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排序方式: 共有127条查询结果,搜索用时 15 毫秒
1.
Jaedong Cho John Blackwell Sergei N. Chvalun Morton Litt Yuan Wang 《Journal of Polymer Science.Polymer Physics》2004,42(13):2576-2585
As‐cast films of poly(2,5‐benzimidazole) exhibit uniplanar orientation in which the planes of the aromatic rings lie parallel to the film surface. Upon doping with phosphoric acid, the original crystalline order is lost, but the doped film can be stretched to produce films with uniaxial orientation. After thermal annealing at 540 °C, nine Bragg reflections are resolved in the fiber diagram, and these are indexed by an orthorhombic unit cell with the dimensions a = 18.1 Å, b = 3.5 Å, and c = 11.4 Å, containing four monomer units of two chains. The absence of odd‐order 00l reflections points to a 21 chain conformation, which is probably planar so that the aromatic units can be stacked along the b axis. The water and phosphoric acid contents of the crystalline structure cannot be determined exactly because of the presence of extensive amorphous regions that probably have different solvation. The best agreement between the observed and calculated intensities is for an idealized structure containing two phosphoric acids and two water molecules per unit cell. However, the phosphoric acid is probably present mainly in the form of pyrophosphoric acid and its higher oligomers. In addition, the X‐ray data are consistent with a more disordered structure containing chains with random (up and down) polarity and a lack of c‐axis registry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2576–2585, 2004 相似文献
2.
The problem of predicting the short-term future behavior of a sequence, after observing it as long as we please, so as to
achieve a specified reliability against all possible sequences is considered. For a particular problem, namely, predicting
when in a sequence of 0's and 1's the pair (1, 0) in that order is not coming next, a reliability of 3/4 can be approximated
as closely as we please, but not achieved.
This article appeared as a Research Memorandum of the Rand Corporation, RM-1570, 12 October 1955. 相似文献
3.
Blackwell HE 《Organic & biomolecular chemistry》2003,1(8):1251-1255
The application of microwave irradiation to expedite solid-phase organic reactions could be the tool that allows combinatorial chemistry to deliver on its promise--providing rapid access to large collections of diverse small molecules. Herein, several different approaches to microwave (MW)-assisted solid-phase reactions and library synthesis are introduced, including the use of solid-supported reagents, multicomponent coupling reactions, solvent-free parallel library synthesis, and spatially addressable library synthesis on planar solid supports. The future impact of MW-assisted organic reactions on solid-phase and combinatorial chemistry could prove to be immense, and methods for further improvement of this strategic combination of technologies are highlighted. 相似文献
4.
Cellulose microfibrils are viewed as imperfect array of elementary fibrils. We have investigated the possible defects in Valonia cellulose microfibrils, which are such that the microfibrils can be broken into elementary fibrils by deformation, but are not sufficient to allow for a small angle maximum corresponding to the elementary fibril dimension. The microfibril has been constructed by convolution of th elementary fibril with a two dimensional point lattice. Defects have been incorporated in the microfibril, first by introduction of gaps between the elementary fibrils. These regular gaps were then replaced by a statistical distribution of the elementary fibrils about the lattice points, modeled by Hosemann distortions of the first type. The cylindrically averaged transforms of such structures show that significant distortions can be incorporated within the microfibril without producing large scale changes in the equatorial intensity distribution. Larger distortions are necessary before a small angle maximum corresponding to the 35 A elementary fibril is predicted, by which stage the wide angle x-ray pattern is unacceptable. 相似文献
5.
6.
Blackwell HE Sadowsky JD Howard RJ Sampson JN Chao JA Steinmetz WE O'Leary DJ Grubbs RH 《The Journal of organic chemistry》2001,66(16):5291-5302
Heptapeptides containing residues with terminal olefin-derivatized side chains (3 and 4) have been treated with ruthenium alkylidene 1 and undergone facile ring-closing olefin metathesis (RCM) to give 21- and 23-membered macrocyclic peptides (5 and 6). The primary structures of peptides 3 and 4 were based upon a previously studied heptapeptide (2), which was shown to adopt a predominantly 3(10)-helical conformation in CDCl(3) solution and an alpha-helical conformation in the solid state. Circular dichroism, IR, and solution-phase (1)H NMR studies strongly suggested that acyclic precursors 3 and 4 and the fully saturated macrocyclic products 7 and 8 also adopted helical conformations in apolar organic solvents. Single-crystal X-ray diffraction of cyclic peptide 8 showed it to exist as a right-handed 3(10)-helix up to the fifth residue. Solution-phase NMR structures of both acyclic peptide 4 and cyclic peptide 8 in CD(2)Cl(2) indicated that the acyclic diene assumes a loosely 3(10)-helical conformation, which is considerably rigidified upon macrocyclization. The relative ease of introducing carbon-carbon bonds into peptide secondary structures by RCM and the predicted metabolic stability of these bonds renders olefin metathesis an exceptional methodology for the synthesis of rigidified peptide architectures. 相似文献
7.
