全文获取类型
收费全文 | 4005篇 |
免费 | 140篇 |
国内免费 | 13篇 |
专业分类
化学 | 2717篇 |
晶体学 | 14篇 |
力学 | 45篇 |
数学 | 543篇 |
物理学 | 839篇 |
出版年
2023年 | 22篇 |
2021年 | 38篇 |
2020年 | 72篇 |
2019年 | 71篇 |
2018年 | 35篇 |
2017年 | 42篇 |
2016年 | 107篇 |
2015年 | 105篇 |
2014年 | 110篇 |
2013年 | 181篇 |
2012年 | 175篇 |
2011年 | 232篇 |
2010年 | 160篇 |
2009年 | 113篇 |
2008年 | 187篇 |
2007年 | 215篇 |
2006年 | 178篇 |
2005年 | 161篇 |
2004年 | 155篇 |
2003年 | 122篇 |
2002年 | 124篇 |
2001年 | 75篇 |
2000年 | 76篇 |
1999年 | 53篇 |
1998年 | 66篇 |
1997年 | 59篇 |
1996年 | 59篇 |
1995年 | 78篇 |
1994年 | 78篇 |
1993年 | 72篇 |
1992年 | 47篇 |
1991年 | 48篇 |
1990年 | 44篇 |
1989年 | 39篇 |
1988年 | 29篇 |
1987年 | 32篇 |
1986年 | 32篇 |
1985年 | 51篇 |
1984年 | 42篇 |
1983年 | 20篇 |
1982年 | 27篇 |
1981年 | 34篇 |
1980年 | 33篇 |
1979年 | 27篇 |
1978年 | 34篇 |
1977年 | 39篇 |
1975年 | 22篇 |
1974年 | 22篇 |
1973年 | 22篇 |
1972年 | 15篇 |
排序方式: 共有4158条查询结果,搜索用时 406 毫秒
91.
The reaction of the 4,4-dialkylated 2-cyclohexenones 1 or 2 with a twofold excess of a secondary amine 3 affords the 2-amino-1,3-cyclohexadienes 4 and 5 , respectively. Irradiation (λ ≧ 300 nm) of the morpholino derivative 4a yields a mixture of the isomeric 3-morpholino-6-methyl-1,3,5-heptatrienes 6 and 7 , while 5 gives only one corresponding product 8 . The reaction of enone 1 with an equimolar amount of pyrrolidine ( 3c ) affords the bis-enamine 9 which is converted to the unsaturated diketone 10 by oxidative hydrolysis. 相似文献
92.
Here we report the first experimental detection of the C(s) symmetric nitroformyl radical, OCNO(X 2A') in a nitrogen-carbon dioxide matrix at 10 K using a Fourier transform infrared spectrometer (FTIR). The nu1 vibrational frequency was observed at 2113 cm(-1). This assignment was confirmed by follow-up experiments using isotopically labeled reactant molecules (15N, 18O, 13C). To synthesize this radical, we irradiated solid nitrogen-carbon dioxide ice mixtures with energetic electrons at 10 K. Suprathermal nitrogen atoms in their electronic ground and/or first electronically excited state were generated via the radiation induced degradation of molecular nitrogen; these atoms could then react with carbon dioxide to eventually yield the nitroformyl radical. We also investigated the kinetics of the formation of the nitroformyl radical and support the arguments with computations on the doublet and quartet OCNO potential energy surfaces (PESs). 相似文献
93.
Crystal and Molecular Structure of N,N′-Diformylaniline N,N′-diformylaniline crystallizes in the monoclinic space group P21/n with Z = 8 and a = 856.1(2), b = 1277.6(3), c = 1306.1(3) pm, β 92.29(2)°, V = 1427.4(5) · 106 pm3. As shown by X-ray structure determination (2642 symmetry independent reflections, RW = 0.034) the molecule exists in two enantiomeric forms. The molecular structure can be described by two planes; the angle between the plane of the aromatic ring and the plane of the N,N′-diformylamino group is 70.1 resp. 108.3°. The results are compared with those obtained for other derivats. 相似文献
94.
Ralf Otte Roland Fröhlich Shuichi Nakamura Takeshi Toru Dieter Hoppe 《Tetrahedron letters》2007,48(49):8636-8642
The enantio- and diastereoselective synthesis of allenyl aryl sulfides by asymmetric lithiation of 2-alkynyl (2-hetero)aryl sulfides is described. A dynamic thermodynamic resolution by selective crystallization of the intermediate lithium complexes derived from deprotonation, applying a bis(oxazoline) ligand, was achieved to give enantioselectivities up to 85% ee. Subsequent stereospecific nickel-catalyzed cross-coupling reactions with arylzinc reagents established a versatile access to threefold carbon-substituted allenes. 相似文献
95.
