全文获取类型
收费全文 | 4325篇 |
免费 | 94篇 |
国内免费 | 24篇 |
专业分类
化学 | 3048篇 |
晶体学 | 47篇 |
力学 | 56篇 |
数学 | 205篇 |
物理学 | 1087篇 |
出版年
2023年 | 20篇 |
2021年 | 37篇 |
2020年 | 48篇 |
2019年 | 55篇 |
2018年 | 39篇 |
2017年 | 23篇 |
2016年 | 57篇 |
2015年 | 69篇 |
2014年 | 66篇 |
2013年 | 179篇 |
2012年 | 181篇 |
2011年 | 245篇 |
2010年 | 106篇 |
2009年 | 147篇 |
2008年 | 244篇 |
2007年 | 241篇 |
2006年 | 242篇 |
2005年 | 215篇 |
2004年 | 202篇 |
2003年 | 178篇 |
2002年 | 186篇 |
2001年 | 136篇 |
2000年 | 110篇 |
1999年 | 75篇 |
1998年 | 55篇 |
1997年 | 63篇 |
1996年 | 58篇 |
1995年 | 44篇 |
1994年 | 59篇 |
1993年 | 64篇 |
1992年 | 76篇 |
1991年 | 60篇 |
1990年 | 51篇 |
1989年 | 58篇 |
1988年 | 47篇 |
1987年 | 57篇 |
1986年 | 50篇 |
1985年 | 64篇 |
1984年 | 57篇 |
1983年 | 40篇 |
1982年 | 33篇 |
1981年 | 44篇 |
1980年 | 32篇 |
1979年 | 54篇 |
1978年 | 45篇 |
1977年 | 27篇 |
1976年 | 35篇 |
1975年 | 30篇 |
1974年 | 32篇 |
1973年 | 25篇 |
排序方式: 共有4443条查询结果,搜索用时 296 毫秒
151.
Knöevenagel-type reaction between diketene and aldehydes proceeded in the presence of Ti(O-i-Pr)4. This reaction proceeded via titanium enolate derived from Ti(O-i-Pr)4 and diketene. As for the stereoselectivity of the products, E-isomers were produced predominantly in the case of aromatic aldehydes. 相似文献
152.
Sadafumi NishiharaTomoyuki Akutagawa Tatsuo Hasegawa Shigeki FujiyamaToshikazu Nakamura Takayoshi Nakamura 《Journal of solid state chemistry》2002,168(2):661-667
Two polymorphs of monovalent [Ni(dmit)2]− (dmit2−=2-thioxo-1,3-dithiole-4,5-dithiolate) crystals A and B, (anilinium)(18-crown-6)[Ni(dmit)2], were prepared, and the structure and magnetic properties were investigated. In these crystals, the [Ni(dmit)2]− molecules form dimers, which arranged into chains between the supramolecular cation structure (anilinium)(18-crown-6). In crystal A, supramolecular cation formed a regular stack, inducing ladder structure of [Ni(dmit)2], whose magnetism had been well fitted by spin ladder equation with the spin gap of Δ=190 K. Crystal B is ca. 3% more densely packed compared to crystal A. Due to the dense packing, supramolecular cation stack is distorted, which prevented the intermolecular interaction between [Ni(dmit)2]− dimers in direction corresponds to the ladder-leg direction in crystal A. Reflecting the [Ni(dmit)2]− arrangement, crystal B showed a temperature dependence of magnetic susceptibility well reproduced by the singlet-triplet thermal activation model, whose antiferromagnetic exchange interaction (2J) was 140 K. 相似文献
153.
The title hydride and its deuteride were successfully synthesized. The crystal structure of the deuteride was determined by time-of-flight neutron powder diffraction. BaAlD(5) crystallizes with a new orthorhombic structure in space group Pna2(1) (No. 33), cell parameters a = 9.194(1) A, b = 7.0403(9) A, and c = 5.1061(6) A, Z = 4. BaAlH(5) is the first example that contains one-dimensional zigzag chains of [AlH(6)] along the crystallographic c axis. 相似文献
154.