Spring DR Krishnan S Blackwell HE Schreiber SL 《Journal of the American Chemical Society》2002,124(7):1354-1363
Diversity-oriented synthesis of structurally complex and diverse small molecules can be used as the first step in a process to explore cellular and organismal pathways. The success of this process is likely going to be dependent on advances in the synthesis of small molecules having natural product-like structures in an efficient and stereoselective manner. The development, scope, and mechanism of the oxidation of organocuprates was investigated and exploited in the atropdiastereoselective synthesis of biaryl-containing medium rings (9-, 10-, and 11-membered rings). The methodology was performed on high-capacity, large polystyrene beads by metalating aryl bromides with i-PrBu(2)MgLi, followed by transmetalating with CuCN x 2LiBr and then oxidizing with 1,3-dinitrobenzene, and was used in a diversity-oriented synthesis of biaryl-containing medium rings (library total theoretical maximum 1412 members). The high capacity beads were arrayed into 384-well plates and, using a process optimized during the development of a one bead/one stock solution technology platform, converted into arrays of stock solutions, with each stock solution containing largely one compound. These stock solutions were used in numerous phenotypic and protein-binding assays. The process described outlines a pathway that we feel will contribute to a comprehensive and systematic chemical approach to exploring biology (chemical genetics). 相似文献
8.
David R. McMullin Mark W. Sumarah Barbara A. Blackwell J. David Miller 《Tetrahedron letters》2013,54(6):568-572
A strain of Chaetomium globosum (DAOM 240359) was isolated from an indoor air sample in Ottawa, Ontario, Canada. When fermented in liquid culture, this strain produced a number of known metabolites including chaetoglobosins A (6), C, and F (7), chaetomugilin D (5), chaetoviridin A (4), and three new nitrogenous azaphilones; 4′-epi-N-2-hydroxyethyl-azachaetoviridin A (1), N-2-butyric-azochaetoviridin E (2), and isochromophilone XIII (3). The structures were elucidated by spectroscopic analysis including; HRMS, 1D and 2D NMR, UV, and ORD. Compounds 2–7 were antimicrobial when tested using quantitative growth inhibition assays. 相似文献
9.
Costa DP Crocker DE Gedamke J Webb PM Houser DS Blackwell SB Waples D Hayes SA Le Boeuf BJ 《The Journal of the Acoustical Society of America》2003,113(2):1155-1165
Changes in the diving behavior of individual free-ranging juvenile northern elephant seals, Mirounga angustirostris, exposed to the acoustic thermometry of the ocean climate (ATOC) sound source were examined using data loggers. Data loggers were attached to the animals and measured swim speed, maximum depth of dive, dive duration, surface interval, descent and ascent rate, and descent and ascent angle along with sound pressure level (SPL). The ATOC sound source was at a depth of 939 m and transmitted at 195 dB re: 1 microPa at 1 m centered at 75 Hz with a 37.5-Hz bandwidth. Sound pressure levels (SPL) measured at the seal during transmissions averaged 128 dB and ranged from 118 to 137 dB re: 1 microPa for the 60-90 Hz band, in comparison to ambient levels of 87-107 dB within this band. In no case did an animal end its dive or show any other obvious change in behavior upon exposure to the ATOC sound. Subtle changes in diving behavior were detected, however. During exposure, deviations in descent rate were greater than 1 s.d. of the control mean in 9 of 14 seals. Dive depth increased and descent velocity increased in three animals, ascent velocity decreased in two animals, ascent rate increased in one animal and decreased in another, and dive duration decreased in only one animal. There was a highly significant positive correlation between SPL and descent rate. The biological significance of these subtle changes is likely to be minimal. This is the first study to quantify behavioral responses of an animal underwater with simultaneous measurements of SPL of anthropogenic sounds recorded at the animal. 相似文献
10.
Jae Dong Cho John Blackwell Sergei N. Chvalun Jungil Jin Yushan Ni 《Journal of polymer science. Part A, Polymer chemistry》1998,36(17):3137-3145
The structure of a thermotropic liquid crystalline copoly(ester-imide) prepared from p-hydroxybenzoic acid (48 mol %), 4,4′-dihydroxybenzophenone (26 mol %), and N,N′-bis(trimellitimide)hexane (26 mol %) has been investigated by X-ray diffraction. X-ray fiber diagrams of as-spun and annealed fibers contain a series of aperiodic layer lines reminiscent of those seen for fibers of other copolymers that have extended chain conformations and completely random monomer sequences. The positions of these layer lines were reproduced approximately in simulation of the X-ray scattering by a fully extended chain of completely random sequence, and the match was improved to within experimental error when we considered a stereochemically acceptable sinuous chain. This agreement was lost when the sequence statistics deviated were completely random. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3137–3145, 1998 相似文献