全文获取类型
收费全文 | 5028篇 |
免费 | 296篇 |
国内免费 | 22篇 |
专业分类
化学 | 3886篇 |
晶体学 | 13篇 |
力学 | 111篇 |
数学 | 665篇 |
物理学 | 671篇 |
出版年
2023年 | 55篇 |
2022年 | 57篇 |
2021年 | 89篇 |
2020年 | 110篇 |
2019年 | 112篇 |
2018年 | 68篇 |
2017年 | 62篇 |
2016年 | 225篇 |
2015年 | 196篇 |
2014年 | 191篇 |
2013年 | 286篇 |
2012年 | 366篇 |
2011年 | 387篇 |
2010年 | 241篇 |
2009年 | 217篇 |
2008年 | 313篇 |
2007年 | 267篇 |
2006年 | 280篇 |
2005年 | 278篇 |
2004年 | 241篇 |
2003年 | 173篇 |
2002年 | 188篇 |
2001年 | 96篇 |
2000年 | 83篇 |
1999年 | 77篇 |
1998年 | 61篇 |
1997年 | 87篇 |
1996年 | 69篇 |
1995年 | 66篇 |
1994年 | 45篇 |
1993年 | 50篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 20篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 13篇 |
1979年 | 8篇 |
1978年 | 15篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1974年 | 13篇 |
1973年 | 14篇 |
排序方式: 共有5346条查询结果,搜索用时 0 毫秒
41.
Markus Lerchi Elmar Reitter Wilhelm Simon 《Analytical and bioanalytical chemistry》1994,349(4):272-276
A pulsed molecular beam Fourier transform microwave spectrometer especially designed for the quantitative analysis of polar pollutants in air is presented. This very fast method has a sensitivity in the ppm range and can be applied to a wide variety of substances. Details of the apparatus and the results of some performance tests are given. 相似文献
42.
Pablo Wessig Philipp Wettstein Bernd Giese Markus Neuburger Margaretha Zehnder 《Helvetica chimica acta》1994,77(3):829-837
The chiral N-(2-benzoylethyl)-N-tosylglycinamides 1a-c were prepared from the C2-symmetric pyrrolidines 5a-c . Irradiation of these ketones 1a-c gave cis-3-hydroxyprolinamides 10-12 in moderate to good yields (Scheme 3). The de of the photocyclizations depended on the size of the substituents in positions C(2) and C(5) of the chiral pyrrolidine auxiliaries. In addition, the de varied with the reaction temperature, allowing the determination of activation-parameter differences. The structure of products 10-12 were established by NMR and X-ray analyses. 相似文献
43.
Acid-Catalyzed Rearrangement of 9-Hydroxyfuranoeremophilanes to Eremophilanlactones 9-Hydroxyfuranoeremophilanes 1–3 are the main components of freshly harvested rhizomes of P. hybridus (furanopetasin chemovar, Table, Fig. 1). They easily and quantitatively rearrange in the presence of traces of acid to give an epimeric mixture of 8-H-eremophilanlactones 4–6 (eremophilenolides, Table, Figs. 4 and 5). Besides subsequent oxidation (see 4 → 7 ), this is the most important reaction occuring during drying and storage of rhizomes of P. hybridus (Figs. 1 and 3). A reasonable mechanism of the rearrangement is presented, and spectroscopic structure elucidation of 8-H-eremophilanlactones is discussed. 相似文献
44.
Siegfried L Comparone A Neuburger M Kaden TA 《Dalton transactions (Cambridge, England : 2003)》2005,(1):30-36
The metal promoted hydrolysis of nitrile groups in the side chains of tetraazamacrocyclic Cu2+ complexes has been studied by stopped-flow techniques. It is shown that the reaction proceeds by an intramolecular attack of an axially coordinated OH- onto the nitrile group to give the corresponding amide. In alkaline solution the amide then deprotonates and binds to the axial position of the Cu2+ thus preventing further coordination of an OH-. This explains mechanistically that in the Cu2+ complexes of macrocycles carrying two nitrile functions only one is selectively hydrolysed. The nitrile hydrolysis has also been used on a preparative scale to synthesize tetraazamacrocycles with two different side chains. X-Ray diffractions of several products are presented to confirm the structures and the results from the kinetics and equilibria measurements. 相似文献
45.
Thoma G Streiff MB Katopodis AG Duthaler RO Voelcker NH Ehrhardt C Masson C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,12(1):99-117
Polyvalent carbohydrate-protein interactions occur frequently in biology, particularly in recognition events on cellular membranes. Collectively, they can be much stronger than corresponding monovalent interactions, rendering it difficult to control them with individual small molecules. Artificial macromolecules have been used as polyvalent ligands to inhibit polyvalent processes; however, both reproducible synthesis and appropriate characterization of such complex entities is demanding. Herein, we present an alternative concept avoiding conventional macromolecules. Small glycodendrimers which fulfill single molecule entity criteria self-assemble to form non-covalent nanoparticles. These particles-not the individual molecules-function as polyvalent ligands, efficiently inhibiting polyvalent processes both in vitro and in vivo. The synthesis and characterization of these glycodendrimers is described in detail. Furthermore, we report on the characterization of the non-covalent nanoparticles formed and on their biological evaluation. 相似文献
46.
