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571.
572.
We report on the first measurement of the F(2) structure function of the neutron from the semi-inclusive scattering of electrons from deuterium, with low-momentum protons detected in the backward hemisphere. Restricting the momentum of the spectator protons to ?100 MeV/c and their angles to ?100° relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The F(2)(n) data collected cover the nucleon-resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65相似文献   
573.
Pyridazino[3,4-h]psoralens and pyridazino[3,4-j]angelicins are prepared in good yield from resorcinols through a direct, easy and generally applicable synthetic route. The key step in this route is the inverse electron-demand Diels-Alder reaction between linear or angular furocoumarins and 3,6-bis(methoxycarbonyl)-1,2,4,5-tetrazine to give the dicarboxymethylated tetracycles. The ester group in the peri position with respect to the oxygen in the furan ring can be regioselectively transformed to give primary or secondary amides. Similarly, the two ester groups in the tetracycle can be transformed in a high-efficiency process to give bis-amides that can be either symmetrical (from the same amine) or unsymmetrical (from two different amines).  相似文献   
574.
The problem is addressed of establishing the satisfiability of prenex formulas involving a single universal quantifier, in diversified axiomatic set theories. A rather general decision method for solving this problem is illustrated through the treatment of membership theories of increasing strength, ending with a subtheory of Zermelo-Fraenkel which is already complete with respect to the ?*? class of sentences. NP-hardness and NP-completeness results concerning the problems under study are achieved and a technique for restricting the universal quantifier is presented. Mathematics Subject Classification: 03B25, 03E30.  相似文献   
575.
5-Methoxy-4-methyl-2H-benzofuro[3,2-g]benzo-1-pyran-2-one was synthesized and its crystal structure was determined and compared with the optimal conformation arrived at by MM and AM1 theoretical calculations. The latter indicated that the tetracyclic skeleton is planar with total length (C2–C8) 9.23 å, and that the line joining the conters of the terminal-benzene and furan rings makes an angle of 30.5 with that joining the centers of the three rings of psoralen. The electronic properties of the reactive double bond in the pyran ring of1 are similar to those of simpler psoralens.  相似文献   
576.
577.
Polycyclic aromatic hydrocarbons (PAHs) are employed to model the Raman features that are generally associated with sp(2) nanostructures in carbon materials or with disorder and defects in graphitic materials. To this end molecular parameters (geometry changes upon electronic excitation, vibrational normal modes, and displacement parameters) are computed with semiempirical quantum-chemical methods for a series of PAHs ranging from 6 to 384 carbon atoms, and Raman intensities are evaluated according to Albrecht's formalism restricted to the A term. The computed preresonance and resonance Raman intensities are compared with available experimental data for hexa-peri-hexabenzocoronene and for pyrene. For the latter compound, simulations carried out at semiempirical and ab initio levels of theory are shown to be of comparable quality. Finally, the collection of displacement parameters computed for the sample of conjugated molecules is used to model the effect of disorder and defects in the Raman response of a carbon material containing sp(2) islands. It is shown that the computed D-band frequency dispersion, with respect to excitation wavelength, reproduces closely the experimental data measured for sp(2) hybridized carbon materials.  相似文献   
578.
Inorganic-organic composite and hybrid films find widespread applications for the development of functional materials. Polymer matrices with embedded inorganic fillers, nanoparticles or clusters are particularly appealing for optical, electronic, dielectric and magnetic applications. In particular, the development of hybrid layers with tailored dielectric properties represents a key issue in many technological fields.In this framework, poly(methyl methacrylate) (PMMA), due to its outstanding chemico-physical properties, represents a particularly suitable polymer component for the embedding of both microscopic and nanoscopic functional inorganic fillers. The wide use of such a matrix has to be traced back to the favourable combination of chemical and physical properties and easy processing. In this review, the main features and properties of PMMA, with a particular focus on dielectric ones, are firstly briefly described. Selected examples to illustrate the state-of-the art of its corresponding use as dielectric matrix are given and several examples are provided and surveyed.Finally, three case studies concerning PMMA-based hybrid films, produced for very different application fields, are described and discussed. The first example deals with the entrapment of micrometric zinc sulphide powders in PMMA, which acts as a host matrix for the electroluminescent particles in thick film-based Alternate Current Powder Electroluminescent Lamps (ACPELs). The second example describes the preparation of low-κ inorganic-organic hybrid dielectric films based on a PMMA-polyvinylchloride(PVC) blend and a hydrophobic silica powder functionalised on the surface with trimethylsiloxane groups (m-SiO2). The composition of the investigated materials is [(PMMA)x(PVC)y]/(m-SiO2)z with z ranging from 0 to 38.3 wt% and x = y = (100 − z)/2. The third case concerns the use of PMMA as a matrix to embed zirconium oxoclusters through the formation of covalent bonds. The obtained material, characterised by a dielectric constant value remarkably lower (1.93 at 1 kHz and 25 °C) than in pristine PMMA (3.0 at 1 kHz and 25 °C), appears as very appealing for the development of microelectronic devices based on low dielectric constant polymer films such as, for instance, field-effect transistor (FET).These three cases are paradigms of three different approaches to composite and hybrid materials based on the embedding of particles in PMMA polymer matrix.  相似文献   
579.
580.
The salt [K(18-crown-6)][Mn(H2O)2Cr(ox)3.0.5(18-crown-6) (1) has been prepared and structurally and magnetically characterized. It crystallizes in the P2(1)/c space group [a = 21.011(2) A, b = 11.265(2) A, c = 15.748(3) A, beta = 105.952(6) degrees , V = 3584(1) A3, and Z = 4] and contains [Mn(H2O)2Cr(ox)3]infinity chains connected through hydrogen bonding to form 2D anionic networks. The magnetic exchange is ferromagnetic [J = +2.23(2) cm(-1)] in the chain and also in between chains, reaching bulk ferromagnetic ordering below 3.5 K.  相似文献   
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