全文获取类型
收费全文 | 3061篇 |
免费 | 45篇 |
国内免费 | 6篇 |
专业分类
化学 | 2140篇 |
晶体学 | 14篇 |
力学 | 29篇 |
数学 | 492篇 |
物理学 | 437篇 |
出版年
2016年 | 29篇 |
2015年 | 23篇 |
2014年 | 30篇 |
2013年 | 101篇 |
2012年 | 42篇 |
2011年 | 82篇 |
2010年 | 54篇 |
2009年 | 41篇 |
2008年 | 82篇 |
2007年 | 83篇 |
2006年 | 77篇 |
2005年 | 101篇 |
2004年 | 88篇 |
2003年 | 68篇 |
2002年 | 60篇 |
2001年 | 54篇 |
2000年 | 47篇 |
1999年 | 37篇 |
1997年 | 25篇 |
1996年 | 21篇 |
1995年 | 26篇 |
1994年 | 27篇 |
1993年 | 33篇 |
1992年 | 40篇 |
1991年 | 33篇 |
1990年 | 37篇 |
1989年 | 36篇 |
1988年 | 55篇 |
1987年 | 52篇 |
1986年 | 50篇 |
1985年 | 51篇 |
1984年 | 38篇 |
1983年 | 59篇 |
1982年 | 38篇 |
1981年 | 41篇 |
1980年 | 36篇 |
1979年 | 42篇 |
1978年 | 44篇 |
1977年 | 33篇 |
1976年 | 57篇 |
1975年 | 39篇 |
1974年 | 25篇 |
1973年 | 22篇 |
1972年 | 20篇 |
1970年 | 28篇 |
1928年 | 31篇 |
1927年 | 23篇 |
1924年 | 25篇 |
1885年 | 30篇 |
1884年 | 19篇 |
排序方式: 共有3112条查询结果,搜索用时 15 毫秒
71.
Athos Bellomo und Otto Klug 《Fresenius' Journal of Analytical Chemistry》1965,207(2):93-98
Zusammenfassung Die Messung der Elektrolytkonzentration in fließenden Lösungen, auch bei solchen, die Niederschläge oder Suspensionen enthalten, wird auf Grund der Veränderung des Dielektrikums einer Zelle, welche von einem elektrischen Hochfrequenzstrom durchflossen wird, durchgeführt. Auf diesem Wege ist die quantitative Bestimmung der Konzentration der gelösten Substanz oder die der durchlaufenden Menge beträchtlich vereinfacht und automatisiert. Dabei werden subjektive und Manipulationsfehler weitgehend ausgeschlossen. Die untersuchte Probe erleidet keinerlei Veränderungen.
Summary The electrolyte concentration of flowing solutions and of suspensions or solutions containing precipitates can be measured by the variation of the dielectrics in a cell, which is coupled with a high-frequency circuit. By means of this method the quantitative determination of the concentration of solutions or the determination of the quantity of ions passed through the tube can be simplified. The method makes possible an automatization and it precludes the possibility of subjective and manipulation errors. During the measurement the sample remains unchanged.相似文献
72.
Nemukhin AV Grigorenko BL Khriachtchev L Tanskanen H Pettersson M Räsänen M 《Journal of the American Chemical Society》2002,124(36):10706-10711
Theoretical and matrix-isolation studies of intermolecular complexes of HXeOH with water molecules are presented. The structures and possible decomposition routes of the HXeOH-(H(2)O)(n)(n = 0, 1, 2, 3) complexes are analyzed theoretically. It is concluded that the decay of these metastable species may proceed through the bent transition states (TSs), leading to the global minima on the respective potential energy surfaces, Xe + (H(2)O)(n+1). The respective barrier heights are 39.6, 26.6, 11.2, and 0.4 kcal/mol for n = 0, 1, 2, and 3. HXeOH in larger water clusters is computationally unstable with respect to the bending coordinate, representing the destabilization effect. Another decomposition channel of HXeOH-(H(2)O)(n), via a linear TS, leads to a direct break of the H-Xe bond of HXeOH. In this case, the attached water molecules stabilize HXeOH by strengthening the H-Xe bond. Due to the stabilization, a large blue shift of the H-Xe stretching mode upon complexation of HXeOH with water molecules is featured in calculations. On the basis of this computational result, the IR absorption bands at 1681 and 1742 cm(-1) observed after UV photolysis and annealing of multimeric H(2)O/Xe matrixes are assigned to the HXeOH-H(2)O and HXeOH-(H(2)O)(2) complexes. These bands are blue-shifted by 103 and 164 cm(-1) from the known monomeric HXeOH absorption. 相似文献
73.
