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21.
Chemical reactions and processes often involve chiral, yet racemic, cationic reagents, intermediates or products. To afford instead non racemic or enantiopure compounds, an asymmetric ion pairing of the cations with enantiopure anions can be considered--the counter ions behaving as asymmetric auxiliaries, ligands or reagents. Detailed herein is a short review of our approach towards gaining reliable and predictable control over stereoselective ion pairing phenomena through the synthesis and the use of novel configurationally stable hexacoordinated phosphate anions. 相似文献
22.
Remington PJ Knight JS Hanna D Rowley C 《The Journal of the Acoustical Society of America》2005,117(1):68-78
A prototype hybrid system consisting of active and passive components for controlling far-field locomotive exhaust noise has been designed, assembled, and tested on a locomotive. The system consisted of a resistive passive silencer for controlling high-frequency broadband noise and a feedforward multiple-input, multiple-output active control system for suppressing low-frequency tonal noise. The active system used ten roof-mounted bandpass speaker enclosures with 2-12-in. speakers per enclosure as actuators, eight roof-mounted electret microphones as residual sensors, and an optical tachometer that sensed locomotive engine speed as a reference sensor. The system was installed on a passenger locomotive and tested in an operating rail yard. Details of the system are described and the near-field and far-field noise reductions are compared against the design goal. 相似文献
23.
24.
The reaction of aryl alkynes with dilute methylene chloride solutions of quaternary ammonium bromide and 20% trifluoroacetic acid produces primarily the syn Markovnikov adducts of hydrogen bromide. At moderate concentrations of the bromide, the principal product is the Markovnikov anti adduct. At high concentrations of bromide, the anti-Markovnikov anti addition product predominates. 相似文献
25.
26.
The mapping class group of a compact oriented surface of genus greater than
one with boundary acts ergodically on connected components of the representation moduli
corresponding to a connected compact Lie group, for every choice of conjugacy class boundary
condition. 相似文献
27.
B. Doug Park 《Proceedings of the American Mathematical Society》2000,128(10):3067-3073
We construct exotic and using the surgery techniques of R. Fintushel and R.J. Stern. We show that these 4-manifolds are irreducible by computing their Seiberg-Witten invariants.
28.
29.
30.
Doug Wingard 《Journal of Thermal Analysis and Calorimetry》2010,102(2):469-476
The Advanced Crew Escape Suit (ACES) is a pressurized suit worn by astronauts during launch and landing phases of Space Shuttle
operations. In 2008, a large tear (12.7–25.4 mm long, between the pinky and ring finger) in the ACES left-hand glove made
of neoprene latex rubber was found during training for Shuttle flight STS-124. An investigation to help determine the cause(s)
of the glove tear was headed by the NASA Johnson Space Center (JSC) in Houston, Texas. Efforts at JSC to reproduce the actual
glove tear pattern by cutting/tearing or rupturing were unsuccessful. Chemical and material property data from JSC such as
GC-MS, FTIR, DSC, and TGA mostly showed little differences between samples from the torn and control gloves. One possible
cause for the glove tear could be a wedding ring/band worn by an astronaut. Even with a smooth edge, such a ring could scratch
the material and initiate the tear observed in the left-hand glove. A decision was later made by JSC to not allow the wearing
of such a ring during training or actual flight. Another possible cause for the ACES glove tear is crystallinity induced by
strain in the neoprene rubber over a long period of time and use. Neoprene is one among several elastomers known to be susceptible
to crystallization, and such a process is accelerated with exposure of the material to cold temperatures plus strain. When
the temperature is lowered below room temperature, researchers have shown that neoprene crystallization may be maintained
at temperatures as high as 7.2–10 °C, with a maximum crystallization rate near −6.7 to −3.9 °C (Kell et al. J Appl Polym Sci
2(4):8–13, 1959 [1]). A convenient conditioning temperature for inducing neoprene crystallization is a typical freezer that is held near −17.8
°C. For work at the NASA Marshall Space Flight Center (MSFC), samples were cut from several areas/locations (pinky/ring finger
crotch, index finger and palm) on each of two pairs of unstrained ACES gloves for DSC and DMA thermal analysis testing. The
samples were conditioned in a freezer for various times up to about 14 days. Some rectangular conditioned samples were unstrained,
while most were subjected to strains up to 250% with the aid of two slotted aluminum blocks and two aluminum clamps per sample.
Trends were observed to correlate DSC data (heat of fusion) and DMA data (linear CTE and stress for iso-strain testing) with
(a) sample location on each glove; and (b) percent strain during conditioning. Control samples cut “as is” from each glove
location were also tested by DSC and DMA. 相似文献