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71.
The methods of synthesis of 1,4-benzodioxin-2(3H)-one have been reexamined. Frequently quoted in the literature the method of Ghosh has been found to give impure (2-hydroxyphenoxy)acetic acid rather than the lactone. Using various methods some simple derivatives of the title lactone substituted in the benzene ring have been prepared and characterized. The bromination of the lactone with NBS gives predominantly 7-bromo-1,4-benzodioxin-2(3H)-one under electrophilic conditions, while 6-bromo-1,4-benzodioxin-2(3H)-one is obtained as a major isomer under photolytic conditions. 相似文献
72.
A reliable preparative-scale synthesis of 1-bromo-1-lithioethene is reported. This reagent undergoes clean 1,2-addition with a range of aldehydes and ketones at -105 degrees C to afford the corresponding 2-bromo-1-alken-3-ols in moderate to excellent yield. Efficient diastereoselective addition to alpha-siloxy and alpha-methylcyclohexanones, as well as protected 3-keto furanose sugars, is achieved in the presence of 10 mol % CeBr(3). The resulting bromoallylic alcohol adducts have considerable potential as synthetic building blocks. [reaction: see text] 相似文献
73.
Allan R Sampson 《Statistics & probability letters》1984,2(2):77-81
A multivariate correlation ratio of a random vector upon a random vector is defined by where Λ, a fixed positive definite matrix, is related to the relative importance of predictability for the entries of . The properties of ηΛ are discussed, with particular attention paid to a ‘correlation-maximizing’ property. Given are applications of ηΛ to the elliptically symmetric family of distributions and the multinomial distribution. Also discussed is the problem of finding those r linear functions of that are most predictable (in a correlation ratio sense) from . 相似文献
74.
Gavin Sampson Dr. Nicholas C. Bristowe Dr. Sam T. Carr Asad Saib Dr. Gavin B. G. Stenning Dr. Ewan R. Clark Dr. Paul J. Saines 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(32):e202200855
This work describes a homometallic spin- tetrabromocuprate adopting a bilayer structure. Magnetic-susceptibility measurements show a broad maximum centred near 70 K, with fits to this data using a Heisenberg model consistent with strong antiferromagnetic coupling between neighbouring copper atoms in different layers of the bilayer. There are further weak intralayer ferromagnetic interactions between copper cations in neighbouring dimers. First-principles calculations are consistent with this, but suggest there is only significant magnetic coupling within one direction of a layer; this would suggest the presence of a spin ladder within the bilayer with antiferromagnetic rung and weaker ferromagnetic rail couplings. 相似文献
75.
The ring-opening metathesis polymerization (ROMP) reaction is extraordinarily useful for the preparation of a large variety of polymers. We report that the length (n = 25-50) of high-substituent-density oligopeptide polymers synthesized by ROMP is dramatically improved upon addition of LiCl to reduce polymer and oligopeptide aggregation. This methodology should significantly expand the variety of polymers that may be prepared by ROMP and be of general use with norbornyl oligopeptides of any sequence. 相似文献
76.
Robert Quintana Olivier Persenaire Yahia Lemmouchi John Sampson Stuart Martin Leïla Bonnaud Philippe Dubois 《Polymer Degradation and Stability》2013
Cellulose acetate (CA) with a degree of substitution (DS) of 2.5 has been plasticized using eco-friendly plasticizers such as triacetin, tripropionin, triethyl citrate, tributyl citrate, tributyl 2-acetyl citrate and poly(ethylene glycol) of low molecular weight. Thermo-mechanical properties and hydrophilicity of the modified CA have been measured and correlated with the content and nature of the plasticizer used and compared with unplasticized CA. The increase in toughening and the change in the hydrophilicity by the plasticization were evaluated in terms of aging and weathering stability under accelerated conditions. Samples were exposed to UV-degradation with water spray periods. The treated samples were removed periodically and characterized by several analytical techniques. The results are discussed with particular emphasis toward the effects of plasticization on enhancement of the degradation rate of CA. The plasticization of CA triggered an increase of the weight loss between 50 and 90%, where low molecular weight plasticizers were shown to be more effective. A right balance between hydrophilicity and plasticization efficiency (reduction of Tg) is needed to increase the degradation rate of CA. 相似文献
77.
