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71.
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R.M. Christensen 《International Journal of Solids and Structures》1976,12(7):537-544
Asymptotic methods are used to obtain simple formulas for use in predicting the effective modulus properties of composite materials containing randomly oriented fibers. Both cases of two and three dimensional random orientation are treated. The theoretical predictions are compared with experimental results in the two-dimensional case. The results extend and simplify an earlier approach to the problem. 相似文献
74.
H.C. Christensen G. Nilsson K.-Å. Thuomas T. Reitberger 《Chemical physics letters》1973,22(3):533-536
A 5 Me V proton accelerator has been used for pulse radiolysis of organic gases. The transient spectra obtained from the alkanes methane, ethane, propane, n-butane and neopentane have tentatively been assigned to alkyl radicals. Some methodological aspects of this new technique are discussed. 相似文献
75.
The morphology of WC grains is examined by ab initio calculations, and by transmission electron microscopy in a WC-Co cemented carbide. Two compositions are studied to determine the effect of carbon chemical potential on the shape development. The WC grains are found to be truncated triangles, and the grain morphology is adequately reproduced by the ab initio calculations. The energy difference between the two types of grain facets is shown to be due to the formation of stronger bonds with more covalent character between low coordinated W and Co atoms at one type of facet. 相似文献
76.
Phipps RK Petersen BO Christensen KB Duus JØ Frisvad JC Larsen TO 《Organic letters》2004,6(20):3441-3443
[structure: see text] Hesseltin A 1, a novel compound of mixed polyketide-terpenoid origins was isolated from the filamentous fungus Penicillium hesseltinei. The structure and stereochemistry were determined from extensive one- and two-dimensional NMR and mass spectral data. 相似文献
77.
Abrahamian E Fox PC Naerum L Christensen IT Thøgersen H Clark RD 《Journal of chemical information and computer sciences》2003,43(2):458-468
Pharmacophore triplets and quartets have been used by many groups in recent years, primarily as a tool for molecular diversity analysis. In most cases, slow processing speeds and the very large size of the bitsets generated have forced researchers to compromise in terms of how such multiplets were stored, manipulated, and compared, e.g., by using simple unions to represent multiplets for sets of molecules. Here we report using bitmaps in place of bitsets to reduce storage demands and to improve processing speed. Here, a bitset is taken to mean a fully enumerated string of zeros and ones, from which a compressed bitmap is obtained by replacing uniform blocks ("runs") of digits in the bitset with a pair of values identifying the content and length of the block (run-length encoding compression). High-resolution multiplets involving four features are enabled by using 64 bit executables to create and manipulate bitmaps, which "connect" to the 32 bit executables used for database access and feature identification via an extensible mark-up language (XML) data stream. The encoding system used supports simple pairs, triplets, and quartets; multiplets in which a privileged substructure is used as an anchor point; and augmented multiplets in which an additional vertex is added to represent a contingent feature such as a hydrogen bond extension point linked to a complementary feature (e.g., a donor or an acceptor atom) in a base pair or triplet. It can readily be extended to larger, more complex multiplets as well. Database searching is one particular potential application for this technology. Consensus bitmaps built up from active ligands identified in preliminary screening can be used to generate hypothesis bitmaps, a process which includes allowance for differential weighting to allow greater emphasis to be placed on bits arising from multiplets expected to be particularly discriminating. Such hypothesis bitmaps are shown to be useful queries for database searching, successfully retrieving active compounds across a range of structural classes from a corporate database. The current implementation allows multiconformer bitmaps to be obtained from pregenerated conformations or by random perturbation on-the-fly. The latter application involves random sampling of the full range of conformations not precluded by steric clashes, which limits the usefulness of classical fingerprint similarity measures. A new measure of similarity, The Stochastic Cosine, is introduced here to address this need. This new similarity measure uses the average number of bits common to independently drawn conformer sets to normalize the cosine coefficient. Its use frees the user from having to ensure strict comparability of starting conformations and having to use fixed torsional increments, thereby allowing fully flexible characterization of pharmacophoric patterns. 相似文献
78.
Ball G Vant K Christensen N 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(2):1299-1311
We present a detailed numerical study of a chaotic classical system and its quantum counterpart. The system is a special case of a kicked rotor and for certain parameter values possesses cantori dividing chaotic regions of the classical phase space. We investigate the diffusion of particles through a cantorus. A quantum analysis confirms that the cantori act as barriers. We numerically estimate the classical phase space flux through the cantorus per kick and relate this quantity to the behavior of the quantum system. We introduce decoherence via environmental interactions with the quantum system and observe the subsequent increase in the transport of quantum particles through the boundary. 相似文献
79.
80.
An increasing number of proficiency testing schemes (PT schemes) related to occupational and environmental health have been
organised. Most schemes emphasise the importance of the validation of analytical results. With regards to harmonisation of
the schemes, there are many differences between the schemes at many levels. These include factors concerning their relationship
with legislation, national status, type and quality of proficiency testing material, analytical range and priorities for future
development. Since differences between PT schemes have been recognised at the European level by organisers of PT schemes and
external quality assessment schemes (EQASs) it seems appropriate to reinforce collaboration between scheme organisers in order
to improve the quality of analytical performance in occupational and environmental health. 相似文献