171,204 research outputs found
Towards a fuller understanding of selected molecular compounds
Get PDFThis 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 zirconocene-mediated synthesis of amines
No full textThe recent appearance of zirconium in organic synthesis is discussed. The formation and elaboration of Group (IV) metallacycles is reviewed, with emphasis on the formation of zirconocene η2-imine complexes. An introduction to the use of silicon as a removable molecular tether, and its oxidative removal to give a hydroxy group is given.The zirconocene(1-butene) cyclisation of 1, n-dienes and enynes is used to prepare a number of different zirconacycles which react with trimethylsilylcyanide to afford zirconocene iminoacyl complexes. A rearrangement to zirconocene η2-imine complexes occurs, and trapping with a range of unsaturated species gives functionalised cyclic amines on work up. The highly functionalised amines are produced in good overall yield from the starting materials in a one-pot transformation. Zirconaindanes were prepared from aryl zirconocenes via formation of zirconocene benzyne complexes through a C-H activation process and trapping with alkenes, and used in the trimethylsilylcyanide insertion reactions. Substituted alkynes insert regiospecifically into the zirconocene η2-imine complexes, giving allylic amines. Alkenes also insert, but show a lower reactivity. Isocyanates insert readily into zirconocene η2-imine complexes to give α-aminoamides on work up, and ketones insert to give 1,2-aminoalcohols.The insertions of isocyanates into zirconocene η2-imine complexes formed using a C-H activation route give a range of α-aminoamides.A range of 1, n-dienes and -enynes containing a silicon in the linking chain were prepared and successfully cyclised using zirconocene(1-butene). Intramolecular trapping os a zirconocene benzyne complex (generated by a C-H activation route) with a silicon tethered alkene was also achieved. Protic work up followed by oxidative cleavage of the silyl group using the Tamao oxidation gives hydroxyl containing products. Further elaboration of the intermediate zirconacycles before protonation/oxidation gave a range of highly functionalised cyclic and acyclic products in good overall yield. This is the first example of the use of a cleavable silicon tether in early transition metal-mediated cyclisations.</p
Langevin dynamics in constant pressure extended systems
No full textThe advantages of performing Langevin dynamics in extended systems are discussed. A simple Langevin dynamics scheme for producing the canonical ensemble is reviewed, and is then extended to the Hoover ensemble. We show that the resulting equations of motion generate the isobaric–isothermal ensemble. The Parrinello–Rahman ensemble is then discussed and we show that despite the presence of intrinsic probability gradients in this system, a Langevin dynamics approach samples the extended phase space in the correct fashion. The implementation of these methods in the ab initio plane wave density functional theory code CASTEP [M. D. Segall, P. L. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clarke, and M. C. Payne, J. Phys.: Condens. Matter 14, 2717 (2003)] is demonstrated
Open questions in ancient Greek phonology: some new evidence from enclitics
No full textRoussou and Probert (2023: 235–297) put forward some new discoveries bearing on the accentuation of sequences of consecutive ancient Greek enclitics, and a new descriptive claim as to the way in which host words plus sequences of consecutive enclitics were accented. This chapter takes some first steps towards a new account of Greek metrical phonology from which the proposed descriptive facts would follow, and argues that doing so helps us with three open questions in ancient Greek phonology: (a) What phonological principles underly the placement of the Greek word accent, including the restrictions known as the “law of limitation”? (b) Did ancient Greek have “resonant diphthongs”? (c) Is the law of limitation sensitive to the weight of the word-final syllable, or the length of its vowel
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
FIGURE 3 V. a in Two new free-living nematode species (Comesomatidae) from the continental slope of New Zealand, with keys and notes on distribution
No full textFIGURE 3 V. a ur a t a n. sp. A. Male posterior body region showing copulatory apparatus B. Female posterior body region showing chords. C. Mid-region of female body showing vulva and genital branches. Scale bar: A, B = 50 Μm; C = 145 Μm.Published as part of D, Leduc, K, Probert P. & D, Nodder S., 2012, Two new free-living nematode species (Comesomatidae) from the continental slope of New Zealand, with keys and notes on distribution, pp. 40-55 in Zootaxa 3348 on page 46, DOI: 10.5281/zenodo.21040
Habitat-correlated seed germination behaviour in populations of wood anemone (Anemone nemorosa L.) from northern Italy
No full textAlthough various aspects of the biology of Anemone nemorosa have been examined, few studies present data on seed germination, and even then information tends to be rather contradictory. A. nemorosa L. is a spring-flowering, woodland geophyte, widely distributed across much of Europe. Germination phenology, including embryo development and radicle and shoot emergence, were investigated in one mountain and three lowland populations from northern Italy. Immediately after harvest, seeds were either sown on agar in the laboratory under simulated seasonal temperatures, or placed in nylon mesh sachets and buried in the wild. Embryos, undifferentiated at the time of dispersal, grew under summer conditions in the laboratory and in the wild. However, seeds did not germinate under continuous summer conditions. Radicle emergence in the field was first recorded at the beginning of autumn, when soil temperatures had dropped to c. 15°C in the case of the three lowland populations, and to c. 10°C at the mountain site. Shoot emergence was delayed under natural conditions until late autumn/early winter, when soil temperatures had dropped to c. 10°C in the lowlands and c. 6°C at the mountain site. In the laboratory, a period of cold stratification was required for shoot emergence, and this requirement was more pronounced in the mountain population. Seeds of the mountain population completed embryo development, radicle emergence and shoot emergence at cooler temperatures compared with the lowland populations. These results suggest that germination in A. nemorosa is highly adapted and finely tuned to local climate. We conclude that seeds of A. nemorosa display deep, simple epicotyl, morphophysiogical dormancy, and this is the first report of such dormancy for the genus Anemone. However, the continuous development and growth of embryos from the time of natural dispersal, and the lack of evidence of developmental arrest under natural conditions, suggests that radicles are non-dormant
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