全文获取类型
收费全文 | 3130篇 |
免费 | 42篇 |
国内免费 | 3篇 |
专业分类
化学 | 2232篇 |
晶体学 | 11篇 |
力学 | 42篇 |
数学 | 480篇 |
物理学 | 410篇 |
出版年
2018年 | 26篇 |
2017年 | 26篇 |
2016年 | 33篇 |
2015年 | 33篇 |
2014年 | 31篇 |
2013年 | 84篇 |
2012年 | 80篇 |
2011年 | 104篇 |
2010年 | 77篇 |
2009年 | 77篇 |
2008年 | 104篇 |
2007年 | 119篇 |
2006年 | 99篇 |
2005年 | 111篇 |
2004年 | 110篇 |
2003年 | 88篇 |
2002年 | 94篇 |
2001年 | 53篇 |
2000年 | 76篇 |
1999年 | 66篇 |
1998年 | 50篇 |
1997年 | 58篇 |
1996年 | 49篇 |
1995年 | 47篇 |
1994年 | 39篇 |
1993年 | 65篇 |
1992年 | 54篇 |
1991年 | 41篇 |
1990年 | 29篇 |
1989年 | 35篇 |
1988年 | 42篇 |
1987年 | 35篇 |
1986年 | 50篇 |
1985年 | 61篇 |
1984年 | 41篇 |
1983年 | 31篇 |
1982年 | 38篇 |
1981年 | 50篇 |
1980年 | 40篇 |
1979年 | 54篇 |
1978年 | 41篇 |
1977年 | 42篇 |
1976年 | 37篇 |
1975年 | 31篇 |
1974年 | 36篇 |
1973年 | 23篇 |
1972年 | 25篇 |
1971年 | 23篇 |
1970年 | 24篇 |
1934年 | 21篇 |
排序方式: 共有3175条查询结果,搜索用时 968 毫秒
61.
(PPh4)2[WO2Cl3]2 · 2 CH2Cl2. Synthesis, Vibrational Spectrum, and Crystal Structure Depending on the stoichiometry and the solvent, dichloromethane or 1.2-dichloroethane, WO2Cl2 reacts with tetraphenylphosphonium chloride affording (PPh4)2[WO2Cl4] or (PPh4)2[WO2Cl3]2, respectively. Both compounds are easily soluble in dichloromethane, from which they can be crystallized under incorporation of two molecules CH2Cl2 per formula unit. The crystalline compounds have been characterized by their IR and Raman spectra. According to the X-ray crystal structure analysis, (PPh4)2[WO2Cl3]2 · 2 CH2Cl2 crystallizes in the triclinic space group P1 with one formula unit per unit cell (986 independent observed reflexions, R = 0.061). Lattice constants: a = 1100.2, b = 1116.9, c = 1238.4 pm, = 69.40, = 80.46 and = 85.62°. The crystals consist of PPh4⊕ ions, centrosymmetric [WO2Cl3]22? anions and CH2Cl2 molecules. In the anions, the tungsten atoms are linked via two oxo bridges with WO distances of 184 and 252 pm. The distorted octahedral coordination around each tungsten atom is completed by three terminal chloro and one terminal oxo ligand (WO bond length 166 pm), the latter being in trans position to the longer WO bridging bond. (PPh4)2[WO2Cl4] · 2 CH2Cl2 also forms triclinic crystals that are isotypic with (PPh4)2[WOCl5] · 2 CH2Cl2 and in which the anions must have orientational disorder. 相似文献
62.
The marine triquinane sesquiterpene Δ9(12)-capnellene () was synthesised from 2,2,5-trimethyl-5-hexenal () by a sequence of all synthetic operations in 5% overall yield. The key steps 3 → 4 → 5 → 6 involve two intramolecular type-I-“Mg-ene” processes. 相似文献
63.
Sun G Kappl M Pakula T Kremer K Butt HJ 《Langmuir : the ACS journal of surfaces and colloids》2004,20(19):8030-8034
Forces across polymer melts are poorly understood despite their importance for adhesion and fabricating composite materials. Using an atomic force microscope (AFM), this interaction was measured for poly(dimethyl siloxane) (PDMS). The structure of the polymer at the surface changed during the first approximately 10 h. Afterward, short-range attractive forces were observed with short-chain PDMS (M(w) = 4200 g/mol). Using PDMS with a molecular weight (M(w) = 18 000 g/mol) above the entanglement limit, we measured a monotonically decaying repulsive force, which indicates that a quasi-immobilized layer had formed at the solid surface. Due to the small radius of curvature of the tip, forces could be measured in equilibrium. 相似文献
64.
