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1.
Albert V. Fratini Elisa M. Cross Joseph F. O'brien W. Wade Adams 《Journal of Macromolecular Science: Physics》2013,52(1-4):159-179
The determination of the crystalline structure of oriented fibers of poly-2,5-benzoxazole (ABPBO) and poly-2,6-ben-zothiazole (ABPBT) is described. Both unit cells are metrically orthorhombic, with the parameters: a = 6.061 (17), b = 3.384 (13), c (fiber axis) = 11.575 (6) Å for ABPBO; and a = 6.044 (6), b = 3.417 (7), c (fiber axis) = 12.194 (18) Å for ABPBT. The fiber repeat consists in each structure of two fused ring groups arranged in a planar, zigzag conformation. The conformational torsion angle and orientation of chains within the unit cells are derived from a linked-atom least squares refinement technique. Polymer chains pack laterally through van der Waals interactions. A plausible disorder model which involves defects in chain direction is presented. Refinement of a static disorder model for ABPBO in which 50% of the chains have their chain directions reversed leads to a lower R residual and sum of constraints. 相似文献
2.
Statine (STA, 8) and its cyclohexyl analog (ACHPA, 9) are obtained by a stereospecific synthesis via tetramic acid chemistry. Amides of STA and ACHPA, useful for the synthesis of antihyperten-sive renin inhibitors, are prepared directly from the intermediate activated lactams 7 by nucleophilic ring opening with amines. 相似文献
3.
Abstract The optically active B-Methy1 substituted tetrahydroisoquinoline (?)? 1 is a major reaction product in the cleavage of the bisbenzylisoquinoline alkaloid repanduline with potassium in liquid ammonia.1 A synthesis of (±) ? 1 by a rather lengthy procedure has been reported.2 In this connection we now wish to report a simple and efficient method of introducing an 8-methyl substituent into a 7-hydroxy-6- methoxy tetrahydroisoquinoline. This approach, which has probably wider applications, is exemplified by the synthesia of 8-methylcorypalline 4 and its O-methylether 5 and involves the thiomethylation reaction of poppeladorf and Holt3 followed by reductive desulfurization of the phenyl thiomethyl intermediate. 相似文献
4.
The compatibility of Euclidean supergravity with curvature satisfying (a) self-duality and (b) a double duality Ansatz are investigated. The Minkowskian version of (b) is also considered. 相似文献
5.
Covariant solutions to the Vierbein (gravitational) field equations are related to solutions of scalar field equations. New solutions to the latter are also given. 相似文献
6.
Moldovan L Melinte S Bayot V Papadakis SJ De Poortere EP Shayegan M 《Physical review letters》2000,85(20):4369-4372
We report very low temperature ( T) thermopower and resistivity ( rho) measurements on variable-density, two-dimensional hole systems confined to GaAs quantum wells. As the hole density is lowered from 1.49x10(11) cm(-2) to 0.14x10(11) cm(-2), the system crosses from an insulating ( drho / dT less, similar0) to a metallic regime ( drho / dT>0) and finally displays insulating behavior ( drho / dT<0). Diffusion thermopower shows a striking sign reversal in a narrow range of density in the metallic regime, suggesting a qualitative change in the conduction or the scattering mechanism. 相似文献
7.
We classify the radical subgroups and chains of the O'Nan and Rudvalis simple groups O'N and Ru, and then verify the Alperin weight conjecture and the Dade final conjecture for the two groups. 相似文献
8.
EP Sheretov VS Gurov MV Dubkov OV Korneeva 《Rapid communications in mass spectrometry : RCM》1999,13(16):1699-1702
In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
9.
Street A. M. Stavrinou P. N. O'brien D. C. Edwards D. J. 《Optical and Quantum Electronics》1997,29(3):349-378
This paper reviews the research effort to date into optical wireless communication systems, both in academic and industrial
contexts. The idea of using the optical medium for wireless communications is not new, having been proposed as a means for
indoor communications almost two decades ago. However, the last few years have seen an explosive interest in the potential
for free space optical systems to provide portable data communications. One of the prime motivators for reconsidering the
use of an optical carrier in the wireless context is the demand for greater transmission bandwidths. The radio frequency spectrum
is already exceedingly congested and frequency allocations of sufficient bandwidths are extremely hard to obtain. Further,
for the high bandwidth services envisaged, the use of microwave or mm-wave systems will be required, where device technology
is currently either expensive or immature. Proponents of optical wireless systems argue that the optical medium is the only
cost-effective way to provide high bit-rate mobile services to volume markets. This paper identifies the technical obstacles
and limitations of indoor optical wireless delivery in addition to techniques for mitigating these effects and shows that
infrared is a viable alternative to radio for certain applications.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
10.
NL Asfandiarov AI Fokin VG Lukin EP Nafikova GS Lomakin VS Fal'ko YV Chizhov 《Rapid communications in mass spectrometry : RCM》1999,13(12):1116-1123
A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd. 相似文献