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121.
Accurate electrolysis current integration is required in controlled-potential coulometry. Analog integrators have been shown to deviate from ideal response by as much as —0.1%. A digital integrator was designed to optimize the linearity of the voltage-to-frequency converter used, minimize the effects of temperature, eliminate integration errors caused by small polarity changes and signal noise, and allow interchangeability with an analog integrator in a state-of-the-art coulometer widely used throughout the nuclear field for controlled-potential coulometry. Evaluation of the digital integrator, calibrated versus the Faraday, showed bias-free determinations of plutonium with a relative standard deviation of approximately 0.02%. The integrator was designed specifically for use in the controlled-potential coulometric determination of plutonium, but many of the features incorporated can be adopted to digital integrators for other uses where accurate integration over long periods of time is required. 相似文献
122.
123.
The HF dimer is believed to exhibit an internal rotation tunneling process between two planar but nonlinear equilibrium configurations, during which tunneling the roles of the hydrogen-bonded and the free hydrogen atom are interchanged. This process can be represented schematically with labeled atoms as HlFaH2Fb ? FaHlFbH2, and gives rise to a permutation-inversion group G4 containing four operations. In the present work the vibration-rotation-tunneling problem in (HF)2 is treated group theoretically in three ways: (i) by allowing tunneling only through a trans planar C2h intermediate, (ii) by allowing tunneling only through a cis planar C2v intermediate, and (iii) by considering the trans and cis tunneling processes both to occur, though not necessarily with the same probability. The molecular symmetry groups used for these treatments are (i) the point group C2h, (ii) the point group C2v, and (iii) a double group, which might be thought of as . Nonplanar tunneling paths are not considered, since the internal axis method (IAM) coordinate system used here cannot easily be adapted to nonplanar internal rotation motions in this molecule. Various-details of energy level diagrams, symmetry species for operators, selection rules for spectroscopic transitions, and statistical weights are presented for the (HF)2 tunneling problem, as well as some speculation on the general question of when point groups, permutation-inversion groups, or double groups are preferable for treating large-amplitude vibrational motion problems. 相似文献
124.
Kevin G. Michlewicz John J. Urh Jon W. Carnahan 《Spectrochimica Acta Part B: Atomic Spectroscopy》1985,40(3):493-499
A microwave induced plasma system capable of maintaining stable plasmas of each of the gases helium, argon, nitrogen and air is presented. The system is capable of operation at powers of up to 500 W. The TM010 cavity design is similar to that previously described in the literature with some modifications. A demountable torch facilitates centering of diffuse plasmas of helium, nitrogen and air by providing 6 flows directed tangentially within the quartz tube. This torch was not useful for argon plasmas. Toroidal argon plasmas were maintained with a threaded quartz tube arrangement. The heat generated by these plasmas was dissipated by an outer sheath of coolant air. Details of the design and preliminary characterization of each plasma system is presented. 相似文献
125.
126.
Because of the extravagant claims made for the usefulness of non-flame atomic absorption spectroscopy, a critical evaluation of its performance in heavy matrix solutions, for selected trace heavy metals, was made. Studies of physical and chemical interference problems are presented for a range of inorganic and organic matrices. Often quoted remedies (e.g. selective volatilization, standard addition etc.) for solution of the more serious problems were found to be of little help in most cases. The influence of variables such as nature and flow rate of purge gas, ashing temperature and atomization temperature are also evaluated. Analyses for Cd, Cu, Pb, Zn, Ni, Co were done on samples by flame and nonflame methods in high solids solutions. While there appears to be an advantage to the non-flame method in dealing with sample solutions of high organic content, no advantage is apparent for either technique with highly inorganic matrices. 相似文献
127.
