全文获取类型
收费全文 | 4315篇 |
免费 | 112篇 |
国内免费 | 5篇 |
专业分类
化学 | 2798篇 |
晶体学 | 56篇 |
力学 | 58篇 |
数学 | 436篇 |
物理学 | 1084篇 |
出版年
2021年 | 33篇 |
2020年 | 35篇 |
2019年 | 38篇 |
2016年 | 58篇 |
2015年 | 66篇 |
2014年 | 73篇 |
2013年 | 142篇 |
2012年 | 137篇 |
2011年 | 163篇 |
2010年 | 111篇 |
2009年 | 82篇 |
2008年 | 142篇 |
2007年 | 137篇 |
2006年 | 143篇 |
2005年 | 141篇 |
2004年 | 129篇 |
2003年 | 129篇 |
2002年 | 120篇 |
2001年 | 116篇 |
2000年 | 117篇 |
1999年 | 61篇 |
1998年 | 43篇 |
1997年 | 47篇 |
1996年 | 86篇 |
1995年 | 62篇 |
1994年 | 81篇 |
1993年 | 61篇 |
1992年 | 61篇 |
1991年 | 59篇 |
1990年 | 58篇 |
1989年 | 46篇 |
1988年 | 50篇 |
1987年 | 64篇 |
1986年 | 58篇 |
1985年 | 58篇 |
1984年 | 63篇 |
1983年 | 48篇 |
1982年 | 52篇 |
1981年 | 60篇 |
1980年 | 42篇 |
1979年 | 46篇 |
1978年 | 53篇 |
1977年 | 56篇 |
1976年 | 58篇 |
1975年 | 57篇 |
1974年 | 57篇 |
1973年 | 42篇 |
1968年 | 27篇 |
1967年 | 24篇 |
1957年 | 30篇 |
排序方式: 共有4432条查询结果,搜索用时 15 毫秒
991.
On the Identity of a so‐called Ammonium Carbonate Sample Commercial samples of so‐called “ammionum carbonate” are shown to contain ammonium carbamate rather than ammonium carbonate. In fact samples may contain varying quantities of α‐ and β‐(NH4)(CO2NH2), of which only the α‐phase is reported in the literature. Mixtures of both phases tend to leak the volatile α‐phase and to react with moisture to form (NH4)(HCO3). The crystal structure of the new β‐(NH4)(CO2NH2) is refined from X‐ray powder data. 相似文献
992.
The [Fe(II)(Cys)(4)](2-) site of rubredoxin from Clostridium pasteurianum (Rd(red)) has been studied by M?ssbauer spectroscopy in both purified protein and whole cells of Escherichia coli overproducing it. Excellent fits were obtained to an S = 2 spin Hamiltonian for D = 5.7(3) cm(-1), E/D = 0.25(2), delta = 0.70(3) mm/s, DeltaE(Q) = -3.25(2) mm/s, eta = 0.75(5), A(x) = -20.1(7) MHz, A(y) = -11.3(2) MHz, and A(z) = -33.4(14) MHz. These parameters were analyzed with crystal-field theory for the (5)D manifold of iron(II), revealing a d(z(2)) orbital ground state that is admixed by approximately 0.21 d(x(2) - y(2)). The spin-Hamiltonian parameters are consistent within the (5)D theory, apart from the zero-field splitting parameter, D. This problem was solved by extending the crystal-field treatment with spin-orbit coupling to spin-triplet d-d excited states of the iron. Theoretical estimates are given for the spin-triplet (D(T)) and spin-quintet contributions (D(Q)) to D based on excitation energies derived from time-dependent density functional theory, TD-DFT. The computational results were interpreted in terms of crystal-field theory, yielding the Racah parameters B = 682 cm(-1) and C = 2583 cm(-1). The theoretical analysis gives the relative magnitudes D(Q):D(T):D(ss) = 51%: 42%:7% (D(ss) originates from spin-spin interaction). The DFT analysis corroborates the pivotal role of the torsion angles (omega(i)) of the C-S(i) bonds in shaping the electronic structure of the iron(II) site. Rd(red) in overexpressing whole cells accounts for 60% of the M?ssbauer absorption. The Rd(red) spectra from whole cells are virtually identical to those of the purified protein. By using the theoretical omega dependence of the spin Hamiltonian parameters, the torsions for Rd(red) in whole cells and purified protein samples are estimated to be the same within 2 degrees. These findings establish M?ssbauer spectroscopy as a structural tool for investigating iron sites in whole cells. 相似文献
993.