Tellurium-related defects inn-type silicon have been analysed by deep level transient spectroscopy (DLTS). The tellurium doping of the samples was performed by ion implantation or during epitaxy. In all the samples, a level having a thermal activation energy around 0.37 eV is observed. This activation energy is found to be dependent on the electric field. Taking into account the Poole-Frenkel (FP) effect and a temperature dependence proportional toT ?2 in the capture cross-section, a value ofE A=0.41eV is obtained. A second Te-related level having an activation energy ofE A=0.14eV without correction of the FP effect could only be measured in the implanted samples. The same concentration is observed for the two levels in the implanted samples thus indicating that both peaks in the DLTS spectra are caused by the identical Te-related defect. 相似文献
96.
K. Hofmann 《Fresenius' Journal of Analytical Chemistry》1970,250(4):256-259
Zusammenfassung Im Hinblick auf die Bestimmung von Thiolgruppen in Muskelgewebe und Fleischerzeugnissen mit Hilfe der amperometrischen Titration mit Silbernitrat wurde der Einfluß folgender Salze, die in dem Untersuchungsmaterial enthalten sein können, geprüft: CaCl2, MgCl2, ZnCl2, MnSO4, CuSO4, FeCl3, NaCl, KCl, NaNO2, Na3PO4, ATP und Lactat. Diese Salze wurden vor der Titration zu SH-Glutathion in 0,14 M Tris-Puffer pH 7,4 zugegeben. CuSO4 verursachte eine starke Erniedrigung des SH-Gehaltes der Lösungen, die — bei nicht zu hoher Konzentration an CuSO4 — durch Zusatz von 0,5% ÄDTA verhindert werden konnte. In Gegenwart von MnSO4 trat ebenfalls eine Abnahme der vorgelegten SH-Menge ein. NaCl verursachte in Konzentrationen von 0,1 M und höher abnorm verlaufende Titrationskurven, wobei die graphische Ermittlung des Äquivalenzpunktes Schwierigkeiten bereitete. Die Titration von SH-Gruppen in 0,6 M KCl-Lösung ergab ebenfalls abweichende Titrationskurven, die das Ergebnis verfälschten. Die übrigen untersuchten Salze übten bei den angewendeten Konzentrationen (bis 3 · 10–4 M) keinen signifikanten Einfluß auf die amperometrische Titration der SH-Gruppen aus.
Effect of salts on the amperometric titration of thiol groups with silver nitrate
The following salts, which can be present in animal tissue and meat products were studied: CaCl2, MgCl2, ZnCl2, MnSO4, CuSO4, FeCl3, NaCl, KCl, NaNO2, Na3PO4, ATP or lactate. They were added to reduced glutathion in 0.14 M tris buffer pH 7.4 before titration. CuSO4 caused a strong decrease of the SH content. This effect could be inhibited by the addition of 0,5% of EDTA in the presence of low concentrations of the copper salt. MnSO4 also decreased the SH content. High concentrations of sodium chloride (0.1 M and more) caused a change in the normal titration curve and, therefore, made the determination of the endpoint difficult. Titration in 0.6 M KCl solution also yielded abnormal curves. The remaining salts investigated had no influence on the amperometric titration of SH groups in the concentrations used (up to 3×10–4 M).相似文献
97.
An analytical method has been developed for the simultaneous extraction and determination of trace tertiary octylphenol (t-OP), technical nonylphenol isomers (NP), nonylphenol monoethoxylate isomers (NP1EO) and seven phthalates in the atmosphere using gas chromatography-mass spectrometry (GC-MS). High volume samples were collected using a high-volume pump equipped with a PUF/XAD-2 column for air and glass fiber filter for particles. The detection limits of the method for alkylphenols (APs) and the phthalates ranged from 0.0006 to 0.034 ng m−3 in air. The recoveries of t-OP, NP, NP1EO and the phthalates for the entire procedure were satisfactory (>69%). The method was successfully applied to the determination of the analytes in the atmosphere samples collected over land and the ocean. The concentrations of t-OP, NP, NP1EO showed decline trends from land to the open sea, and the phthalates present over land and the North Sea were comparable. It is suggested that the atmosphere is a significant pathway for the transport of alkylphenols and the phthalates in the environment. 相似文献
98.
