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81.
83.
A series of acrylate monomers with alkoxy tails of varying lengths are synthesised and polymerised. The butoxy analogue had a stable enantiotropic cholesteric liquid crystalline phase which formed a grandjean texture when prepared as a thin film between glass slides. The polymer was mixed with a low molar mass nematic liquid crystal in various proportions and the pitch of the chiral nematic phases were determined using a cano‐wedge cell technique. The polymer prepared from (S)‐2‐(4‐butoxyphenyl‐4′‐benzoyloxy)‐1‐methyl ethyl acrylate had a pitch length of 113 nm which indicates that the polymer film could be employed in optical devices requiring selective reflection of light with short wavelengths in the region of 170 nm. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
84.
85.
The performance of one-dimensional (1D) coupled cavities photonic crystal (PC) filters has been analyzed by finite-difference time-domain (FDTD) simulation. It is shown that the addition of tapered Bragg mirrors at each side of the cavities, to create near-Gaussian field profiles for the cavity modes, results in the prediction of near flat-top passband filters with high out-of-band rejection ratio and near unity transmission. The tapered structures suppress the vertical radiation loss to allow optimization of the number of mirror periods for the best filter response whilst guaranteeing high transmission. A critical coupling condition (k = 2Lout/Lin = 1) for flat-top responses in doubly coupled cavities filters is proposed in the tapered structures. An optimized filter for 100 GHz optical communication system are demonstrated with 1 dB bandwidth of 0.17 nm, roll-off of 0.6 dB/GHz, out-of-band signal rejection of 33 dB and transmission of 95%. Further improvement of roll-off and out-of-band rejection is demonstrated in a triply coupled cavities filter. 相似文献
86.
87.
The asymmetric total synthesis of the α-glucosidase inhibitor (+)-castanospermine is reported. The central theme in our approach to this polyhydroxylated alkaloid is the simultaneous generation of the piperidine ring and the C-1/8a erythro stereodiad through the diastereoselective, oxamidation of an unsaturated O-alkyl hydroxamate. This process is believed to proceed sequentially via singlet acylnitrenium and aziridinium ion intermediates. 相似文献
88.
Alyssa S. Hickert Andrew C. Durgan Duncan A. Patton Samantha A. Blake Matthew E. Cremeens 《Theoretical chemistry accounts》2011,130(4-6):883-889
Carbon?Cdeuterium labeled amino acids can serve as sensitive probes for biophysical characterization. Although multiple research groups have used infrared spectroscopy in conjunction with alkyl backbone or side-chain deuterated amino acids for the biophysical characterization of conformational and/or environmental changes, it was not entirely clear to the authors that perdeuterated aryl rings would demonstrate a similar sensitivity toward conformational or environmental changes. In an effort to evaluate the sensitivity of aryl carbon?Cdeuterium (C?CD) IR frequencies, a B3LYP investigation of the sensitivity of aryl C?CD frequencies toward conformational and environmental changes was conducted for phenylalanine (Phe) and tryptophan (Trp). To compensate for the low molar absorptivity of C?CD frequencies, perdeuterated aryl rings were investigated, which are commercially available and can be readily compared to experimental data. B3LYP results suggest that aryl-deuterated Phe and Trp will exhibit moderate sensitivities toward conformational and environmental changes with frequency shifts upward of 13 and 26?cm?1 for Phe and Trp, respectively. B3LYP predicts that conformational sensitivity arises from dipole changes and not orbital alignment changes. In an effort to mimic what might be observed experimentally, simulated IR spectra were created and show absorption band changes with conformational and environmental changes, which indicate that IR characterization of perdeuterated aryl rings in amino acids could serve as a biophysical tool. 相似文献
89.
We point out that the quasiparticle spectrum of the Landau Fermi liquid theory has an extra Z2 symmetry, local in momentum space, which is not generic to the Hamiltonian with interactions. Thus the Fermi liquid is in this sense a (quantum) zero-temperature critical point. 相似文献
90.
Duncan Temple Lang 《Computational Statistics》2009,24(2):271-281
The R computing environment has become an important part of the statistical community and fostered the development of over
a thousand add-on packages, many representing state-of-the-art research in statistical methodology. Although it is relatively
easy to develop functionality on top of the system, it is very difficult for developers to directly extend the core system
itself—the language, the interpreter and the internal data structures. Yet the ability to easily introduce new core, first-class
data structures into the system that are customized and efficient is becoming essential in this era of large, complex data
sets and innovative algorithms and data structures. While the community that might use such a facility to introduce new data
types may be small, it is potentially very talented and important, and may lead to significant innovations that allow us to
continue to leverage R for the next 5 years or more in rich new ways. I describe some of the difficulties that people encounter
in extending the system and suggest that an object-oriented architecture for the internal implementation of R (or any system)
would make such low-level internals extensible by package developers and not just the core development team. This would promote
potentially rich experimentation that would allow us and others to approach new styles of computation in R, while simultaneously
maintaining the existing important community which provides so much value-added to the R environment. Specifically, transforming
the R implementation from a representation-specific architecture to a C++ abstract/virtual interface-based architecture may
be the least disruptive approach to the continued evolution of R, and would bring many advantages and some technical challenges.
Such an approach involves many technical details and potential degradations in performance. Due to the length of the this
paper, I do not explore these issues in great detail but introduce the basic concepts. I do, however, refer to some technical
aspects that are best understood with some knowledge of the implementation of R at the level of using the .Call () interface in R. 相似文献