Margit L. Svendsen Jesper Wengel Otto DahlFinn Kirpekar Peter Roepstorff 《Tetrahedron》1993,49(48):11341-11352
Novel Oligodeoxynucleotide analogues containing 3′-C-threo-methylene phosphodiester internucleoside linkages were synthesized on automated DNA-synthesizers using the phosphoramidite approach. The sugar modified phosphoramidite building block 5 was obtained by phosphitylation of 1-(2,3-dideoxy-5-O-(4,4′-dimethoxytrityl)-3-C-hydroxymethyl-β-D-threo-pentofuranosyl)thymine (4) which was synthesized in only three steps from 5′-O-(4,4′-dimethoxytrityl)thymidine (1). The hybridization properties and enzymatic stability of the oligonucleotide analogues were studied by UV experiments. 17-Mers having one or three modifications in the middle or two modifications in each end hybridized to DNA with moderate lowered affinity compared to unmodified 17-mers (ΔTm 1–3°C per modification). Furthermore, the end-modified and all-modified oligonucleotides were stable towards snake venom phosphodiesterase. 相似文献
74.
Ernst Schaumann Erwin Kausch Karl-Heinz Klaska Rolf Klaska Otto Jarchow 《Journal of heterocyclic chemistry》1977,14(5):857-860
The crystal and molecular structures of the cycloadduct from isopropyl[α-(dimethylthio-carbamoyl)isopropyl]carbodiimide and methyl isothiocyanate were determined by single-crystal X-ray methods. The product was established to be 2-(1-isopropyl-3,5-dimethyl-4,6-dithioxo-hexahydro-1,3,5-triazine-2-ylidenamino)-N,N-dimethylthioisoburyramide, in which contrary to expectations based on hybridisation the heterocyclic ring adopts a boat conformation. 相似文献
75.
A review is given on optical means for single shot testing (probing) as well as continuous monitoring (sensing) of heavy metal ions (HMs). Following an introduction into indicator based approaches, we discuss the types of indicator dyes and polymeric supports used, as well as existing sensing schemes for HMs. The wealth of information is compiled in the form of tables and critically reviewed. Notwithstanding the tremendous work performed so far, it is obvious that still severe limitations do exist in terms of selectivity, limits of detection, dynamic ranges, applicability to specific problems, and reversibility. On the other hand, such sensors have found — and will find — their application whenever rapid and cost-effective testing is required, where personnel is scarce or unskilled, and in field tests. Despite their limitations, the number of such sensors (and of irreversible probes) for HMs is likely to increase in future. 相似文献
76.
Zusammenfassung Die Stammsubstanzen des Lignins gehören der Gruppe der p-Hydroxyzimtalkohole an. Sie werden durch wäßrige Mineralsäure polymerisiert. Um das Wesen dieser Reaktion kennenzulernen, wurde der Zimtalkohol selbst in verdünnter wäßriger Lösung der Wirkung von Säure ausgesetzt. Er bildet ein Dimeres der Konstitution VIII, die durch Abbau und Synthese bewiesen wurde. Es ist anzynehmen, daß die Polymerisation der p-Hydroxyzimtalkohole nacl, einem Schema verläuft, von dem die Dimerisation des Zimtalkohols die erste Stufe ist.Institut für die Chemie dei Holzes und der PolysaccharideKollegenA. Wacek in alter Freundschaft zum 60. Geburtstag gewidmet.K. Freudenberg. 相似文献
77.
