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81.
We consider possible physics beyond the standard model that could account for the large recently reported [Muon g-2 Collaboration, H. N. Brown et al., Phys. Rev. Lett. 86, 2227 (2001)] effect in g(mu)-2. If the underlying theory can be treated perturbatively, then the only possibilities are models that contain particles that yield "direct" contributions, e.g., supersymmetric models, models with unexpectedly light leptoquarks, or with a rather exotic heavy vector boson. If the underlying theory involves strong interactions, as in technicolor models, then the discrepancy could be accounted for by a variety of mechanisms. 相似文献
82.
83.
Yaxin Li Raina Dano Cathy Li Wenjing Zhang Justin D. Lathia Bingcheng Wang Bin Su 《Biomedical chromatography : BMC》2022,36(3):e5310
Previously compound I showed great anti-glioblastoma activity without toxicity in a mouse xenograft study. In this study, a sensitive and rapid high-performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) method was developed and validated to investigate the pharmacokinetics and brain distribution of compound I in mice. The protein precipitation method was applied to extract the compound from mouse plasma and brain homogenates, and it was then separated using a Kinetex C18 column with a mobile phase consisting of acetonitrile–0.1% formic acid water (50:50, v/v). The analytes were detected with multiple reaction monitoring for the quantitative response of the compounds. The inter- and intra-day precisions were <8.29 and 3.85%, respectively, and the accuracy range was within ±7.33%. The method was successfully applied to evaluate the pharmacokinetics of compound I in mouse plasma and brain tissue. The peak concentration in plasma was achieved within 1 h. The apparent elimination half-life was 4.06 h. The peak concentration of compound I in brain tissue was 0.88 μg/g. The results indicated that compound I was rapidly distributed and could cross the blood–brain barrier. The pharmacokinetic profile summarized provides valuable information for the further investigation of compound I as a potential anti-glioblastoma agent. 相似文献
84.
The use of nano- or microfibrillar cellulose (NFC or MFC) in papermaking is generally hampered by high cost and potentially
wasteful use in typical wet end applications. The solubility and fines nature of the material makes it inefficient to retain,
and when retained it is generally inefficiently applied within the spatial distribution of the paper fibre matrix. To illustrate
the benefits of capturing the important NFC in a layer structure to enhance surface and stiffness properties of paper and
board, we present a study whereby NFC is entrapped at the surface of a fibrous web by forming an in situ composite using a
porous coating layer, consisting in the exemplified case of modified calcium carbonate. It is shown that NFC can integrate
itself within the porous structure providing excellent holdout and thin layer continuity essential in developing an efficient
concentration of the NFC at the surface of the substrate. The effect is likened to the well-known I-beam construction. An
additional feature is the potential for recycling the remaining fibrous content in the NFC or, more particularly, MFC product
after the nanocrystalline cellulose (NCC) gel fraction has been absorbed, allowing for further efficient processing if needed
and hence providing a potential cost reduction in the overall NFC/MFC production. The increased smoothness and uniformity
obtained is illustrated by confocal laser profilometry and electron microscopy. The effect on permeability is also illustrated. 相似文献
85.
H. Watanabe Kunihiro Osaki Mutsuo Matsumoto Dobrin P. Bossev Cathy E. McNamee Masaru Nakahara Ming-Long Yao 《Rheologica Acta》1998,37(5):470-485
Nonlinear rheological features were investigated for an aqueous solution of tetraethylammonium perfluorooctyl sulfonate (C8F17SO3
–N+(C2H5)4; abbreviated as FOSTEA). In the solution (c=0.045 mol/l; 28.3 g/l), spherical micelles of FOSTEA were connected with each other to form threads of pearl-necklace shape.