E. Anklam Maria Rosa Bassani Thomas Eiberger Stefan Kriebel Markus Lipp Reinhard Matissek 《Analytical and bioanalytical chemistry》1997,357(7):981-984
Pyrolysis Mass Spectrometry (Py-MS) was used for the discrimination of cocoa butters from other vegetable fats. Mass spectra
ranging from 50 amu to 250 amu were analyzed by principal component analysis (PCA) and with neural nets. The application of
neural nets leads to a good discrimination between the two classes. Detailed analysis of the nets revealed that only the first
60 masses were used within the net. The use of PCA requires a careful selection of the number of masses included in the calculation.
Canonical variance analysis was applied to determine the significant masses. Optimal performance of PCA was observed only
using the first 22 significant masses. Most of these masses were different from the ones used by the neural net. It seems
that the mass spectra obtained by Py-MS contain sufficient information for the discrimination of pure cocoa butter from other
vegetable fats, but none of the methods seems to be able to extract all information available. Neural net provides a very
robust method for this task and no prior data selection was necessary.
Received: 13 May 1996 / Revised: 7 August 1996 / Accepted: 7 August 1996 相似文献
47.
Martin Sukopp Luciana Marinelli Markus Heller Trixi Brandl SimonL. Goodman ReinhardW. Hoffmann Horst Kessler 《Helvetica chimica acta》2002,85(12):4442-4452
The (3R,5S,6E,8S,10R)‐11‐amino‐3,5,8,10‐tetramethylundec‐6‐enoic acid (ATUA; 1 ), which was designed as a βII′‐turn mimic according to the concepts of allylic strain and 2,4‐dimethylpentane units, was incorporated into a cyclic RGD peptide. The three‐dimensional structure of cyclo(‐RGD‐ATUA‐) (=cyclo(‐Arg‐Gly‐Asp‐ATUA‐)) 4 in H2O was determined by NMR techniques, distance geometry calculations and molecular‐dynamics simulations. The RGD sequence of 4 shows high conformational flexibility but some preference for an extended conformation. The structural features of the RGD sequence of 4 were compared with the RGD moiety of cyclo(‐RGDfV‐) (=cyclo(‐Arg‐Gly‐Asp‐D ‐Phe‐Val‐)). In contrast to cyclo(‐RGDfV‐), which is a highly active αvβ3 antagonist and selective against αIIbβ3, cyclo(‐RGD‐ATUA‐) shows a lower activity and selectivity. The structure of the ATUA residue in the cyclic peptide resembles a βII′‐turn‐like conformation. Its middle part, adjacent to the C?C bond, strongly prefers the designed and desired structure. 相似文献
48.
The addition of carboxylic acids to dimethylamino-propinal ( 1a ) and 4-dimethyl-amino-but-3-in-2-on ( 1b ) gives, after rearrangement of the very instable primary adducts ( 2 ), Z-3-acetoxy-N,N-dimethylacrylamides and -crotonamides 3 to 8 in excellent yields and in a stereospecific manner. Similarly, the adducts of HCl and HBr to the alkynes 1a and 1b may be rearranged at low temperature by traces of acid to cis/trans equilibria of 3-halo-acrylamides and -crotonamides 9 and 10 . - On the other hand, treatment of 3-alkoxy-3-dimethylaminoacrolein with traces of acid yields alkylesters of E-3-dimethylaminoacrylic acid ( 12 , X = OR). - The preparative aspects of the rearrangement are discussed, and a brief outline of the spectroscopic properties of the compounds 3 to 8 is given. 相似文献
49.
50.
The energetics and dynamics of double proton transfer (DPT) is investigated theoretically for the Watson-Crick conformation of the guanine-cytosine (GC) base pair. Using semiempirical density functional theory the isolated and DNA-embedded GC pair is considered. Differences in the energetics and dynamics of DPT thus addresses the question of how relevant studies of isolated base pairs are for the understanding of processes occurring in DNA. Two-dimensional potential energy surfaces involving the transferring hydrogen atoms and the proton donors and acceptors are presented for both systems. The DPT reaction is accompanied by a contraction of the distance between the two bases with virtually identical energetic barriers being 18.8 and 18.7 kcal/mol for the isolated and DNA-embedded system, respectively. However, the transition state for DPT in the DNA-embedded GC pair is offset by 0.1 A to larger N-H separation compared to the isolated GC pair. Using activated ab initio molecular dynamics, DPT is readily observed for the isolated base pair with a minimal amount of 21.4 kcal/mol of initial average kinetic energy along the DPT normal mode vector. On a time scale of approximately 100 fs DPT has occurred and the excess energy is redistributed. For the DNA-embedded GC pair considerably more kinetic energy is required (30.0 kcal/mol) for DPT and the process is completed within one hydrogen vibration. The relevance of studies of isolated base pairs and base pair analogs in regard of reactions or properties involving DNA is discussed. 相似文献