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31.
Reactive copper solutions have been prepared by a cold temperature, two equivalent lithium naphthalenide reduction of copper(I) complexes. One equivalent of organic halides react with one equivalent of the anionic copper to yield organocopper reagents. 相似文献
32.
33.
Behrouzian B Savile CK Dawson B Buist PH Shanklin J 《Journal of the American Chemical Society》2002,124(13):3277-3283
The novel product profile obtained by incubating chiral fluorinated substrate analogues with castor stearoyl-ACP Delta(9) desaturase has been rationalized through a series of labeling studies. It was found that the introduction of the Z-double bond between C-9 and C-10 of the parent substrate occurs with pro-R enantioselectivity--a result that accounts for the observed stereochemistry of oxidation products derived from (9R)- and (9S)-9-fluorostearoyl-ACP. Oxidation of (9R)-9-fluorostearoyl-ACP occurs via at least two rapidly interchanging substrate conformations in the active site as detected by reaction pathway branching induced by deuteration at C-10 and C-11. Hydroxylation and desaturation of this substrate share the same site of initial oxidative attack. 相似文献
34.
Lawlor A Reagan D McCullagh GD Gregorio PD Tartaglia P Dawson KA 《Physical review letters》2002,89(24):245503
We introduce an order parameter for dynamical arrest. Dynamically available volume (unoccupied space that is available to the motion of particles) is expressed as holes for the simple lattice models we study. Near the arrest transition the system is dilute in holes, so we expand dynamical quantities in a series of hole density. Unlike the situation when presented in particle density, all cases of simple models that we examine have a quadratic dependence of the diffusion constant on hole density. This observation implies that in certain regimes ideal dynamical arrest transitions may possess a hitherto unnoticed degree of universality. 相似文献
35.
36.
P.H. Dawson 《Surface science》1979,81(2):464-478
The interaction of oxygen with polycrystalline niobium has been studied using both Auger electron spectroscopy and low-energy secondary ion mass spectrometry in the temperature range from 300–1250 K. At higher temperatures there is oxygen dissolution into the bulk but a preferential surface segregation on recooling. Between 300 and 1250 K, there is a rapid initial adsorption into a very stable state which is associated with increases in the Nb+ and NbO+ yields that are linear with coverage. At 1250 K, further changes are very slow. At 900 K, the initial stage is followed by the adsorption with a lower sticking coefficient (<0.1) as coverage increases from θ = 0.5 to 0.7. This produces an additional larger increase in the yield of Nb+ but a much smaller change in NbO+. At 300 K, the sticking probability falls more slowly with coverage above θ = 0.5 and the amount of oxygen continues to increase slowly with exposure. The SIMS spectrum shows dramatic increases in Nb+, NbO+ and NbO+2 yields and the successive appearance of small yields of ions such as Nb2O+2 and Nb2O+3 as oxide formation begins. The Nb+ yield slowly decreases as further oxidation occurs. Each stage of oxidation has a characteristic secondary ion mass spectrum. 相似文献
37.
David F. Dawson 《Annali di Matematica Pura ed Applicata》1971,88(1):1-8
Summary Conditions on a complex matrix A are given which are necessary and sufficient for A to have the property that if x belongs
to the complex linear space generated by the set of all convergent real sequences having nonnegative kth differences, then
Ax belongs to the complex linear space generated by the set of all convergent real sequences having nonnegative jth differences.
Entrata in Redazione il 18 giugno 1970. 相似文献
38.
Donald A. Dawson 《Stochastic Processes and their Applications》1980,10(1):1-31
Stochastic partial differential equations with polynomial coefficients have many applications in the study of spatially distributed populations in genetics, epidemiology, ecology, and chemical kinetics. The purpose of this paper is to describe some methods for investigating the qualitative behavior of spatially homogenous solutions of nonlinear stochastic partial differential equations. The stability and bifurcation of solutions, as well as the behavior near critical points are discussed. The methods employed are the nonlinear Markov approximation, moment density inequalities for identifying invariant sets of behaviors and rescaling and quasi-linearization around critical points. 相似文献
39.
A model is presented to describe the propagation of positive corona streamers in the low field region of a non-uniform field gap in atmospheric air. It has been assumed that the growth is a property solely of the streamer tip, uninfluenced by the channel conductivity. Calculations from the model indicate that the criterion for propagation of a streamer in zero external field is that the number of ions in the tip be 108 and the radius about 3×10?3 cm. It is proposed that the streamer ceases to propagate as a result of the loss of energy of the tip due to the formation of ion pairs in the channel. The results of previous experimental observations of streamers are compared with calculations derived from the model, and a prediction from the model of the lifetime of streamers after voltage removal is discussed. 相似文献
40.
The general question of the properties of light, neutral colored spin-zero particles in QCD is examined. Models with spontaneous breaking of QCD at very large distances, such as that of DeRújula, Giles, and Jaffe and the SO(3) scheme of Slansky, Goldman, and Shaw, require such light colored Higgs scalars. These scalars will form color-singlet hadronic bound states at short distances and estimates are given of bound state masses, decay widths, and production rates in processes such as ψ → γ + X within the MIT bag model. The resulting states are expected in the mass neighborhood ~1.5 GeV and should resemble glueballs. 相似文献