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61.
The intrinsic viscosities of styrene-isoprene two- and three-block copolymers have been studied in a number of solvents as a function of molecular weight, composition and temperature. The behaviour of the three-block copolymers is similar in all points to that of the two-block. No conformational transition was observed between 10 and 60°. The intrinsic viscosities measured at 35° are close to the weighted average of the corresponding homopolymers of equal molecular weight. The Stockmayer-Fixman interaction parameters B are also close to the weighted average of the corresponding homopolymers. These results seem to indicate a structure with a limited number of heterocontacts. 相似文献
62.
Nga Sze Ieong Author Vitae Author Vitae 《Journal of organometallic chemistry》2008,693(5):802-807
Although extensive previous studies have shown that strained [1]ferrocenophanes predominantly undergo ring-opening chemistry at the bridging atom E-cyclopentadienyl (E-Cp) bond, recent reports have highlighted that reactivity at the Fe-Cp bond can also occur, especially on photoactivation. We provide an overview of recent results from our group and those of other researchers. In addition, the development of photocontrolled living polymerizations of sila[1]ferrocenophanes using Fe-Cp bond cleavage chemistry is described. 相似文献
63.
An analytical solution for buckling of an eccentrically stiffened sandwich truncated conical shell is investigated. The shell consists of two functionally graded material (FGM) coating layers and a core layer which are metal or ceramic subjected to an axial compressive load and an external uniform pressure. Shells are reinforced by stringers and rings, in which the material properties of shells and stiffeners are graded in the thickness direction following a general sigmoid law distribution. Two models of coated shell-stiffener arrangements are investigated. The change of the spacing between stringers in the meridional direction is taken into account. A couple set of three-variable-coefficient partial differential equations in terms of displacement components are solved by the Galerkin method. A closed-form expression for determining the buckling load is obtained. The numerical examples are presented and compared with previous works. 相似文献
64.
Jean Pascal Eloundou Ohandja Ayina Hippolyte Ntede Nga Jean Franois Gerard Jean Pierre Pascault Gisle Boiteux Grard Seytre 《Journal of Polymer Science.Polymer Physics》1998,36(16):2911-2921
Three reactive epoxy–amine systems based on diglycidyl ether of bisphenol A (DGEBA) with 4,4′-diaminodiphenylsulfone (DDS), 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA), and 4,4′-methylenebis [2,6-diethylaniline] (MDEA), were studied during isothermal curings at 140 and 160°C. The simultaneous kinetic and dielectric studies allow to express conductivity, σ, in terms of conversion, x, and of glass transition temperature, Tg. The conductivity, σ0, of the initial monomer mixture and, σ∞ of the fully cured network are measured. It is found that:
- The glass transition temperature, Tg, versus conversion, x, curves follows the equation of Di Benedetto modified by Pascault and Williams
- There exists a linear relation between log σ/log σ0 and Tg.
65.