A total synthesis of the antitumor sesquiterpene coriolin ( 9 ; racemic) in 11 steps from 3,3,6-trimethylbicyclo[2.2.2]oct-7-ene-2,5-dione ( 2a / 2b ) is described (yield 2a / 2b → 8: 28%). The sequence is unprecedentedly short and avoids difficult separation problems. The key step in the scheme is a novel facet of oxadi-π-methane photochemistry, i.e., the steering by subtle steric effects of the β,γ-unsaturated ?-diketone to undergo a regioselective photorearrangement involving one β,γ-enone partial chromophore. Furthermore, the overall phototransformation, which can be carried out at unusually high concentrations (≥20% solutions), involves also a Norrish type I process equilibrating the two epimeric starting enediones 2a and 2b in favour of the desired stereoisomer. 相似文献
65.
66.
67.
Reaction of Trithiazyl Chloride with Titanium Tetrachloride. Crystal Structure of (S4N5)2[Ti2Cl10] In the reaction of trithiazyl chloride with titanium tetrachloride, chlorine is abstracted and the brown-yellow adduct TiCl4(N2S2) is obtained. In this compound — according to its i.r. spectrum — a N2S2 ring is bonded to the titanium via the N atoms, thus forming a polymer. As a by-product, brown crystalline (S4N5)2[Ti2Cl10] forms. Its crystal structure was determined and refined with X-ray diffraction data (R = 0.042 for 812 reflexions). It crystallizes in the monoclinic space group P21/c with two formula units per unit cell. The lattice constants are a = 670, b = 1 633, c = 1108 pm, β = 97.24°. The structure consists of S4N5⊕ cations, which are nearly equal to those in [S4N5]Cl, and [Ti2Cl10]2? anions, which are nearly identical with those in (PCl4)2[Ti2Cl10]. 相似文献
68.
Atom transfer radical polymerization conditions with copper(I) bromide/2,2′-bipyridine (Cu/2,2′-bpy) as the catalyst system were employed for the homopolymerization and random copolymerization of 1-phenoxycarbonyl ethyl methacrylate (PCMA) with methyl methacrylate (MMA). Temperature studies indicated that the polymerizations occurred smoothly in bulk at 110 °C. Poly(PCMA)(polydispersity index=1.27) homopolymer was characterized and then used as macroinitiator for increasing its molecular weight. The homopolymerization of PCMA was also carried out under free radical conditions using 2,2′-azobisisobutyronitrile as an initiator.The monomer and polymers were characterized by FT-IR and 1H and 13C-NMR techniques. The glass transition temperatures, the solubility parameters and average-molecular weights of the polymers were determined. Thermal stabilities of the polymers were given as compared with each other by using TGA curves. Thermal degradation products of poly(PCMA)s obtained by ATRP and free radical polymerization were compared with each other by using 1H-NMR technique. 相似文献
69.
Morcombe CR Gaponenko V Byrd RA Zilm KW 《Journal of the American Chemical Society》2005,127(1):397-404
(13)C CPMAS NMR has been investigated in application to protein samples with a variety of deuteration patterns. Samples were prepared with protons in either all hydrogen positions, only in the exchangeable sites, or in the exchangeable sites plus select methyl groups. CP dynamics, T(1) relaxation times, and (13)C line widths have been compared. Using ubiquitin as a model system, reasonable (1)H-(13)C CP transfer is observed for the extensively deuterated samples. In the absence of deuterium decoupling, the (13)C line widths observed for the deuterated samples are identical to those observed for the perprotio samples with a MAS rate of 20 kHz. Extensive deuteration has little effect on the T(1) of the exchangeable protons. On the basis of these observations, it is clear that there are no substantive compromises accompanying the use of extensive deuteration in the design of (1)H, (15)N, or (13)C solid-state NMR methods. 相似文献
70.
Wally L. Chang Kurt C. Frisch Kaneyoshi Ashida 《Journal of polymer science. Part A, Polymer chemistry》1989,27(11):3619-3635
Two component interpenetrating networks (IPNs) of the SIN type (simultaneous interpenetrating networks), composed of a polyisocyanurate network and a star-shaped nylon 6, were made. The overall polymerization rates and physical properties for reaction injection molding (RIM) have been studied by the quasi-adiabatic process. In order to model the actual rapid molding conditions, time versus temperature reaction profiles were measured, and the relative rates of polymerization subsequently determined from these data. 相似文献