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91.
92.
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
93.
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
94.
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
95.
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
96.
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
97.
K. G. Johnston 《Semigroup Forum》1994,49(1):131-135
Communicated by Norman R. Reilly 相似文献
98.
99.
The influence of an inhibitor (CF3Br or Halon 1301) on the propagation of high-speed turbulent flames, quasi-detonations and the transition to detonation has
been investigated for methane-air, propane-air and acetylene-air mixtures. The experiments are carried out in a 13 m tube
(15 cm diameter) filled with regularly spaced orifice plates (blockage ratio of 0.39) to ensure rapid flame acceleration.
In all cases, the addition of the inhibitor reduces the turbulent flame velocity and extinguishes the flame with sufficient
inhibitor concentration (2.7% and 7.5% for methane-air and propane-air, respectively). For acetylene-air mixtures, the quasi-detonation
speed is progressively reduced with increasing inhibitor concentration and eventually causes the failure of the quasi-detonation
and transition back to a fast turbulent flame. The inhibitor also narrows the propagation limits in all cases. To elucidate
the inhibition mechanism, detailed modelling of both the turbulent flame structure as well as the chemical kinetics are required. 相似文献
100.
The vortex formed at the tip of a propeller interacting with the vortex formed at the tip of a stator vane provides a unique environment for the study of vortex interactions. Changes in the relative vortex strengths and vortex rotational directions were determined to impact the resulting vortex structures and are easily implemented with the experimental apparatus described herein. Study of the development of the vortex interaction was determined to be possible by increasing the initial separation between the two vortices. Vortex interaction phenomenon has been observed using smoke flow visualization.The authors would like to thank the NASA Lewis Research Center for their funding of propeller related research from which this experiment evolved and the National Sciences and Engineering Research Council of Canada for R. Johnston's Post Graduate Scholarships. 相似文献