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981.
We consider the minimum number of edges in ak-edge-connected graph of ordern with chromatic number at leastc, obtaining the optimal bounds in most cases. 相似文献
982.
Sulfide cluster-derived ensembles are promising models of the active sites in commercial hydrotreatment catalysts. A series of sulfide clusters were adsorbed intact onto high-surface-area γ-alumina, magnesium oxide and activated carbon supports, then pretreated to produce highly dispersed catalytic ensembles with sizes similar to those of their precursor clusters. The activities of the bimetallic cluster-derived catalysts were significantly higher than those of the monometallic catalysts. We took this as evidence that direct interactions between molybdenum and the promoter element cause the promotional effect observed in commercial hydrotreatment catalysts. The hydrodesulfurization and hydrodenitrogenation activities correlated with the extent of molybdenum reduction. Our results suggested that the active sites in promoted hydrotreatment catalysts are centered on molecular-scale ensembles containing molybdenum, sulfur and the promoter element. 相似文献
983.
We make a thorough non-covariant analysis of the derivation of the equation of motion for a charged particle, including radiation reaction, by means of a simple model for the charge: a dumbbell. This model and our method to expand retarded quantities, based on complex-variable theory, allow us to avoid some of the usual approximations, so we can show several features of the radiation reaction problem. 相似文献
984.
Guantao Chen Ronald J. Gould Michael S. Jacobson Richard H. Schelp 《Journal of Graph Theory》1994,18(7):663-672
For several years, the study of neighborhood unions of graphs has given rise to important structural consequences of graphs. In particular, neighborhood conditions that give rise to hamiltonian cycles have been considered in depth. In this paper we generalize these approaches to give a bound on the smallest number of cycles in G containing all the vertices of G. We show that if for all x, y ? V(G), |N(x) ∩ N(y)| ≧ 2n/5 + 1, then V(G) is coverable by at most two cycles. Several related results and extensions to t cycles are also given. 相似文献
985.
2,6- and 2,7-Dimethylanthracenes were reduced seriatim with lithium in liquid ammonia and lithium in methylamine to their 1,4,5,8,9,10-hexahydro derivatives. Two-fold epoxidation of these tetraenes provided the cis and trans diepoxides resulting from electrophilic attack at the inner double bonds. Trans diepoxide 1 has the very unusual characteristic of lacking a σ plane of symmetry but possessing an inversion center. As a consequence, the 1H NMR spectrum of C1-symmetric 1 is simplified relative to those of its isomers 5, 9, and 10. 相似文献
986.
The self-diffusion of benzene, toluene, and ethylbenzene in polystyrene have been analyzed using the Vrentas/Duda free-volume diffusion model. Diffusion coefficient predictions suggest an exponential concentration dependence of the activation energy required to overcome attractive forces, E. Without the use of any diffusion data approximating E as zero over the entire concentration range yields self-diffusion coefficient predictions which are in good agreement with experimental data. © 1992 John Wiley & Sons, Inc. 相似文献
987.
M. D. Zipper G. P. Simon P. Cherry A. J. Hill 《Journal of Polymer Science.Polymer Physics》1994,32(7):1237-1247
Homopolymers and blends of polycarbonate/ThermX have been investigated by differential scanning calorimetry, dynamic mechanical analysis, density measurements, and positron annihilation lifetime spectroscopy. The study focuses on the chain mobility and free volume in the amorphous miscible regions of the blends and how this mobility and free volume are affected by increasing crystallinity. It is proposed that judicious annealing, which results in increasing crystallinity (while avoiding ageing or transesterification), induces a constraint on the amorphous region leading to an increase in Tg and coalescence of free volume sites. © 1994 John Wiley & Sons, Inc. 相似文献
988.
Thermotropic copolyester fibers of oxynaphthoate and oxybenzoate have been subjected to conditions that promote solid-state polymerization as well as annealing. The annealing process causes the crystals to perfect with a simultaneous increase in heat of fusion and melting temperature. Solid-state polymerization, a reaction rate-controlled process, causes the polymer viscosity average molecular weight to increase by chain extension from about 14,000 g/mole to more than 87,000 g/mole with a simultaneous impressive increase in tenacity from about 10 g/d (1.2 GPa) to almost 30 g/d (3.7 GPa). To understand the changes in mechanical properties, we have modeled the fiber structure as short rod-like molecules poorly bonded to a continuous matrix of parallel molecules. Lengthening of the reinforcing molecules facilitates better transfer of load from matrix to molecules, resulting in higher tenacity fibers. © 1994 John Wiley & Sons, Inc. 相似文献
989.
The nature of the propagation center in the cationic polymerization of N-benzoyl-8-octanelactam initiated by octanoylium hexachlcroantimonate, SbCl5, and Ph3CAsF6 in perdeuterated tetrachloroethane or its mixture with o-dichlorobenzene was studied using 1H, 13C, 19F, 31P, 75As, and 121Sb nuclear magnetic resonance (NMR) of model oligomers and the products of their end-capping with triphenylphosphine. In all cases, the nature of the propagation center has been found to be of an acylium ion pair with an SbCl6? or AsF6? counterion coordinated with the nearest benzoylamide group and cosolvated by the solvent. © 1994 John Wiley & Sons, Inc. 相似文献
990.