全文获取类型
收费全文 | 4040篇 |
免费 | 98篇 |
国内免费 | 13篇 |
专业分类
化学 | 2665篇 |
晶体学 | 39篇 |
力学 | 55篇 |
数学 | 850篇 |
物理学 | 542篇 |
出版年
2020年 | 61篇 |
2019年 | 51篇 |
2017年 | 34篇 |
2016年 | 59篇 |
2015年 | 84篇 |
2014年 | 72篇 |
2013年 | 123篇 |
2012年 | 141篇 |
2011年 | 140篇 |
2010年 | 116篇 |
2009年 | 103篇 |
2008年 | 174篇 |
2007年 | 163篇 |
2006年 | 134篇 |
2005年 | 151篇 |
2004年 | 120篇 |
2003年 | 95篇 |
2002年 | 77篇 |
2001年 | 43篇 |
2000年 | 57篇 |
1999年 | 44篇 |
1998年 | 39篇 |
1997年 | 38篇 |
1996年 | 35篇 |
1995年 | 40篇 |
1994年 | 41篇 |
1993年 | 51篇 |
1992年 | 49篇 |
1991年 | 34篇 |
1989年 | 36篇 |
1988年 | 42篇 |
1987年 | 47篇 |
1985年 | 53篇 |
1984年 | 62篇 |
1983年 | 40篇 |
1982年 | 30篇 |
1981年 | 43篇 |
1980年 | 53篇 |
1979年 | 34篇 |
1978年 | 37篇 |
1977年 | 36篇 |
1976年 | 29篇 |
1975年 | 33篇 |
1974年 | 33篇 |
1956年 | 40篇 |
1934年 | 33篇 |
1932年 | 37篇 |
1931年 | 29篇 |
1930年 | 30篇 |
1926年 | 28篇 |
排序方式: 共有4151条查询结果,搜索用时 15 毫秒
141.
Armstrong TA Bettoni D Bharadwaj V Biino C Borreani G Broemmelsiek D Buzzo A Calabrese R Ceccucci A Cester R Church M Dalpiaz P Dalpiaz PF Dibenedetto R Dimitroyannis D Fabbri MG Fast J Gianoli A Ginsburg CM Gollwitzer K Hahn A Hasan M Hsueh S Lewis R Luppi E Macrí M Majewska AM Mandelkern M Marchetto F Marinelli M Marques J Marsh W Martini M Masuzawa M Menichetti E Migliori A Mussa R Palestini S Pallavicini M Pastrone N Patrignani C Peoples J Pesando L Petrucci F Pia MG Pordes S Rapidis P 《Physical review letters》1992,69(16):2337-2340
142.
143.
1,3-Dithiane-1-N-p-chlorophenylimides (1,4-9) were prepared and their configuration and conformation was determined by 1H and 13C NMR. The compounds were rearranged to the corresponding 2-(2'-amino-5'-chlorophenyl)-1, 3-dithianes (1U,4U,9U). The rearrangement reactions took place with ?95% stereospecifity. The mechanism of the reaction was investigated with the aid of analogs specifically deuterated at C-2. 相似文献
144.
The luminescence of solid xenon at 20 K has been excited optically in the wavelength range 1550Åto 1150Å. The 1760Å emission band due to a transition of the molecule is found in agreement with α? or e?-excitation. The excitation spectrum yields a relatively high quantum efficiency of the Γ(3/2) exciton series indicating efficient self-trapping. The Γ(1/2) exciton series and conduction band states have a low quantum efficiency. 相似文献
145.
Zheng X Mulcahy ME Horinek D Galeotti F Magnera TF Michl J 《Journal of the American Chemical Society》2004,126(14):4540-4542
We describe the preparation of a compound whose molecules consist of two metal sandwich stands carrying tentacles with affinity to metal surfaces and holding an axle that carries a dipolar or a nonpolar rotator. The dipolar rotor exists as three pairs of enantiomers, rapidly interconverting at room temperature. When mounted on a gold surface, each molecule represents a chiral altitudinal rotor, with the rotator axle parallel to the surface. The surface-mounted rotor molecules are characterized by several spectroscopic and imaging techniques. At any one time, in about one-third of the dipolar rotors the rotator is free to turn and the direction of its dipole can be flipped by the electric field applied by an STM tip, as revealed by differential barrier height imaging. Molecular dynamics calculations suggest that electric field normal to the surface causes members of one pair of enantiomers to rotate unidirectionally. 相似文献
146.
The capabilities of the recently developed multireference, general-model-space (GMS), state-universal (SU) coupled-cluster (CC) method have been extended in order to enable the handling of any excited state that represents a single (S) or a double (D) excitation relative to the ground state. A series of calculations concerning the ground and excited states of the CH(+), HF, F(2), H(2)O, NH(2), and CH(2) molecules were carried out so as to assess the performance of the GMS SU CCSD method. For diatomics we have computed the entire potential energy curves, while for triatomics we have focused on vertical excitation energies. We demonstrate how a systematic enlargement of the model space enables a consideration of a larger and larger number of excited states. A comparison of the CC and full configuration interaction or large-scale CI results enables an assessment of the accuracy and reliability of the GMS SU CCSD method within a given basis set. In all cases very good results have been obtained, including highly excited states and those having a doubly-excited character. 相似文献
147.