Ondeykal JG Herath KB Jayasuriya H Polishook JD Bills GF Dombrowski AW Mojena M Koch G DiSalvo J DeMartino J Guan Z Nanakorn W Morenberg CM Balick MJ Stevenson DW Slattery M Borris RP Singh SB 《Molecular diversity》2005,9(1-3):123-129
The chemokines (CXCL9, CXCL10 and CXCL11) and associated CXCR3 receptor are expressed during the inflammatory process from multiple sclerosis, atherosclerosis or organ transplantation resulting in the recruitment of lymphocytes leading to tissue damage. It is hypothesized that blocking of the ligand/CXCR3 receptor interaction has potential to provide opportunity for development of agents that would block tissue rejection. In this paper, four classes of natural product inhibitors (IC50 ranging 0.1–41 M) have been described that block the CXCR3 receptor interaction of IP-10 ligand. These include a cyclic thiopeptide (duramycin), polyketide glycosides (roselipins), steroidal glycosides (hypoglausin A and dioscin) and a novel alkyl pyridinium alkaloid that were isolated by bioassay-guided fractionation of the organic extracts derived from actinomycete, fungal, plant and marine sources and discovered using 125 I IP-10/CXCR3 binding assay. Duramycin was the most potent with an IC50 of 0.1 M. Roselipins 2A, 2B and 1A showed IC50 values of 14.6, 23.5, and 41 M, respectively. Diosgenin glycosides dioscin, hypoglaucin A and kallstroemin D exhibited IC50 values of 2.1, 0.47 and 3 M, respectively. A novel cyclic 3-alkyl pyridinium salt isolated from a sponge displayed a binding IC50 of 0.67 M. 相似文献
128.
Biancardo M Quinn AJ Floyd L Mendes PM Briggs SS Preece JA Bignozzi CA Redmond G 《The journal of physical chemistry. B》2005,109(18):8718-8722
We report on charge transport measurements through laterally contacted assemblies of Au nanoparticles capped with 11-mercaptoundecanoic acid ligands. Both alternating- and direct-current data indicate that although the nanoparticles behave as electrically isolated metallic islands, there is a significant influence from the nanoparticle environment, indicating the existence of a slow reorganization process linked to charge transport. On the basis of the observation of temperature-dependent hysteresis of charge tunneling, we propose that this process is due to proton transfer between the carboxylic acid tails of the ligands. 相似文献
129.
Glidle A Hadyoon CS Gadegaard N Cooper JM Hillman AR Wilson RW Ryder KS Webster JR Cubitt R 《The journal of physical chemistry. B》2005,109(30):14335-14343
We describe in situ neutron reflectivity (NR) and RAIRS studies of the chemical modification of films of a polypyrrole-based conducting polymer derived from the pentafluorophenyl ester of poly(pyrrole-N-propanoic acid) (PFP) electrodeposited on electrode surfaces. We explore the role of the solvent in controlling the rate of reaction with solution-based nucleophiles (amines, which react with the ester to form amides). By varying the identity of the solvent (water vs acetonitrile) and the neutron contrast (deuteration), we find that both the identity of the solvent and its population within the film are paramount in determining chemical reactivity and electroactivity. IR signatures allow monitoring of the reaction of solution-based amine-tagged species such as amino-terminated poly(propylene glycol), ferrocene ethylamine, and lysine with film-based ester functionalities: the carbonyl bands show ester/amide interconversion and some hydrolysis to acid. Time-dependent spectral analysis shows marked variations in reaction rate with (i) (co-)polymer composition (replacement of some fluorinated ester-functionalized pyrrole with unfunctionalized pyrrole), (ii) the solvent to which the polymer film is exposed, and (iii) the rate of polymer deposition. NR data provide solvent profiles as a function of distance perpendicular to the interface, the variations of which provide an explanation for film reactivity patterns. Homopolymer films are relatively hydrophobic, thus hindering reaction with species present in water solutions. Incorporating pyrrole groups raises the solvent population-dramatically for water-thereby facilitating entry and reaction of aqueous-based lysine. Changing film deposition rate yields films with different absolute levels of solvent and reactivity patterns that are dependent on the size of the reactant molecules: more rapid deposition of polymer gives films with a more open structure leading to a higher solvent content and thence increased reactivity. These results, supported by XPS and AFM data, allow assembly of composition-structure-reactivity correlations, in which the controlling feature is film solvation. 相似文献
130.
Diversifying vancomycin via chemoenzymatic strategies 总被引:4,自引:0,他引:4
[reaction: see text] The rapid diversification of glycopeptides via glycorandomization reveals that significantly diverse substitutions are tolerated and suggests there may be a synergistic benefit to the construction of mechanistically related natural product core scaffold fusions. This work also further highlights the utility of chemoenzymatic approaches to diversify complex natural product architectures. 相似文献