These threads were further organized into a transient network to exhibit linear relaxation characteristic of living polymers,
single-mode terminal relaxation widely separated from faster relaxation processes. Nonlinear relaxation experiments against
large step-strains γ(≤8) revealed that the terminal relaxation mode had a γ-insensitive relaxation time but its relaxation
intensity exhibited significant damping (much stronger than that for entangled polymers). In contrast, the relaxation time
and intensity for the fast relaxation modes first increased and then decreased with increasing γ. Under shear flow, the FOSTEA
threads exhibited strong thinning of the viscosity. These nonlinear features of the FOSTEA threads were compared with those
of other threadlike micelles, analyzed on the basis of an empirically introduced constitutive equation, and discussed in relation
to strain/low-induced scission of the living threads.
Received: 20 February 1998 Accepted: 30 July 1998 相似文献
86.
Chin YH Buda C Neurock M Iglesia E 《Journal of the American Chemical Society》2011,133(40):15958-15978
Kinetic and isotopic data and density functional theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH(4) oxidation reactions using O(2), H(2)O, or CO(2) as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilibrium constants for different elementary steps. Transitions among regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O* coverages are given, in turn, by a virtual O(2) pressure, which represents the pressure that would give the prevalent steady-state O* coverages if their adsorption-desorption equilibrium was maintained. The virtual O(2) pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling between C-H and O═O activation steps. O* coverages and virtual pressures depend on O(2) pressure when O(2) activation is equilibrated and on O(2)/CH(4) ratios when this step becomes irreversible as a result of fast scavenging of O* by CH(4)-derived intermediates. In three of these kinetic regimes, C-H bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve from oxygen-oxygen (O*-O*), to oxygen-oxygen vacancy (O*-*), and to vacancy-vacancy (*-*) site pairs as O* coverages decrease. On O*-saturated cluster surfaces, O*-O* site pairs activate C-H bonds in CH(4) via homolytic hydrogen abstraction steps that form CH(3) groups with significant radical character and weak interactions with the surface at the transition state. In this regime, rates depend linearly on CH(4) pressure but are independent of O(2) pressure. The observed normal CH(4)/CD(4) kinetic isotope effects are consistent with the kinetic-relevance of C-H bond activation; identical (16)O(2)-(18)O(2) isotopic exchange rates in the presence or absence of CH(4) show that O(2) activation steps are quasi-equilibrated during catalysis. Measured and DFT-derived C-H bond activation barriers are large, because of the weak stabilization of the CH(3) fragments at transition states, but are compensated by the high entropy of these radical-like species. Turnover rates in this regime decrease with increasing Pt dispersion, because low-coordination exposed Pt atoms on small clusters bind O* more strongly than those that reside at low-index facets on large clusters, thus making O* less effective in H-abstraction. As vacancies (*, also exposed Pt atoms) become available on O*-covered surfaces, O*-* site pairs activate C-H bonds via concerted oxidative addition and H-abstraction in transition states effectively stabilized by CH(3) interactions with the vacancies, which lead to much higher turnover rates than on O*-O* pairs. In this regime, O(2) activation becomes irreversible, because fast C-H bond activation steps scavenge O* as it forms. Thus, O* coverages are set by the prevalent O(2)/CH(4) ratios instead of the O(2) pressures. CH(4)/CD(4) kinetic isotope effects are much larger for turnovers mediated by O*-* than by O*-O* site pairs, because C-H (and C-D) activation steps are required to form the * sites involved in C-H bond activation. Turnover rates for CH(4)-O(2) reactions mediated by O*-* pairs decrease with increasing Pt dispersion, as in the case of O*-O* active structures, because stronger O* binding on small clusters leads not only to less reactive O* atoms, but also to lower vacancy concentrations at cluster surfaces. As O(2)/CH(4) ratios and O* coverages become smaller, O(2) activation on bare Pt clusters becomes the sole kinetically relevant step; turnover rates are proportional to O(2) pressures and independent of CH(4) pressure and no CH(4)/CD(4) kinetic isotope effects are observed. In this regime, turnover rates become nearly independent