B. Srinivasan K. J. Mathew U. I. Narayanan W. F. Guthrie T. E. Sampson 《Journal of Radioanalytical and Nuclear Chemistry》2009,282(3):963-970
The calorimetry exchange (CALEX) program is administered by New Brunswick Laboratory (NBL). The main objective of the program
is to provide an independent verification of the internal quality control practices in nuclear material safeguards facilities
making plutonium accountability measurements by non-destructive calorimetry/gamma spectrometry techniques. Facilities measure
the calorimetric power, and plutonium and 241Am isotope abundances of CALEX program standards using routine accountability procedures. The measurement results as well
as two other quantities (effective specific power and plutonium mass) calculated from these results are evaluated for accuracy
(or bias) and precision. In this paper, a limited number of measurement results of a CALEX program standard (identified as
Calex I) are evaluated with specific goals to identify a suitable method for uncertainty estimation and to identify the major
contributors to the uncertainties. In order to achieve the goals, the Calex I measurement results were evaluated using two
different methods: the first method confined to uncertainty estimation from random variations of the measurement results alone,
and the second method providing a more comprehensive evaluation of uncertainties from both the measurements and the characterized
values of the measured standard according to the Guide to the Expression of Uncertainty in Measurement (GUM). The results
of this study, and a subsequent study extended to a larger number of results in the CALEX program database, are expected to
provide relevant input for developing the International Target Values for plutonium measurements by the calorimetry/gamma
spectrometry method. 相似文献
78.
The sperm protein fertilinbeta, a member of the ADAM family of proteins, is implicated in sperm-egg binding in all mammals studied to date. Multivalent inhibitors containing the three amino acid binding sequence of fertilinbeta, ECD, have been shown previously to be more effective inhibitors of fertilization than their monovalent counterparts. Here, we probed sperm-egg interactions with ruthenium-catalyzed ring-opening metathesis polymers that contained from 3 to 70 ECD pharmacophores in densities ranging from 10% to 100%. Evaluation of the polymer potencies, and synthesis of a triblock copolymer from two building blocks, revealed that two multivalent contacts are sufficient for maximal inhibition, and that the distance between ECD pharmacophores required is 7-9 monomers spanning 4-5 nm. We conclude that inhibition requires recruitment of two receptors on the egg surface into an inhibitory complex. 相似文献
79.
Yun Zhang Allan Sampson Kai Hwang Beatriz Luna 《Journal of computational and graphical statistics》2019,28(2):362-373
Stochastic block model (SBM) and its variants are popular models used in community detection for network data. In this article, we propose a feature-adjusted stochastic block model (FASBM) to capture the impact of node features on the network links as well as to detect the residual community structure beyond that explained by the node features. The proposed model can accommodate multiple node features and estimate the form of feature impacts from the data. Moreover, unlike many existing algorithms that are limited to binary-valued interactions, the proposed FASBM model and inference approaches are easily applied to relational data that generate from any exponential family distribution. We illustrate the methods on simulated networks and on two real-world networks: a brain network and an US air-transportation network. 相似文献
80.
Numerous organic and inorganic laboratory standards were gathered from nine European and North American laboratories and were analyzed for their delta(18)O values with a new on-line high temperature pyrolysis system that was calibrated using Vienna standard mean ocean water (VSMOW) and standard light Antartic precipitation (SLAP) internationally distributed reference water samples. Especially for organic materials, discrepancies between reported and measured values were high, ranging up to 2 per thousand. The reasons for these discrepancies are discussed and the need for an exact and reliable calibration of existing reference materials, as well as for the establishment of additional organic and inorganic reference materials is stressed. Copyright 1999 John Wiley & Sons, Ltd. 相似文献