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61.
A. J. Campbell C. A. Fyfe D. Harold-smith K. R. Jeffrey 《Molecular Crystals and Liquid Crystals》2013,570(1-2):1-23
Pulse nuclear magnetic resonance measurements have been used to determine the activation enthalpies for the reorientation of the organic rings in ferrocene, ruthenocene and dibenzenechromium. The contribution from non-bonded interactions to the total potential for reorientation of the rings has been calculated and is compared to the experimental activation enthalpies. The results indicate that there is no contribution to the potential from the non-bonded interactions between the two rings on a single molecule and that the observed potential barrier is due to a combination of crystal packing forces and bonding forces within the molecule. The crystal structure of ferrocene is thought to be disordered above 135[ddot]K, and a model is proposed for the order-disorder transition in the solid state. On this basis, dynamic models are proposed for the structures of the three metallocenes in solution. 相似文献
62.
Kevin J. Schwarz Chao Yang Dr. James W. B. Fyfe Prof. Thomas N. Snaddon 《Angewandte Chemie (International ed. in English)》2018,57(37):12102-12105
The first asymmetric cooperative Lewis base/palladium catalyzed benzylic alkylation of acyclic esters is reported. This reaction proceeds via stereodefined C1‐ammonium enolate nucleophiles. Critical to its success was the identification of benzylic phosphate electrophiles, which were uniquely reactive. Alkylated products were obtained with very high levels of enantioselectivity, and this method has been applied toward the synthesis of the thrombin inhibitor DX‐9065a. 相似文献
63.
M.K. Islam S. Biswas N.A. Chowdhury A. Mannan M. Salahuddin AA Mamun 《等离子体物理论文集》2022,62(1):e202100073
A theoretical investigation has been carried out on the propagation of non-linear ion-acoustic shock waves (IASHWs) in a magnetized degenerate quantum plasma system composed of inertial non-relativistic positively charged light and heavy ions, inertialess non-relativistically or ultra-relativistically degenerate electrons and positrons. The reductive perturbation method has been employed to derive the Burgers' equation. It has been observed that under consideration, our plasma model supports only positive potential shock structure. It is also found that the amplitude and steepness of the IASHWs have been significantly modified by the variation of ion kinematic viscosity, oblique angle, number density, and charge state of the plasma species. The results of our present investigation will be helpful for understanding the propagation of IASHWs in white dwarfs and neutron stars. 相似文献
64.
Colin A. Fyfe Peter J. Stephenson Richard P. Veregin Gordon K. Hamer Robert H. Marchessault 《Journal of carbohydrate chemistry》2013,32(4):663-673
Abstract The chemical shifts and multiplicities of the high-resolution 13C CP/MAS NMR spectrum of cellulose II are quite diagnostic of the lattice structure of this polymorph. Particularly important is the chemical shift of C-1 and its clear splitting into two lines of equal intensity. Similar chemical shifts and multiplicities are seen in the spectrum of cellotetraose. Thus cellotetraose is considered to be a good model for the lattice structure of the polymer. A detailed investigation of the multiplicity of the C-1 resonance of cellotetraose shows that the two peaks are of equal intensity in this case also. Because of the limited number of repeat units in the tetramer, this observation implies that the unit cell contains two independent chains rather than a “double” repeat unit. This gives support for a similar lattice structure, with two independent chains, for cellulose II itself. 相似文献
65.
Commensurability between inter-vortex distance and crystal lattice constant is investigated by angular dependent magnetization
in very pure twinned and twin-free NdBa2 Cu3 Oy single crystals. With increasing temperature the incommensurate states split up and become finally commensurate with half
the vortex distance. These new commensurate states are related to a substructure of the intrinsic pinning potential within
the unit cell and discussed with respect to temperature, field, anisotropy, and twin structure. 相似文献
66.