of Pt dispersion, because the O(2) activation step is essentially barrierless. In the absence of O(2), alternate weaker oxidants, such as H(2)O or CO(2), lead to a final kinetic regime in which C-H bond dissociation on *-* pairs at bare cluster surfaces limit CH(4) conversion rates. Rates become first-order in CH(4) and independent of coreactant and normal CH(4)/CD(4) kinetic isotope effects are observed. In this case, turnover rates increase with increasing dispersion, because low-coordination Pt atoms stabilize the C-H bond activation transition states more effectively via stronger binding to CH(3) and H fragments. These findings and their mechanistic interpretations are consistent with all rate and isotopic data and with theoretical estimates of activation barriers and of cluster size effects on transition states. They serve to demonstrate the essential role of the coverage and reactivity of chemisorbed oxygen in determining the type and effectiveness of surface structures in CH(4) oxidation reactions using O(2), H(2)O, or CO(2) as oxidants, as well as the diversity of rate dependencies, activation energies and entropies, and cluster size effects that prevail in these reactions. These results also show how theory and experiments can unravel complex surface chemistries on realistic catalysts under practical conditions and provide through the resulting mechanistic insights specific predictions for the effects of cluster size and surface coordination on turnover rates, the trends and magnitude of which depend sensitively on the nature of the predominant adsorbed intermediates and the kinetically relevant steps. 相似文献
87.
Cathy Y. Wong 《Journal of luminescence》2011,131(3):366-22844
The origin of the fine structure of the ground state single- and biexciton of CdSe nanocrystals is reviewed, along with the theoretical framework used to describe these states. Calculations were performed to determine the transition dipole moments of optically allowed transitions from the single- to biexciton fine structure states. Two-dimensional photon echo spectroscopy measurements for a sample of CdSe nanocrystals are reported. The two-dimensional electronic spectrum at a population time of 0 fs is analyzed using a simulation based on k.p theory predictions of the exciton and biexciton manifolds of states. The analysis suggests that a particular excited state absorption transition from the single- to biexciton fine structure dominates the 2D spectra. These excited state absorptions are clearly resolved in 2D spectra and the method therefore has promise for gaining clearer insights into quantum dot spectroscopy. 相似文献
88.
89.
Alberto J. Rodriguez Cathy Zozakiewicz Randy Yerrick 《School science and mathematics》2005,105(7):352-362
Recent science and teacher education reports continue to stress the need for radical changes in the way teachers are prepared to teach science to diverse learners. In response, a three‐year intervention project was developed to help teachers in culturally diverse schools transform their science teaching practices using learning technologies. Many challenges arose that called for strategies to further manage the progress of the project. This paper describes how one of those strategies, “prompted praxis,” was used to manage two of the main challenges encountered, the teachers' following through with their professional development goals and our own sense of urgency to effect change. 相似文献
90.
Ghanem A Bados P Perreau F Benabdallah R Plagellat C de Alencastro LF Einhorn J 《Analytical and bioanalytical chemistry》2008,391(1):345-352
A multiresidue method has been developed to analyze atrazine (ATZ), diuron (DIU), and their major degradation products, desethylatrazine
(DEA), desisopropylatrazine (DIA), and dichlorophenylmethylurea in sewage sludge. Liquid chromatography coupled to electrospray
tandem mass spectrometry (LC–ESI-MS–MS) allowed, in the multiple-reaction monitoring mode, the simultaneous analysis of these
pesticides in only one run after their extraction with ethyl acetate–dichloromethane 90:10 (v/v) and a cleanup on a Florisil column. Stable isotopically labeled ATZ and DIU were used as internal standards to overcome
matrix effects during the pesticide quantification. Using fortified samples, the method gave rise to 86–115% as mean recovery
values depending on the analyte. Limits of detection (LODs) and of quantification (LOQs) ranging from 0.3 (DIA) to 1.5 (DEA)
μg kg−1 dw and from 0.4 (DIA) to 2.0 (DEA) μg kg−1 dw, respectively, were sufficient to achieve the monitoring of these molecules in sludge from wastewater treatment plants
of the Ile-de-France region. 相似文献