Bauer O Guerasimova A Sauer S Thamm S Steinfath M Herwig R Janitz M Lehrach H Radelof U 《Rapid communications in mass spectrometry : RCM》2004,18(16):1821-1829
Peptide nucleic acid (PNA) is a novel class of DNA analogues in which the entire sugar-phosphate backbone is replaced by a pseudopeptide counterpart. Owing to its neutral character and the consequent lack of electrostatic repulsion, PNA exhibits very stable heteroduplex formation with complementary nucleic acid that is essentially ionic strength independent and enables hybridization under minimum salt conditions. This feature as well as its superior ion stability and easy ionization compared to DNA renders PNA very attractive for hybridization-based matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) applications. We have developed an approach to DNA characterization that takes advantage of multiplexed PNA hybridizations analyzed by MALDI-TOFMS. Our motivation was the further development of oligonucleotide fingerprinting, an efficient technique for cDNA and genomic DNA library characterization. Through positive 'charge-tagging' of PNA the efficiency of detection in MALDI-TOFMS was considerably enhanced permitting an unparalleled degree of multiplexing. Results from the simultaneous hybridization of 21 charge-tagged PNA hexamer oligonucleotides showed that genomic DNA and cDNA clones are successfully characterized on the basis of their hybridization profiles. The degree of multiplexing achieved may render a significant increase in throughput and hence efficiency of oligonucleotide fingerprinting possible. 相似文献
99.
We have extended to molecules a density functional previously parametrized for atomic computations. The Coulomb-hole–Hartree–Fock functional, introduced by Clementi in 1963, estimates the dynamical correlation energy by the computations of a Hartree–Fock-type single-determinant wave function, where the Hartree–Fock potential was augmented with an effective potential term, related to a hard Coulomb hole enclosing each electron. The method was later revisited by S. Chakravorty and E. Clementi [Phys. Rev. A 39 , 2290 (1989)], where a Yukawa-type soft Coulomb hole replaced the previous hard hole; atomic correlation energies, computed for atoms with Z = 2 to Z = 54 as well as for a number of excited states, validated the method. In this article, we parametrized a function, which controls the width of the soft Coulomb hole, by fitting the first and second atomic ionization potentials of the atoms with 1 ? Z ? 18. The parametrization has been preliminarily validated by computing the dissociation energy for a number of molecules. A few-determinant version of the Coulomb-hole–Hartree–Fock method, necessary to account for the nondynamic correlation corrections, is briefly discussed. © 1994 John Wiley & Sons, Inc. 相似文献
100.
Ralf Garbe Jürgen Pebler Kurt Dehnicke Dieter Fenske Helmut Goesmann Gerhard Baum 《无机化学与普通化学杂志》1994,620(4):592-598
Phosphorane Iminato Complexes of Antimony. The Crystal Structures of [Sb2Cl5(NPMe3)2][SbCl6] · CH3CN and [SbCl(NPPh3)]2[SbCl6]2 · 6 CH3CN The title compounds are formed by reaction of antimony pentachloride in acetonitrile solution with the phosphorane iminato complexes SbCl2(NPMe3) and SbCl2(NPPh3), respectively, which themselves are synthesized by reaction of antimony trichloride with Me3SiNPR3 (R = Me, Ph). The complexionic compounds are characterized by 121Sb Mössbauer spectroscopy and by crystal structure determinations. [Sb2Cl5(NPMe3)2][SbCl6] · CH3CN: Space group P41, Z = 4, 3 698 observed unique reflections, R = 0.022. Lattice dimensions at ?60°C: a = b = 1 056.0(1), c = 2 709.6(2) pm. The structure consists of SbCl6? ions and cations [Sb2Cl5(NPMe3)2(CH3CN)]+, in which one SbIII atom and one SbV atom are bridged by the N atoms of the phosphorane iminato ligands. [SbCl(NPPh3)]2[SbCl6]2 · 6 CH3CN: Space group P1 , Z = 2, 5 958 observed unique reflections, R = 0.033. Lattice dimensions at ?60°C: a = 989.4(11), b = 1 273(1), c = 1 396(1) pm, α = 78.33(7), β = 77.27(8)°, γ = 86.62(8)°. The structure consists of SbCl6? ions and centrosymmetric cations [SbCl(NPPh3)(CH3CN)2]22+, in which the antimony atoms are bridged by the N atoms of the phosphorane iminato ligands. 相似文献