全文获取类型
收费全文 | 1252篇 |
免费 | 41篇 |
国内免费 | 7篇 |
专业分类
化学 | 802篇 |
晶体学 | 8篇 |
力学 | 53篇 |
数学 | 283篇 |
物理学 | 154篇 |
出版年
2023年 | 8篇 |
2021年 | 11篇 |
2020年 | 17篇 |
2019年 | 12篇 |
2018年 | 11篇 |
2017年 | 16篇 |
2016年 | 31篇 |
2015年 | 36篇 |
2014年 | 24篇 |
2013年 | 54篇 |
2012年 | 73篇 |
2011年 | 81篇 |
2010年 | 43篇 |
2009年 | 44篇 |
2008年 | 61篇 |
2007年 | 74篇 |
2006年 | 63篇 |
2005年 | 68篇 |
2004年 | 69篇 |
2003年 | 53篇 |
2002年 | 45篇 |
2001年 | 22篇 |
2000年 | 13篇 |
1999年 | 17篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 21篇 |
1995年 | 14篇 |
1994年 | 15篇 |
1993年 | 17篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 17篇 |
1984年 | 24篇 |
1983年 | 14篇 |
1982年 | 28篇 |
1981年 | 15篇 |
1980年 | 13篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 10篇 |
1975年 | 4篇 |
1974年 | 8篇 |
1973年 | 4篇 |
1969年 | 6篇 |
排序方式: 共有1300条查询结果,搜索用时 109 毫秒
31.
Off-lattice Monte Carlo simulations employing the PIVOT algorithm are used to generate ideal and excluded volume linear polymers in two dimensions. The form factor at small and large wave vectors is calculated from the resulting configurations and compared to the exact equation for ideal chains and to both scaling and renormalization group predictions for excluded volume chains. It is found that using the des Cloizeaux form for the distance distribution function in an analytic calculation of the form factor leads to close agreement with the Monte Carlo data and that simple expressions for both the small and large wave vector expansions reproduce the essential features of the form factor. 相似文献
32.
设计合成了一个新型吡啶基二氮环配体L(N,N'-双(吡啶基-3-亚甲基)-1,5-二氮环辛烷)及其铜配位聚合物{[Cu(滋-H2L)(L)Cl](ClO4)3}∞(1),研究了此配合物的晶体结构、光谱及磁性质。该配合物属正交晶系,Pnma空间群,晶胞参数为a=3.5170(17)nm,b=1.0440(5)nm,c=1.1966(6)nm,V=4.394(4)nm3,Z=4。配体L以顺式螯合和反式桥联两种不同的配位形式将铜离子连接起来形成一维链状阳离子结构。 相似文献
33.
The electrical conductivities of sodium chloride and sodium iodide solutions in formamide-water mixtures of different composition have been measured at 25°C. From these data, the limiting equivalent conductances for these salts have been determined. The correspondingWalden products are compared together with the one reported for sodium bromide in the same solvent mixtures, and they are analysed as functions of the anion size. Further, the limiting ionic equivalent conductances are estimated for the different ions. The variation of the ionicWalden products with solvent composition is discussed in terms of ionic solvation and solvent structure.
Elektrische Leitfähigkeit von Halogensalzen in wäßrigen Formamid-Lösungen
Zusammenfassung Es wurden die elektrischen Leitfähigkeiten von Natriumchlorid- und Natriumjodidlösungen in Formamid-Wasser-Mischungen verschiedener Zusammensetzung bei 25°C gemessen. Aus diesen Daten wurden die Grenzäquivalenzleitfähigkeiten für diese Salze bestimmt. Die entsprechendenWalden-Produkte wurden mit den für Natriumbromid in den gleichen Lösungsmittelgemischen berichteten verglichen und hinsichtlich der Anionengrößen analysiert. Außerdem wurden die ionischen Grenzäquivalenzleitfähigkeiten für die einzelnen Ionen abgeschätzt. Die Variation der ionischenWalden-Produkte mit der Lösungsmittelzusammensetzung wurde bezüglich Ionensolvatation und Lösungsmittelstruktur diskutiert.相似文献
34.
Cyclodehydration of achiral or racemic aryl-delta-oxoacids with (R)-phenylglycinol stereoselectively affords chiral non-racemic bicyclic lactams, from which the enantiodivergent synthesis of (R)- and (S)-2-phenylpiperidine, the diastereodivergent synthesis of cis- and trans-3-ethyl-2-phenylpiperidine, and the enantioselective synthesis of the piperidine alkaloid (-)-anabasine is reported. 相似文献
35.
Munir M. Ahmad Martin R. Bryce Joan Halfpenny Larry Weiler 《Tetrahedron letters》1984,25(38):4275-4278
Bis(triphenylphosphoranylidene)ammonium iodide (PPN+I?) forms a 2:3 complex with TCNQ [(PPN)2(TCNQ)3(CH3CN)2] that provides an example of a TCNQ complex containing acetonitrile in the crystal lattice; the material is a semi-conductor with trimerised TCNQ stacks. 相似文献
36.
Time shift correction in second-order liquid chromatographic data with iterative target transformation factor analysis 总被引:4,自引:0,他引:4
Enric Comas R. Ana Gimeno Joan Ferr Rosa M. Marc Francesc Borrull F. Xavier Rius 《Analytica chimica acta》2002,470(2):607-173
When the generalized rank annihilation method (GRAM) is applied to liquid chromatographic data with diode-array detection, an important problem is the time shift of the peak of the analyte in the test sample. This problem leads to erroneous predictions. This time shift can be corrected if a time window is selected so that the chromatographic profile of the analyte in the test sample is trilinear with the peak of the analyte in the calibration sample. In this paper we present a new method to determine when this condition is met. This method is based on the curve resolution with iterative target transformation factor analysis (ITTFA). The calibration and test matrices are independently decomposed into profiles and spectra, and aligned before GRAM is applied. Here we study two situations: first, when the calibration matrix has one analyte and second, when it has two analytes. When the calibration matrix has two analytes, we selectively determine the time window for the analyte to be quantified. There were considerably fewer prediction errors after correction. 相似文献
37.
