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81.
82.
In a confined two dimensional system of non-interacting electrons in a normal constant magnetic field, the current is calculated along the sample as a response to an electric field in the same direction which is switched on and then off. The resulting stationary current is shown to be a step-like function of the electron density. 相似文献
83.
The localization length of one-dimensional disordered systems with statistically correlated random potentials is studied both numerically and analytically. The results indicate that the localization length generally increases when the correlation function is positive. In the presence of anticorrelation effects the localization length may be shorter than in the uncorrelated case. 相似文献
84.
85.
Despite Nature's longstanding ability to use a proton, the most prevalent Lewis acid, to both activate and orient a substrate during an enantioselective reaction, this work represents the first example of this phenomenon outside of a protein. A chiral, nonracemic BisAMidine (BAM) ligand was designed, synthesized, and complexed to the proton of a Br?nsted acid. The resulting coordination compound catalyzed the production of enantioenriched product from the combination of a Schiff base and nitroalkane (the aza-Henry reaction). This particular reaction is also considered a model for many analogous carbon-carbon bond-forming reactions catalyzed by enzymes (e.g., the Mannich reaction). This discovery suggests the use of ionic hydrogen bonds in asymmetric catalysis may not only be more general than previously thought, but also a viable "green" approach to single-enantiomer organic compounds. 相似文献
86.
Numerical and experimental techniques are used to model the flow and pressure distribution around the forebody of the HYFLEX
hypersonic flight vehicle. We compare numerical simulation results with modified Newtonian theory and flight data to determine
the accuracy of the computational fluid dynamics (CFD) technique used. The numerical simulations closely match the trends
in flight data, and show that real gas effects have a small but significant influence on the nose pressure distribution. We
also present pressure results from a scale-model tested in a shock tunnel, and compare them with simulation results. For the
shock tunnel experiment, the model was placed such that part of the upper surface was in a region of the test flow where nonuniformities
were significant, and it was shown that the numerical simulation could adequately capture these experimental flow features.
The binary scaling parameter (describing the similarity in species dissociation between flight and model) was used to design
the scale-model tests in the shock tunnel, and its effectiveness is discussed. We find that matching the flight Mach number
in the shock tunnel experiment is not critical for reproducing flight pressure data, so long as flight velocity is matched,
and binary scaling is maintained.
Received 11 June 1998 / Accepted 1 September 1998 相似文献
87.
Wilmin P. Bartolini Catherine M. Bentzley Murray V. Johnston Barbara S. Larsen 《Journal of the American Society for Mass Spectrometry》1999,10(6):521-528
Elucidating structure function relationships of DNA in cellular processes requires fast, reliable methods that can be applied to picomole amounts of sample. Higher order structure can be inferred by distinguishing paired and unpaired regions. It is shown here that enzymatic digestion coupled with product analysis by matrix-assisted laser desorption ionization (MALDI) is able to identify unpaired bases within structured DNA regions. The method is demonstrated with DNA duplexes having a five nucleotide mismatch as a 5' overhang, a 3' overhang, and an internal loop. Exo- and endonuclease digestions are performed under solution conditions (temperature, annealing, and enzyme buffers) which promote base pairing and specific enzyme activity. For each type of mismatch, the length and sequence of the single stranded region can be inferred from MALDI spectra taken as a function of digestion time. 相似文献
88.
Ghizzo A. Bertrand P. Begue M.L. Johnston T.W. Shoucri M. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1996,24(2):370-378
High-frequency beatwave simulations relevant to the University of California at Los Angeles (UCLA) experiment with relativistic eulerian hybrid Vlasov code are presented. These Hilbert-Masov simulations revealed a rich variety of phenomena associated with the fast particle dynamics induced by beatwave experiment for a high ratio of driver frequency to plasma frequency ωpump/ωplasma ≈33. The present model allows us to extend detailed modeling to frequency ratios greater than the current practical maximum of 10 or so, for Vlasov or particle-in-cell (PIC) codes, by replacing the Maxwell equations by mode equations for the electromagnetic Vlasov code. Numerical results, including beat frequency chirping (i.e., pump frequency linearly decreasing with time), show that the amplitude limit due to relativistic detuning can be enhanced with accelerated particles up to the ultrarelativistic energies with a high-acceleration gradient of more than 25 GeV/m 相似文献
89.
Bruce A. Kimball Rori K. Craver John J. Johnston Dale L. Nolte 《Journal of separation science》1995,18(4):221-225
In order to investigate the relationship between terpenoid content and black bear foraging preference, an analytical method was required to quantify mono- and sesquiterpenoids present in Douglas-fir sapwood. Sapwood samples were scraped from trees, immediately frozen in liquid nitrogen, and then homogenized. A simple extraction requiring no clean-up step was performed with ethyl acetate. Extracts were analyzed by gas chromatography with mass selective detection versus external standards. The recoveries of 22 terpenoids from fortified controls were approximately 90% with good precision (relative standard deviations of approximately 10%). 相似文献
90.
K. G. Johnston 《Semigroup Forum》1994,49(1):131-135
Communicated by Norman R. Reilly 相似文献