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21.
Zhou L Thompson R Reamer RA Miller C Welch C Ellison DK Wyvratt JM 《Journal of chromatography. A》2003,987(1-2):409-420
The possible mechanisms for the chiral recognition of 2(S)-(3,5-bis-trifluoromethyl-phenyl)-2-[3(S)-(4-fluorophenyl)-4-(1H-[1,2,4]triazol-3-ylmethyl)-morpholin-2(R)-yloxy]-ethanol (compound A) and its enantiomer with native gamma-cyclodextrin (gamma-CD) were investigated using capillary electrophoresis (CE), reversed-phase liquid chromatography (RPLC), proton (1H), fluorine (19F) and carbon (13C) nuclear magnetic resonance spectroscopy (NMR), electrospray mass spectrometry (ESI-MS) and circular dichroism (CD). All experiments provided clear evidence of the formation of diastereomeric complexes between the enantiomers and gamma-CD. Proton, fluorine and carbon NMR spectra suggested that both aromatic rings, with mono-fluoro and bis-tri-fluoro functional groups, on the guest molecule were partially included into the cavity of the gamma-CD. ESI-MS spectra indicated that the diastereomeric complexes have a 1:1 stoichiometric ratio. The binding constants of the diastereomeric complexes obtained by CE, RPLC and CD were compared. The effects of the gamma-CD concentration, organic modifiers and temperature on the CE-chiral separation were also investigated. 相似文献
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T. L. Dawson R. D. Lundberg F. J. Welch 《Journal of polymer science. Part A, Polymer chemistry》1969,7(1):173-181
Trialkylboron–oxygen, an active, low-temperature free-radical initiator, has been employed to investigate the effects of very low temperatures on the copolymerizations of vinyl acetate with cis and trans-1,2-dichloroethylenes. The low temperatures favor the propagation rate relative to the transfer rate, such that high molecular weight copolymers containing substantial quantities of 1,2-dichloroethylene can be prepared. The molecular weights of the copolymers depend only on the amounts of 1,2-dichloroethylene in the copolymers, regardless of the isomer which takes part in the copolymerization. Since the double bond of the trans isomer is about six times as reactive as that of the cis isomer, this indicates that the dominating chain transfer reaction occurs by chlorine atom elimination subsequent to the addition of the dichloroethylene unit to the growing free radical chain. It is suggested that a similar chain-transfer mechanism occurs in the polymerization of vinyl chloride, wherein an infrequent head-to-head placement of monomer unit is followed by ejection of a chlorine atom to form an olefinic bond and termination of that growing chain. The presence of the 1,2-dichloroethylene unit in the copolymer increases the glass transition temperature approximately 1°C per weight per cent copolymerized with the vinyl acetate. 相似文献
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D. I. Welch C. D. Watts 《International journal of environmental analytical chemistry》2013,93(2):185-198
Abstract The collection of rainwater, aerosol and vapour samples at a semi-rural site in the UK was achieved using a PTFE-lined wet-only rainfall collector and a high-volume filter/adsorption trap air sampler, respectively. Analysis of atmospheric deposition revealed the presence of several hundred compounds, many of which were of anthropogenic origin, e.g.: PAH, phenols and alkylbenzenes. Amounts of compounds varied from low nanograms to tens of micrograms per litre in rainwater samples and from low picograms to high nanograms per cubic metre in aerosols. Phenolic compounds were the most abundant group of organics identified in rainwater and were present at total concentrations of >20μg1?1 in some of the samples analysed. In the high-volume air samples most anthropogenic compounds were detected in the adsorbent rather than the filter extract. Seasonal variations in the PAH content of the adsorbent extracts were observed. The presence of siloxanes in the air samples was thought to be the result of contamination. 相似文献
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Richard Coupe Heather Welch Angela Pell E. Michael Thurman 《International journal of environmental analytical chemistry》2013,93(15):1127-1140
During 1996-1997, water samples were collected from five sites in the Yazoo River Basin and analysed for 14 herbicides and nine degradates. These included acetochlor, alachlor, atrazine, cyanazine, fluometuron, metolachlor, metribuzin, molinate, norflurazon, prometryn, propanil, propazine, simazine, trifluralin, three degradates of fluometuron, two degradates of atrazine, one degradate of cyanazine, norflurazon, prometryn, and propanil. Fluxes generally were higher in 1997 than in 1996 due to a greater rainfall in 1997 than 1996. Fluxes were much larger from streams in the alluvial plain (an area of very productive farmland) than from the Skuna River in the bluff hills (an area of small farms, pasture, and forest). Adding the flux of the atrazine degradates to the atrazine flux increased the total atrazine flux by an average of 14.5%. The fluometuron degradates added about 10% to the total fluometuron flux, and adding the norflurazon degradate flux to the norflurazon flux increased the flux by 82% in 1996 and by 171% in 1997. 相似文献
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Jason P. Seeley Dr. Mircea Cotlet Aileen M. Eagleton Seiichiro Higashiya Prof. John T. Welch 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(4):961-965
The fluorescence intensity of a C-terminal acceptor chromophore, N-(7-dimethylamino-4-methyl coumarin (DACM), increased proportionally with 280 nm irradiation of an increasing number of donor tryptophan residues located on a β-sheet forming polypeptide. The fluorescence intensity of the acceptor chromophore increased even as the length of the β-sheet edge approached 256 Å, well beyond the Förster radius for the tryptophan–acceptor chromophore pair. The folding of the peptides under investigation was verified by circular dichroism (CD) and deep UV resonance Raman experiments. Control experiments showed that the enhancement of DACM fluorescence occurred concomitantly with peptide folding. In other control experiments, the DACM fluorescence intensity of the solutions of tryptophan and DACM did not show any enhancement of DACM fluorescence with increasing tryptophan concentrations. Formation of fibrillar aggregates of the substrate peptides prepared for the fluorescence studies was undetectable by thioflavin T (ThT) fluorescence. 相似文献
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The flux and fluence dependence of disorder produced in silicon during the implantation of 11B has been investigated at room temperature, -50°C. and -120°C. Implantations were carried out with 200 keV 11B ions using current densities in the range from 0.06μA/cm2 to 15μA/cm2, and the disorder monitored by measuring the energy spectra of backscattered protons which were incident on the sample at 450 keV parallel to a (110) axis. Significant differences in the dependence of the disorder on 11B flux and fluence were observed between the implantations performed at room temperature and those carried out at the two lower temperatures. 相似文献