Multinuclear Diffusion NMR Spectroscopy and DFT Modeling: A Powerful Combination for Unraveling the Mechanism of Phosphoester Bond Hydrolysis Catalyzed by Metal‐Substituted Polyoxometalates
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Dr. Thi Kim Nga Luong Prof. Dr. Pavletta Shestakova Dr. Tzvetan T. Mihaylov Dr. Gregory Absillis Prof. Dr. Kristine Pierloot Prof. Dr. Tatjana N. Parac‐Vogt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(11):4428-4439
A detailed reaction mechanism is proposed for the hydrolysis of the phosphoester bonds in the DNA model substrate bis(4‐nitrophenyl) phosphate (BNPP) in the presence of the ZrIV‐substituted Keggin type polyoxometalate (Et2NH2)8[{α‐PW11O39Zr(μ‐OH) (H2O)}2] ? 7 H2O (ZrK 2:2) at pD 6.4. Low‐temperature 31P DOSY spectra at pD 6.4 gave the first experimental evidence for the presence of ZrK 1:1 in fast equilibrium with ZrK 2:2 in purely aqueous solution. Moreover, theoretical calculations identified the ZrK 1:1 form as the potentially active species in solution. The reaction intermediates involved in the hydrolysis were identified by means of 1H/31P NMR studies, including EXSY and DOSY NMR spectroscopy, which were supported by DFT calculations. This experimental/theoretical approach enabled the determination of the structures of four intermediate species in which the starting compound BNPP, nitrophenyl phosphate (NPP), or the end product phosphate (P) is coordinated to ZrK 1:1. In the proposed reaction mechanism, BNPP initially coordinates to ZrK 1:1 in a monodentate fashion, which results in hydrolysis of the first phosphoester bond in BNPP and formation of NPP. EXSY NMR studies showed that the bidentate complex between NPP and ZrK 1:1 is in equilibrium with monobound and free NPP. Subsequently, hydrolysis of NPP results in P, which is in equilibrium with its monobound form. 相似文献
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Dennis Rottschäfer Nga Kim T. Ho Beate Neumann Dr. Hans‐Georg Stammler Dr. Maurice van Gastel Dr. Diego M. Andrada Priv.‐Doz.Dr. Rajendra S. Ghadwal 《Angewandte Chemie (International ed. in English)》2018,57(20):5838-5842
Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C6H4)(IPr)] and [(IPr)(C6H4)2(IPr)] ( 4 and 5 , respectively; IPr=C{N(2,6‐iPr2C6H3)}2CHCH), are reported. In a nickel‐catalyzed double carbenylation of 1,4‐Br2C6H4 and 4,4′‐Br2(C6H4)2 with IPr ( 1 ), [(IPr)(C6H4)(IPr)](Br)2 ( 2 ) and [(IPr)(C6H4)2(IPr)](Br)2 ( 3 ) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC8. Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet?triplet energy gap ΔES?T of 10.7 kcal mol?1, whereas 4 features more quinoidal character with a rather large ΔES?T of 25.6 kcal mol?1. In view of the low ΔES?T, 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character. 相似文献
69.
The paper investigates DC programming and DCA for both modeling discrete portfolio optimization under concave transaction costs as DC programs, and their solution. DC reformulations are established by using penalty techniques in DC programming. A suitable global optimization branch and bound technique is also developed where a DC relaxation technique is used for lower bounding. Numerical simulations are reported that show the efficiency of DCA and the globality of its computed solutions, compared to standard algorithms for nonconvex nonlinear integer programs. 相似文献
70.
Dr. Kyung Seob Song Patrick W. Fritz Dr. Daniel F. Abbott Lok Nga Poon Cristiano M. Caridade Dr. Felipe Gándara Prof. Dr. Victor Mougel Prof. Dr. Ali Coskun 《Angewandte Chemie (International ed. in English)》2023,62(38):e202309775
Phthalocyanines (PCs) are intriguing building blocks owing to their stability, physicochemical and catalytic properties. Although PC-based polymers have been reported before, many suffer from relatively low stability, crystallinity, and low surface areas. Utilizing a mixed-metal salt ionothermal approach, we report the synthesis of a series of metallophthalocyanine-based covalent organic frameworks (COFs) starting from 1,2,4,5-tetracyanobenzene and 2,3,6,7-tetracyanoanthracene to form the corresponding COFs named M-pPPCs and M-anPPCs, respectively. The obtained COFs followed the Irving–Williams series in their metal contents, surface areas, and pore volume and featured excellent CO2 uptake capacities up to 7.6 mmol g−1 at 273 K, 1.1 bar. We also investigated the growth of the Co-pPPC and Co-anPPC on a highly conductive carbon nanofiber and demonstrated their high catalytic activity in the electrochemical CO2 reduction, which showed Faradaic efficiencies towards CO up to 74 % at −0.64 V vs. RHE. 相似文献