MJ Lazaro AA Herod M Domin Y Zhuo CA Islas R Kandiyoti 《Rapid communications in mass spectrometry : RCM》1999,13(14):1401-1412
Three methods for determining a 'safe' estimate for high-mass limits of MALDI spectra of coal derived liquids were explored, using a sample of coal-tar pitch and its pyridine-insoluble fraction. Co-addition of increasing numbers of single-shot spectra (10, 30, 50 and 100 pulses) showed visually observable reductions in noise levels, consistent with robust and statistically meaningful signals. Three separate types of post-acquisition calculation were used to identify high-mass limits of the spectra. (i) A literature method indicated high-mass limits similar to those observed visually-as a shift from baseline at the highest masses, nearly 350 000 u for the coal tar pitch and about 390 000 u for its pyridine insoluble fraction. (ii) Comparing instrument signal with pre-selected multiples of the standard deviation, upper mass estimates of between 40-60 000 u for the coal-tar pitch and about 95 000 u for its pyridine-insoluble fraction were found. (iii) Calculation of the slope was used to identify 'lift-off' of the spectrum from baseline. The angle between the smoothed spectrum and the baseline was matched to a pre-selected value (e.g. 0.5 degrees and 1 degrees ). However, the arbitrary specification of the key parameter did not establish this last method on a firm basis. The choice of a criterion for estimating high-mass limits of MALDI spectra remains a semi-quantitative procedure; a reasonably conservative high-mass limit may be estimated by comparison of signal with five times the standard deviation. However, evaluation of size exclusion chromatograms of the present samples using polystyrene standards suggests that molecular mass distributions of pitch samples arrived at by MALDI mass spectrometry are, at least partly, determined by the limitations of available instruments. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
67.
Li F Wang Z Ergang NS Fyfe CA Stein A 《Langmuir : the ACS journal of surfaces and colloids》2007,23(7):3996-4004
Monolithic pieces of hierarchically structured silica, containing both periodic macropores and mesopores with well-controlled architecture, are synthesized by dual templating methods. Colloidal crystal templating with close-packed arrays of poly(methyl methacrylate) spheres yields regular, highly interconnected macropores a few hundred nanometers in diameter, and templating with nonionic surfactants produces mesoporosity (2.5-5.1 nm pore diameters) in the macropore walls. Several distinct mesostructures can be achieved within the silica skeleton, depending on the choice of surfactant, co-surfactant, and processing conditions. In the three-dimensional (3D) confinement of the colloidal crystal template, wormlike channels, cubic (Pm3n), or two-dimensional (2D) hexagonal (P6mm) mesostructures are produced with the surfactant Brij 56 (C16H33(OCH2CH2)nOH (n approximately 10) and dodecane as cosurfactant. In the 2D hexagonal structure, channels are oriented perpendicular to the polymer spheres, thereby connecting adjacent macropores through the silica walls. This orientation contrasts with channel alignment parallel to latex spheres when the polymeric surfactant Pluronic P123 (EO20PO70EO20) is used. On the basis of high-resolution 3D transmission electron microscopy, scanning electron microscopy, small-angle X-ray scattering, and nitrogen sorption measurements, structural and textural properties of the monoliths are described in detail as a function of the synthesis parameters. The control over the mesoarchitecture of these silica-surfactant systems in 3D confinement is explained by considering the relative dimensions of the mesostructures with respect to the interstitial space in the latex template, interfacial interactions, entropic effects, and structural frustration. 相似文献
68.
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70.
This paper presents the application of Artificial Immune Systems to the design of classifier ensembles. Ensembles of classifiers
are a very interesting alternative to single classifiers when facing difficult problems. In general, ensembles are able to
achieve better performance in terms of learning and generalisation errors.
Several papers have shown that the processes of classifier design and combination must be related in order to obtain better
ensembles. Artificial Immune Systems are a recent paradigm based on the immune systems of animals. The features of this new
paradigm make it very appropriate for the design of systems where many components must cooperate to solve a given task. The
design of classifier ensembles can be considered within such a group of systems, as the cooperation of the individual classifiers
is able to improve the performance of the overall system.
This paper studies the viability of Artificial Immune Systems when dealing with ensemble design. We construct a population
of classifiers that is evolved using an Artificial Immune algorithm. From this population of classifiers several different
ensembles can be extracted. These ensembles are favourably compared with ensembles obtained using standard methods in 35 real-world
classification problems from the UCI Machine Learning Repository. 相似文献