A convenient and one-pot synthetic method of lanthanide thiolate compounds was developed. An excess of metallic samarium, europium, and ytterbium directly reacted with diaryl disulfides in THF to give selectively Ln(II) thiolate complexes, [Ln(SAr)(&mgr;-SAr)(thf)(3)](2) (1, Ln = Sm; 2, Ln = Eu; Ar = 2,4,6-triisopropylphenyl), Yb(SAr)(2)(py)(4) (3, py = pyridine), and [{Ln(hmpa)(3)}(2)(&mgr;-SPh)(3)][SPh] (6, Ln = Sm; 7, Ln = Eu; 8, Ln = Yb; hmpa = hexamethylphosphoric triamide). Reaction of metallic lanthanides with 3 equiv of disulfides afforded Ln(III) thiolate complexes, Ln(SAr)(3)(py)(n)()(thf)(3)(-)(n)() (9a, Ln = Sm, n = 3; 9b, Ln = Sm, n = 2; 10, Ln = Yb, n = 3) and Ln(SPh)(3)(hmpa)(3) (11, Ln = Sm; 12, Ln = Eu; 13, Ln = Yb). Thus, Ln(II) and Ln(III) thiolate complexes were prepared from the same source by controlling the stoichiometry of the reactants. X-ray analysis of 8 revealed that 8 has the first ionic structure composed of triply bridged dinuclear cation and benezenethiolate anion [8, orthorhombic, space group P2(1)2(1)2(1) with a = 21.057(9), b = 25.963(7), c = 16.442(8) ?, V = 8988(5) ?(3), Z = 4, R = 0.040, R(w) = 0.039 for 5848 reflections with I > 3sigma(I) and 865 parameters]. The monomeric structures of 11 and 13 were revealed by X-ray crystallographic studies [11, triclinic, space group P&onemacr; with a = 14.719(3), b = 17.989(2), c = 11.344(2) ?, alpha = 97.91(1), beta = 110.30(2), gamma = 78.40(1) degrees, V = 2751.9(9) ?(3), Z = 2, R = 0.045, R(w) = 0.041 for 7111 reflections with I > 3sigma(I) and 536 parameters; 13, triclinic, space group P&onemacr; with a = 14.565(2), b = 17.961(2), c = 11.302(1) ?, alpha = 97.72(1), beta = 110.49(1), gamma = 78.37(1) degrees, V = 2706.0(7) ?(3), Z = 2, R = 0.031, R(w) = 0.035 for 9837 reflections with I > 3sigma(I) and 536 parameters]. A comparison with the reported mononuclear and dinuclear lanthanide thiolate complexes has been made to indicate that the Ln-S bonds weakened by the coordination of HMPA to lanthanide metals have ionic character. 相似文献
155.
Uchida K Saito M Murakami A Kobayashi T Nakamura S Irie M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(2):534-542
A photochromic polymer film containing three different diarylethene derivatives, that is, 1,2-bis(3,5-dimethyl-2-thienyl)perfluorocyclopentene (1), 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (2), and 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (3) was prepared. Upon UV irradiation, the three derivatives changed to their closed-ring isomers having different colors, yellow, red, and blue. They showed different spectra not only in UV/Vis region but also in the IR spectral region. Upon irradiation with visible light of appropriate wavelengths, each closed-ring isomer was selectively bleached, and three bits eight states recording was performed. The eight states could be read out nondestructively by using IR light of appropriate wavenumbers. 相似文献
156.
Fujitani T Nakamura I Kobayashi Y Takahashi A Haneda M Hamada H 《The journal of physical chemistry. B》2005,109(37):17603-17607
Adsorption and reactions of NO on clean and CO-precovered Ir(111) were investigated by means of X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HR-EELS), infrared reflection absorption spectroscopy (IRAS), and temperature-programmed desorption (TPD). Two NO adsorption states, indicative of fcc-hollow sites and atop sites, were present on the Ir(111) surface at saturation coverage. NO adsorbed on hollow sites dissociated to Na and Oa at temperatures above 283 K. The dissociated Na desorbed to form N2 by recombination of Na at 574 K and by a disproportionation reaction between atop-NO and Na at 471 K. Preadsorbed CO inhibited the adsorption of NO on atop sites, whereas adsorption on hollow sites was not affected by the coexistence of CO. The adsorbed CO reacted with dissociated Oa and desorbed as CO2 at 574 K. 相似文献
157.