Wolfgang Stadlbauer Otto Schmut Prof. Dr. Thomas Kappe 《Monatshefte für Chemie / Chemical Monthly》1977,108(2):367-379
The reaction of N-(2-pyridyl)-carbaminates (3), 1-phenyl-3-(2-pyridyl)-urea (5 a) or alkyl and aryl substituted pyridopyrimidinediones (2) synthesized from 2-amino-pyridine with arylisocyanates (4 a, b) yields 3-aryl substituted pyrido-[1,2-a]-s-triazine-2,4-diones (1). 2-Aminopyridine and azamalonic derivatives (ethoxycarbonyl isocyanate, ethyl iminodicarboxylate) react to give the 3-unsubstituted triazine system (1 c). The isolation of monocyclic triazinediones obtained from the reaction of N-phenylbenzamidine (9) with aryl isocyanate (4 a) or ethoxycarbonyl isocyanate failed because of hydrolytic ring opening. The mechanism of the reaction of 3-substituted pyrido-pyrimidinediones (2) and phenyl isocyanate (4 a) is discussed. 相似文献
78.
Thomas E. Sorensen Walter B. England David M. Silver E. Otto Steinborn 《Theoretical chemistry accounts》1992,84(1-2):1-19
Summary Many-body perturbation theory is derived for chemical bonds. Paired quasiparticles represent the bonds. Products of the paired quasiparticles define a model Bardeen-Cooper-Schrieffer function. The pairing force is added as a model interaction to the self-consistent problem. The starting model is based on valency and adiabatic symmetry correlation. Symmetries are enforced by the model Hamiltonian. Perturbative corrections are expressed as ordinary Feynman diagrams. The number of diagrams needed is the same as for particle-hole theory.This work was supported in part by the U.S. Department of the Navy, Space and Naval Warfare Systems Command under Contract N00039-89-C-0001, and in part by NATO Research Grant 1861. It was presented, in part, at the A.C. Wahl Memorial Session, Molecular Spectroscopy Symposium, Columbus, Ohio, 1984; and Midwest Theoretical Chemistry Conference, Milwaukee, Wisconsin, 1985. 相似文献
79.
Otto Knab 《manuscripta mathematica》1978,24(3):295-322
Let z=∞ be the only irregular singular point of the linear differential equation $$(D)w^{(n)} + P_{n - 1} (z)w^{(n - 1)} + \cdots + P_o (z)w = 0$$ with rational coefficients pj(z). If w is a multivalent and irregular solution of (D), we define the index I(τr,W), τ ∈ (0,1), of a branch W of w in the plane cut along a half ray. If r→∞, I(τr,W) possesses a finite number of aymptotic directions being exactly the asymptotic directions of the points z with maximal possible modulus of W(z). It follows that each branch W is of mean type σ(W)=|d|/λ in each sector containing an asymptotic direction of I. The possible values of the order of growth λ=λ(W)<∞ and the constant d are given by the PUISEUX-diagram of (D). 相似文献
80.
Hans-Christian Mez Günther Rist Jaroslav Kalvoda Otto Ermer Ljubinka Lorenc Mihailo Lj. Mihailovi 《Helvetica chimica acta》1976,59(4):1273-1293
X-ray structure determination of the p-bromobenzoate 2b of one of the (E/Z)-isomeric 3β-hydroxy-5,10-seco-1(10)-cholesten-5-ones confirmed the (E)-configuration proposed previously and showed the cyclodecenone ring to adopt an extended crown conformation of type A 1 (Fig. 9). Analysis of the 1H- and 13C-NMR. spectra of acetate 2a revealed that in solution the ten-membered ring of this steroid exists in at least two distinct forms, the predominant (about 85%) corresponding to the solid-state conformation of 2b (= A 1), and the minor most likely to B 2 (Fig. 9). From NMR. data the energy difference between the two species and the relevant activation energy were estimated. A number of conformational force field calculations using a simplified partial structural model was performed; but the computed energy differences between the various possible conformations do not reproduce the effective situation, neither in solution nor in the crystal lattice, indicating that additional effects such as the transannular interaction of the double bond with the carbonyl group may strongly influence the thermodynamic stability of the system. The conformations deduced were used to rationalize the stereochemical course of different chemical reactions of 2a . 相似文献