation of rates of addition with and without different catalysts to develop an understanding of the thio-Michael system.
Further studies with asymmetric catalysts were carried out. Stage Two involved the use of adiabatic calorimetry and advanced reaction modelling to provide a complete understanding of the thio-Michael reaction under study. From this, the mechanism we propose is a new, self-accelerating process, in which the product may catalyse collapse of one observable intermediate to the product. The exact mode of this autocatalytic step is not clear at present, but is the subject of ongoing studies as is the generality of this new mechanistic insight into the Michael addition reaction
Application of Zinc(II)-Binol for the Formal Aza-Diels-Alder Reaction of N-Arylimines with Danishefsky's Diene: CD-Based Absolute Stereochemistry Determination, Origin of Asymmetric Induction and Mechanistic Considerations
No full textZinc(II)–binol has been employed as an efficient Lewis acid catalyst (10 or 100 mol-% loading) for the formal aza-Diels–Alder reaction of ester, furyl and dimethyl acetal-substituted N-aryl imines. Asymmetric induction varies from poor to good, with the major enantiomer obtained being (S) when the (S)-binol complex has been employed. The absolute stereochemistry of the dimethyl acetal-substituted cycloaduct was determined by CD, and both efficient aza-Diels–Alder reaction and asymmetric induction are proposed to be dependent upon the formation of bidentate zinc-imine complexes. These complexes have been modelled using semiempirical methods
The ‘pure’ relationship, sham marriages and immigration control
No full textThis is the author pre-print version. The final version is available from Hart Publishing via the link in this record.This chapter investigates the circumstances in which a marriage involving a non- EEA migrant spouse is designated a sham marriage so that residence rights are refused. It analyses the problems of understanding and defining a sham marriage and argues that controls over sham marriages often regulate a much wider range of marriages than those entered for the sole purpose of obtaining residence rights
R v Hall and the changing perceptions of the crime of bigamy
No full textThis is the author accepted manuscript. The final version is available from Cambridge University Press via the DOI in this record.In 1845, the conviction of Thomas Hall for bigamy was reported as an example of the unequal way in which the law operated, with great play being made of the steps that Hall could have taken to free himself from his first wife by a divorce, were it not for the cost involved. Since then, virtually every account of nineteenth-century bigamy or divorce has included some version of the judge's apparently ‘brilliantly sarcastic’ speech. But what the judge was reported as saying at the time differs in a number of crucial particulars from what later commentators have reported him as saying. Later accounts have played up the misconduct of the first wife, inflated the cost of obtaining a divorce, and exaggerated the poverty and lowly status of Hall, while playing down the sentence he received and ignoring his deception of his second wife. This paper traces the evolution of the account over time, and identifies the timing of the various changes that were made. It illustrates how history is used – by politicians, reformers, and scholars – to support both a particular view of the past and to bolster claims as to how the law should change for the future
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