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191.
The reaction of styrene oxides with the malononitrile anion leads to the formation of 2-amino-3-eyano-5-aryl-4,5-dihydrofurans (III), while a similar reaction with β,β-dimethylstyrene oxides leads to 2-amino-3-cyano-4-aryl-5,5-dimethyI-4,5-dihydrofurans (V). Therefore steric factors may play a significant role in the site of attack of the malononitrile anion on the oxide. Compounds of type V are more conveniently obtained from the cyanolaetones (VI) by rearrangement and dehydration. 相似文献
192.
193.
Using the plane-wave approximation we derive analytical expressions for both the real and imaginary parts of the polarization potential arising from nuclear inelastic scattering. These potentials and the resulting elastic and inelastic cross sections are compared with exact coupledchannel calculations for 13C on 40Ca at 68 MeV. The agreement, for the most part, is good. We also briefly discuss the numerical non-local potentials for this system and the imaginary polarization potential for 16O on 208Pb at 104 MeV. 相似文献
194.
We consider the cosmological constraints on unstable particles with masses O(mW). Constraints from the energy density during primordial nucleosynthesis, from subsequent entropy generation, and from perturbations of the microwave background are not as restrictive as bounds from the dissociation of light elements. Most restrictive is the need to avoid over-production of D and 3He through the photo-dissociation of 4He. When applied to gravitinos, this bound imposes an upper limit of O(108) GeV on the reheating temperature after inflation, which could create problems for baryosynthesis in supersymmetric theories. These problems can be avoided if the photino is heavier than the gravitino. 相似文献
195.
John Ellis Azar Mustafayev Keith A. Olive 《The European Physical Journal C - Particles and Fields》2010,69(1-2):219-233
In the context of phenomenological models in which the soft supersymmetry-breaking parameters of the MSSM become universal at some unification scale, M in, above the GUT scale, M GUT, it is possible that all the scalar mass parameters m 0, the trilinear couplings A 0 and the bilinear Higgs coupling B 0 vanish simultaneously, as in no-scale supergravity. Using these no-scale inputs in a renormalisation-group analysis of the minimal supersymmetric SU(5) GUT model, we pay careful attention to the matching of parameters at the GUT scale. We delineate the region of M in, m 1/2 and tan?β where the resurrection of no-scale supergravity is possible, taking due account of the relevant phenomenological constraints such as electroweak symmetry breaking, m h ,b→s γ, the neutralino cold dark matter density Ω χ h 2 and g μ ?2. No-scale supergravity survives in an L-shaped strip of parameter space, with one side having m 1/2?200 GeV, the second (orthogonal) side having M in?5×1016 GeV. Depending on the relative signs and magnitudes of the GUT superpotential couplings, these may be connected to form a triangle whose third side is a hypotenuse at larger M in, m 1/2 and tan?β, whose presence and location depend on the GUT superpotential parameters. We compare the prospects for detecting sparticles at the LHC in no-scale supergravity with those in the CMSSM and the NUHM. 相似文献
196.
We present a direct experimental comparison of the helium-surface interaction potential for two unreconstructed metal surfaces. We analyze phase shifts in helium atom scattering from a nanostructured bimetallic surface to yield the relative shape and position of an adsorbate's potential with respect to the reference defined by the substrate. In our prototype system, submonolayer growth of Ni on Cu(100), the He-Ni/Cu(100) potential has an attractive well that is 1.6+/-0.4 meV shallower, and a repulsive wall 0.11+/-0.03 A closer to the ion cores, compared to the He-Cu(100) potential. Our observations provide a ready test of state-of-the-art theoretical calculations. 相似文献
197.
Ellis DE Terra J Warschkow O Jiang M González GB Okasinski JS Bedzyk MJ Rossi AM Eon JG 《Physical chemistry chemical physics : PCCP》2006,8(8):967-976
Characterization of lead substitution for calcium in hydroxyapatite (CaHA) is carried out, using experimental techniques and Density Functional theoretical (DFT) analyses. Theoretical modeling is used to obtain information of the Pb chemical environment for occupancy at either Ca(I) or Ca(II) sites of CaHA. Effects of the larger ionic radius of Pb(+2) compared to Ca(+2) are apparent in embedded cluster calculations of local chemical bonding properties. DFT periodic planewave pseudopotential studies are used to provide first-principles predictions of local structural relaxation and site preference for Pb(x)Ca(10-x)HA over the composition range x< or = 6. General characteristics of the polycrystalline material are verified by X-ray diffraction and FTIR analysis, showing the presence of a single phase of CaHA structure. A short range structure around lead is proposed in order to interpret the Pb L-edge EXAFS spectrum of the solid solution Ca(6.6)Pb(3.4)HA. In this concentration we observe that lead mainly occupies Ca(II) sites; the EXAFS fit slightly favors Pb clustering, while theory indicates the importance of Pb-Pb avoidance on site (II). 相似文献
198.
A new type of 2-D separation material was synthesized and studied. The material is suitable for 2-D chromatography utilizing both covalent and noncovalent interactions. The first dimension is boronate affinity chromatography, and the second dimension is RP chromatography (or vice versa). The polymeric media were prepared using p-vinylphenylboronic acid as the functional monomer. This monomer was selected due to the presence of the boronic acid group for the cis-diol/boronate interaction in boronate chromatography. Two crosslinkers were evaluated, namely ethylene glycol dimethacrylate and divinylbenzene. The crosslinker content was varied to maximize the polymer strength and the RP performance of the packed column. Several parameters were evaluated to define the optimum for polymer strength and column performance including crosslinker, porogen, initiator, and column-packing parameters. The polymer-based HPLC columns were successful in separating phenol, catechol, dimethylphthalate, and hydroquinone under RP conditions, and thus can be used as an RP HPLC column. The columns were also successful in separating catechol and adenosine under boronate chromatography conditions, and thus can be used as a boronate affinity column. Moreover, the two types of chromatography can be performed consecutively on the same column during one complete chromatographic run, making it a 2-D chromatography. Under these 2-D conditions, the catechol was separated from a mixture of phenol, catechol, dimethylphthalate, and hydroquinone; the adenosine ribonucleoside was separated from a mixture of adenosine ribonucleoside, adenosine deoxyribonucleoside, and uridine deoxyribonucleoside. This type of single-column 2-D HPLC eliminates the requirement of a complex and expensive multidimensional HPLC instrument and provides increased peak capacity for separation. 相似文献
199.
We revisit the constraints that the non-observation of ultra-high-energy photons due to the GZK cutoff can impose on models of Lorentz violation in photon propagation, following recent work by Maccione, Liberati and Sigl (2010) [arXiv:1003.5468] that carries further an earlier analysis by the present authors [J. Ellis et al., Phys. Rev. D 63 (2001) 12402, hep-th/0012216]. We argue that the GZK cutoff constraint is naturally evaded in the D-brane model of space–time foam presented recently by the present authors [J. Ellis et al., Phys. Lett. B 665 (2008) 412, arXiv:0804.3566], in which Lorentz-violating effects on photon propagation are independent of possible effects during interactions. We also note a novel absorption mechanism that could provide a GZK-like cutoff for photons in low-scale string models. 相似文献
200.