Aromí G Ribas J Gamez P Roubeau O Kooijman H Spek AL Teat S MacLean E Stoeckli-Evans H Reedijk J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(24):6476-6488
Coordination complexes of the ligand H3L [1,3-bis(3-oxo-3-phenylpropionyl)-2-hydroxy-5-methylbenzene] with Cu(II) are reported. Clusters showing various nuclearities or modes of supramolecular organization have been prepared by slightly changing the reaction conditions and have been crystallographically characterized. The reaction of H3L with one equivalent of Cu(OAc)2 in DMF yields the dinuclear complex [Cu2(HL)2(dmf)2] (1). Reaction in MeOH of H3L with an increased amount of metal, in the form of Cu(NO3)2, and excess strong base (nBu4NOH) affords the cluster [Cu8(L)2(OMe)8(NO3)2] (2). Complex 2 is a dimer of two linear [Cu4] arrays bridged by methoxide ligands, where the polynucleating ligand is fully deprotonated. The [Cu4]2 clusters are linked to each other by NO3- bridges to form one-dimensional coordination polymers. The link between [Cu8] units and their relative spatial positioning can be modified by changing the anion of the Cu(II) salt, as demonstrated by the synthesis of the cluster polymers [Cu8(L)2(OMe)8Cl2] (3) and [Cu8(L)(OMe)7.86Br2.14] (4), where only NO3- has been replaced by Cl- or Br-, respectively. Similarly, when ClO4- is used, compound [Cu8(L)2(OMe)8(ClO4)2(MeOH)4] (5) can be isolated. It contains independent [Cu8] units. A slight change in the stoichiometry of the reaction leading to 2 affords the related complex catena-[Cu4(L)(OMe)3(NO3)2(H2O)0.36] (6). This polymer contains essentially the same [Cu4] moiety as 2, albeit organized in a completely different arrangement. Each [Cu4] unit in 6 is linked by OMe- ligands to two such equivalent groups to form an infinite chain. Magnetic susceptibility measurements reveal weak antiferromagnetic exchange between Cu(II) centers in 1 (J = -0.73 cm(-1)) and strong antiferromagnetic coupling within [Cu4] chains in 2, 5, and 6 (most negative J values of -113.8 and -177.3 cm(-1) for 2 and 6, respectively). 相似文献
38.
Mukherjee PS Konar S Zangrando E Mallah T Ribas J Chaudhuri NR 《Inorganic chemistry》2003,42(8):2695-2703
Two novel inorganic-organic hybrid 3D extended networks of Ni(II) and Mn(II) having molecular formulas [(maleate)(2)Ni(3)(bpe)(4)(H(2)O)(4)](NO(3))(2).H(2)O (1) and [(adipate)Mn(bpe)] (2) (bpe = 1, 2-bis(4-pyridyl)ethane), respectively, have been synthesized and characterized by single-crystal X-ray diffraction studies and low-temperature (300-2 K) magnetic measurements. Compound 1 crystallizes in the monoclinic system, space group C2/c (No. 15), with chemical formula C(56)H(62)N(10)Ni(3)O(19), a = 30.955(4) A, b = 12.705(3) A, c = 17.058(5) A, beta = 117.26(2) degrees, and Z = 4. Compound 2 crystallizes in the triclinic system, space group Ponemacr; (No. 2), with chemical formula C(18)H(20)MnN(2)O(4), a = 8.492(2) A, b = 9.444(2) A, c = 11.533(3) A, alpha = 97.19(1) degrees, beta = 94.64(1) degrees, gamma = 105.02(1) degrees, and Z = 2. The structure determination reveals for both a 3D network. Compound 1 contains two crystallographically independent Ni(II) ions in different octahedral environments. Ni(1) lies on an inversion center, and its coordination environment comprises two chelating maleate anions and two bpe nitrogen donors, while the Ni(2) ion is surrounded by meridionally disposed three bpe N atoms, two water molecules, and one oxygen donor from the dicarboxylate anion. Of the three crystallographic independent bpe ligand, one presents an anti and the others a gauche conformation. The corresponding N-to-N distances are 9.344, 6.543, and 6.187 A. Variable-temperature magnetic susceptibility measurement of the complex reveals the existence of a dominant ferromagnetic interaction within the molecule. Compound 2 is composed of Mn(2) dimer units linked by adipate anions to form corrugated 2D sheets which, on interconnection through bpe (anti conformation, N-to-N distance of 9.391 A), produces an interpenetrated 3D alpha-polonium-related type net. Complex 2 reveals to be antiferromagnetic fitting data using a dimeric Mn(II) model that considers negligible magnetic transmission through the carbon skeleton of adipate and the bpe pathway. 相似文献
39.
Versatile and stereocontrolled synthetic entries to novel types of cyclopropyl carbocyclic nucleosides are described. The target products have been synthesized from suitable cyclopropane precursors obtained, in turn, from olefinic compounds derived from D-glyceraldehyde as a chiral precursor. Selective manipulation of the functional groups has allowed the preparation of enantiopure nucleosides, some of them displaying opposite chirality. All these molecules contain a quaternary stereogenic carbon at C-1 or C-3 of the cyclopropane ring and bear an amino, a hydroxymethyl, or a methyl group as an additional substituent. In one instance, thymine is directly linked to the cyclopropane. A methylene unit serves as the spacer in the other synthesized nucleosides. 相似文献
40.