The phase behavior of the system water, n-heptane and the nonionic surfactant Igepal® CA520 has been studied by visual inspection, high-performance liquid chromatography, polarizing microscopy and freeze-fracture electron microscopy. The phase diagram at 25?°C contains two large homogeneous microemulsion phases, an extended region of a lamellar liquid crystalline structure and some two- and three-phase regions. The oil-rich part of the phase diagram has been investigated by static and dynamic light scattering in order to examine the behavior of the collective diffusion coefficient and the scattering intensity in the presence of increasing concentrations of water, starting from the binary system of n-heptane and Igepal® CA520. The results suggested that at a very low water content the aggregates of the microemulsion are small. With the exception of this region the structure is bicontinuous. 相似文献
994.
Synthesis, Structure and Thermolysis of NH4[Re3Br10] NH4[Re3Br10] crystallizes as dark brown single crystals upon slow cooling of a hot, saturated hydrobromic-acid solution of [Re3Br9(H2O)2] after the addition of NH4Br. The crystal structure (monoclinic, C2/m (Nr. 12); Z = 4; a = 1461.6(7), b = 1 085.6(4), c = 1030.3(7) pm, β = 92.63(4)°, Vm = 245.9(4)cm3/mol; R = 0.097, Rw = 0.043) contains [Re3Br12]? units that share two common edges. These chains run along [010] and are held together by NH4+ ions. Each NH4+ is surrounded by eight Br? from four different chains. The first step of the thermal decomposition at 290°C is the disproportionation to ReBr3 (ReCl3 type), rhenium metal and (NH4)2[ReBr6]. Secondly, the internal reduction of (NH4)2[ReBr6] at 390°C to rhenium metal takes place. 相似文献
995.
Giffin D. Jones W. C. Meyer N. B. Tefertiller D. C. MacWilliams 《Journal of polymer science. Part A, Polymer chemistry》1970,8(8):2123-2138
2-Chloromethylbutadiene has been converted to quaternary ammonium and sulfonium monomers which have been polymerized at room temperature. They show a very great tendency to dimerize on heating in water solution. The aqueous quaternary monomer dimerized 25 times as fast as the aqueous sulfonium monomer and nearly 105 times as fast as neat isoprene at 50°C. The quaternary monomer dimerized with itself in a water solution to which 2-hydroxymethylbutadiene has been added as an example of a nonionic diene. The latter monomer did not dimerize rapidly in water, nor did 2-aminomethylbutadiene. The hydrochlorides of 2-aminomethylbutadiene and 2-dimethylaminomethylbutadiene dimerized at rates comparable to that of the sulfonium monomer. Poly 2-chloromethylbutadiene contains reactive chlorine except for the structure resulting from the minor extent of 1,2 addition. Water-soluble derivatives have been made from it with nucleophilic tertiary amines and sulfides. Cationic polymers are substantive to paper pulp, and the sulfonium polymers can be cured in paper to give improved wet strength. 相似文献
996.
H.-Jürgen Meyer 《无机化学与普通化学杂志》1994,620(1):81-84
The Electronic Structure of Nb3Br8 The bonding in Nb3Br8 was investigated with the aid of extended Hückel and molecular orbital calculations to gain some understanding concerning stability and possible chemical altering. Seven electrons occupy metal—metal bonding states of a Nb3Br8 unit. Six of them are responsible for strong σ bonds within the Nb3 cluster. One to two additional electrons may be incorporated to occupy a1 orbitals, which slightly increase the bonding in a Nb3 cluster, rather than between adjacent clusters. 相似文献
997.
998.
Investigation on the Thermal Decomposition of Ammonium Yttrium Halides. III. Ammonium Yttrium Bromide, (NH4)3YBr6 The decomposition equilibria of NH4Br and (NH4)3YBr6 were determined by total pressure measurements. It was shown by high temperature X-ray patterns and DTA that (NH4)3YBr6 and YBr3 show phase transitions in the measured temperature region. An endothermic transition of YBr3 starts near 300°C and is very slow. By interpretation of the thermal decomposition of (NH4)3YBr6, the enthalpy of formation, and the standard entropy was derived. In the system NH4Br/YBr3 only the described phase (NH4)3YBr6 exists. 相似文献
999.
1000.
Julius Meyer 《Fresenius' Journal of Analytical Chemistry》1914,53(3):145-154
Zusammenfassung Das Eindampfen von Lösungen der selenigen Säure auf dem Wasserbade führt infolge der bei dieser Temperatur schon merklichen Flüchtigkeit des Selendioxyds zu Selenverlusten, die bei Anwesenheit von Salzsäure sehr gross werden und durch den häufig empfohlenen Zusatz von Kalium- oder Natriumchlorid nicht verhindert werden können. Die störende Anwesenheit von Salpetersäure lässt sich durch Zusatz von Ammoniak und etwas Salzsäure fast unschädlich machen. Zur Reduktion der Lösungen von seleniger Säure eignet sich am besten Hydrazinhydrat, bei dessen Anwendung aber auf das Auffangen des verstäubten Selens Rücksicht genommen werden muss. 相似文献