Izabela Jendrzejewska Tomasz Goryczka Ewa Pietrasik Joanna Klimontko Josef Jampilek 《Molecules (Basel, Switzerland)》2020,25(24)
Studies carried out by X-ray and thermal analysis confirmed that acetaminophen (paracetamol), declared by the manufacturers as an Active Pharmaceutical Ingredient (API), was present in all studied medicinal drugs. Positions of diffraction lines (2θ angles) of the studied drugs were consistent with standards for acetaminophen, available in the ICDD PDF database Release 2008. values were lower than 0.2°, confirming the authenticity of the studied drugs. Also, the values of interplanar distances dhkl for the examined samples were consistent with those present in the ICDD. Presence of acetaminophen crystalising in the monoclinic system (form I) was confirmed. Various line intensities for API were observed in the obtained diffraction patterns, indicating presence of the preferred orientation of the crystallites in the examined samples. Thermal analysis of the studied substances confirmed the results obtained by X-ray analysis. Drugs containing only acetaminophen as an API have melting point close to that of pure acetaminophen. It was found that presence of other active and auxiliary substances affected the shapes and positions of endothermal peaks significantly. A broadening of endothermal peaks and their shift towards lower temperatures were observed accompanying an increase in the contents of additional substances being “impurities” in relation to the API. The results obtained by a combination of the two methods, X-ray powder diffraction (XRPD) and differential scanning calorimetry/thermogravimetry (DSC/TGA), may be useful in determination of abnormalities which can occur in pharmaceutical preparations, e.g., for distinguishing original drugs and forged products, detection of the presence of a proper polymorphic form or too low content of the active substance in the investigated drug. 相似文献
148.
Antunes MA Ferrence GM Domingos A McDonald R Burns CJ Takats J Marques N 《Inorganic chemistry》2004,43(21):6640-6643
Reaction of [(Tp(Me)2)(2)UI] with KNR(2) (R = C(6)H(5), SiMe(3)) in tetrahydrofuran (THF) afforded the monomeric trivalent actinide amide complexes [(Tp(Me)2)(2)U[N(C(6)H(5))(2)]], 1, and [(Tp(Me)2)(2)U[N(SiMe(3))(2)]], 2. The complexes have been fully characterized by spectroscopic methods and their structures were confirmed by X-ray crystallographic studies. In the solid state 1 and 2 exhibit distorted pentagonal bipyramidal geometries. The U-NR(2) bond lengths in both complexes are the same but in complex 2 the greater steric demands of the N(SiMe(3))(2) ligand led to elongated U-N(pz) bonds, especially those opposite the amido ligand. 相似文献
149.
(PPh4)2[MoN(N3)3Cl]2; Synthesis, IR Spectrum, and Crystal Structure The title compound is formed in the reaction of molybdenum (II) benzoate with trimethylsilyl azide and PPh4Cl in dichloro methane forming dark red single crystals. A PPh3Me⊕ salt of the ion [MoN(N3)3Cl]22? is obtained from (PPh3Me)2MoNCl4] treated with silver azide in CH2Cl2 suspension. The solvent CH2Cl2 participates in both reactions as oxidizing agent. (PPh4)2[MoN(N3)3Cl2 is characterized by a structural analysis based upon X-ray data: space group P1 , Z = 1, a = 1050.7 pm; b = 1185.4 pm; c = 1190.8 pm; α = 98.90°; β = 106.87°; γ = 103.97° (4505 independent, observed reflexions, R = 0.039). The compound consists of PPh4⊕ cations and centrosymmetric anions [MoN(N3)3Cl22? in which the molybdenum atoms are bridged by the Nα atoms of two azide groups; the resulting Mo? N bond lengths are 208 pm and 260 pm. In trans position to the long Mo? N bond the terminal nitrido ligand is situated, the Mo?N distance of 164 pm corresponds to a triple bond. Two terminal azido ligands and the chloro ligand are filling up the coordination sphere of the molybdenum atoms to a coordination number of six. The i.r. spectrum is reported and assigned. 相似文献
150.
Stíbr B Holub J Bakardjiev M Pavlík I Tok OL Císarová I Wrackmeyer B Herberhold M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(10):2239-2244
The reaction of nido-[7,8,9-PC(2)B(8)H(11)] (1) with [[CpFe(CO)(2)](2)] (Cp=eta(5)-C(5)H(5) (-)) in benzene (reflux, 3 days) gave an eta(1)-bonded complex [7-Fp-(eta(1)-nido-7,8,9,-PC(2)B(8)H(10))] (2; Fp=CpFe(CO)(2); yield 38 %). A similar reaction at elevated temperatures (xylene, reflux 24 h) gave the isomeric complex [7-Fp-(eta(1)-nido-7,9,10-PC(2)B(8)H(10))] (3; yield 28 %) together with the fully sandwiched complexes [1-Cp-closo-1,2,4,5-FePC(2)B(8)H(10)] 4 a (yield 30%) and [1-Cp-closo-1,2,4,8-FePC(2)B(8)H(10)] 4 b (yield 5%). Compounds 2 and 3 are isolable intermediates along the full eta(5)-complexation pathway of the phosphadicarbaborane cage; their heating (xylene, reflux, 24 h) leads finally to the isolation of compounds 4 a (yields 46 and 52%, respectively) and 4 b (yields 4 and 5%, respectively). Moreover, compound 3 is isolated as a side product from the heating of 2 (yield 10%). The structure of compound 4 a was determined by an X-ray structural analysis and the constitution of all compounds is consistent with the results of mass spectrometry and IR spectroscopy. Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B]-COSY, and (1)H[(11)B(selective)] magnetic resonance measurements led to complete assignments of all resonances and are in excellent agreement with the structures proposed. 相似文献