The electronic structure of azulene molecule has been studied. We have obtained the optimized structures of ground and singlet excited states by using the complete active space self-consistent-field (CASSCF) method, and calculated vertical and 0-0 transition energies between the ground and excited states with second-order M?ller-Plesset perturbation theory (CASPT2). The CASPT2 calculations indicate that the bond-equalized C(2v) structure is more stable than the bond-alternating C(s) structure in the ground state. For a physical understanding of electronic structure change from C(2v) to C(s), we have performed the CASSCF calculations of Duschinsky matrix describing mixing of the b(2) vibrational mode between the ground (1A(1)) and the first excited (1B(2)) states based on the Kekule-crossing model. The CASPT2 0-0 transition energies are in fairly good agreement with experimental results within 0.1-0.3 eV. The CASSCF oscillator strengths between the ground and excited states are calculated and compared with experimental data. Furthermore, we have calculated the CASPT2 dipole moments of ground and excited states, which show good agreement with experimental values. 相似文献
158.
Masaaki?HanedaEmail author Isao?Nakamura Tadahiro?Fujitani Hideaki?Hamada 《Catalysis Surveys from Asia》2005,9(4):207-215
Comprehensive studies combining surface science and real catalyst were performed to get further insight into catalytic active
site and reaction mechanism for NO decomposition over supported palladium and cobalt oxide-based catalysts. On palladium single-crystal
model catalysts, adsorption, dissociation and desorption behavior of NO was found to be closely related to the surface structures,
the stepped surface palladium being active for dissociation of NO. In accordance with this result, the activity of powder
Pd/Al2O3 catalysts for NO decomposition was directly related to the number of step sites exposed on the surface, suggesting that the
step sites act as the catalytic active site for NO decomposition on Pd/Al2O3. NO decomposition over cobalt oxide was found to be significantly promoted by addition of alkali metals. Surface science
study and catalyst characterization led to the same conclusion that the interface between the alkali metal and Co3O4 serves as the catalytic active site. From the results of in situ Fourier transform infrared (FT-IR) spectroscopy and isotopic transient kinetic analysis, a reaction mechanism was proposed
in which the reaction is initiated by NO adsorption onto alkali metals to form NO2− species and then NO2− species react with the adsorbed NO species to form N2 over the interface between the alkali metal and Co3O4. 相似文献
159.
Phospholipase A(2) (PLA(2))-catalyzed membrane leakage can be detected by immobilized liposomes containing a self-quenching fluorescent dye, calcein, on an open column using off-line analysis with a fluorescent spectrophotometer. The calcein release was found to be affected by the pH value, incubation time, and liposome compositions. The fluorescent signal from the negatively charged liposomes hydrolyzed by PLA(2) was 5 times higher than that from neutral liposomes. We utilized this enzymatic reaction to amplify signal to detect polychlorinated biphenyls (PCBs). To achieve this goal, we conjugated an analogue of PCB, 3,4-dichloroaniline, to PLA(2). The competitive immunoreaction between the 3,4-dichloroaniline-PLA(2) conjugate and PCB samples on the anti-PCB antibody column caused the release of the bound PLA(2) conjugates in proportion to the PCB concentration. The released PLA(2) conjugates was then passed through the tandem fluorescent liposome column causing release of fluorescent dye from the liposomes. Therefore, the signal of immunocompetitive assay was amplified on the fluorescent liposome column. The tandem column system achieves a high sensitivity by detecting the PCB concentration as low as 0.5 ng/mL in less than 20 min. It has great potential in detecting other pollutants, and has been used for sensitive immunoassays. 相似文献
160.
Itaru Nakamura 《Tetrahedron letters》2004,45(14):2903-2906
Ytterbium-catalyzed tandem carboalkoxylation/Friedel-Crafts reaction of arylidenecyclopropanes 1 with acetals 2 afforded the corresponding indene derivatives 3 in good to high yields. For example, in the presence of 10 mol % of Yb(OTf)3 the reaction of 1-phenylbenzylidenecyclopropane 1a with the dimethyl acetals of benzaldehyde 2a, p-tolualdehyde 2b, and p-anisaldehyde 2c gave 1,3-diphenyl-2-(2-methoxyethyl)indene 3a, 2-(2-methoxyethyl)-3-phenyl-1-(p-tolyl)indene 3b, and 1-(p-anisyl)-2-(2-methoxyethyl)-3-phenylindene 3c in 82%, 80%, and 80% yields, respectively. 相似文献