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Ostashev VE Collier SL Wilson DK Aldridge DF Symons NP Marlin D 《The Journal of the Acoustical Society of America》2007,122(1):107-112
Formulation and implementation of time-domain boundary conditions (TDBCs) at the surface of a reactive porous material are made challenging by the slow decay, complexity, or noncausal nature of many commonly used models of porous materials. In this paper, approaches are described that improve computational efficiency and enforce causality. One approach involves approximating the known TDBC for the modified Zwikker-Kosten impedance model as a summation of decaying exponential functions. A second approach, which can be applied to any impedance model, involves replacing the characteristic admittance with its Padé approximation. Then, approximating fractional derivatives with decaying exponentials, a causal and recursive TDBC is formulated. 相似文献
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W. Mark Barnett Geng Lin Harvest L. Collier Russell G. Baughman 《Journal of chemical crystallography》1997,27(7):423-427
1,1′-Di(methylacetato)-2,2′-biimidazole, C12H14N4O4, crystallizes from methanol in the space groupP2 1/c, wherea=9.535(2),b=13.385(2),c=5.1208(8) Å,V=652.2(2) Å3, andZ=4.1,1′-Di(chloroethoxyethyl)-2,2′-biimidazole, C14H20Cl2N4O2, crystallizes from cyclohexane in the space groupPbca, wherea=12.372(2),b=8.959(2),c=14.840(2) Å,V=1644.9(5) Å3, andZ=8. The structures were refined toR=0.041 (1380 observed reflections) andR=0.043 (3243 observed reflections), respectively. Both molecules crystallize with coplanar rings and the substituents assume atrans configuration with a center of inversion between the bridging carbon atoms. 相似文献
25.
Calculations based on Pople's independent electron model, with CNDO/S and INDO/S parameterizations, are reported for some ketenimines and carbodiimides. The unusually large shielding of the central carbon uncleus is accounted for by means of the various electronic transitions which contribute to the paramagnetic shielding term. Changes in carbon charge densities tend to predict a chemical shift trend opposite to that observed. 相似文献
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Lundström E Brodin G Lundin J Marklund M Bingham R Collier J Mendonça JT Norreys P 《Physical review letters》2006,96(8):083602
The properties of four-wave interaction via the nonlinear quantum vacuum is investigated. The effect of the quantum vacuum is to generate photons with new frequencies and wave vectors, due to elastic photon-photon scattering. An expression for the number of generated photons is derived, and using state-of-the-art laser data it is found that the number of photons can reach detectable levels. In particular, the prospect of using the high-repetition Astra Gemini system at the Rutherford Appleton Laboratory is discussed. The problem of noise sources is reviewed, and it is found that the noise level can be reduced well below the signal level. Thus, detection of elastic photon-photon scattering may for the first time be achieved. 相似文献
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Raghunath O. Ramabhadran Mohamed Husaini bin Abdul Rahman Steven J. Collier Richard C. Storr Roderick W. Bates 《Tetrahedron》2018,74(1):1-5
Trapping of a pyrimidinone-derived o-quinodimethane with tetraphenylcyclopentadienone gives a mixture of the [4 + 2] and formal [4 + 4] adducts. Computational studies show that the latter arises by a sequence of [4 + 2] cycloaddition, homolysis, radical recombination to a spiro-tetrahydrofuran and Claisen rearrangement under the thermolytic conditions. 相似文献
30.
Voehler MW Collier G Young JK Stone MP Germann MW 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2006,183(1):102-109
The pursuit for more sensitive NMR probes culminated with development of the cryogenic cooled NMR probe. A key factor for the sensitivity is the overall resistance of RF circuitry and sample. Lowering the coil temperature to approximately 25 K and the use of superconducting coil material has greatly reduced the resistance contribution of the hardware. However, the resistance of a salty sample remains the same and evolves as the major factor determining the signal-to-noise ratio. Several approaches have been proposed to reduce the resistance contribution of the sample. These range from encapsulating proteins in a water cavity formed by reverse micelles in low viscosity fluids to the optimal selection of low mobility, low conductivity buffer ions. Here we demonstrate that changing the sample diameter has a pronounced effect on the sample resistance and this results in dramatic improvements of the signal-to-noise ratio and shorter pi/2 pulses. We determined these parameters for common 5 mm NMR tubes under different experimental conditions and compared them to the 2, 3 and 4 mm tubes, in addition, 5mm Shigemi tubes were included since these are widely used. We demonstrate benefits and applicability of studying NMR samples with up to 4M salt concentrations in cryogenic probes. Under high salt conditions, best results in terms of short pi/2 pulses and high signal-to-noise ratios are obtained using 2 or 3mm NMR tubes, especially when limited sample is available. The 4 mm tube is preferred when sample amounts are abundant at intermediate salt conditions. 相似文献