全文获取类型
收费全文 | 3017篇 |
免费 | 115篇 |
国内免费 | 5篇 |
专业分类
化学 | 2218篇 |
晶体学 | 7篇 |
力学 | 62篇 |
数学 | 472篇 |
物理学 | 378篇 |
出版年
2022年 | 23篇 |
2021年 | 19篇 |
2020年 | 43篇 |
2019年 | 45篇 |
2018年 | 29篇 |
2017年 | 26篇 |
2016年 | 72篇 |
2015年 | 64篇 |
2014年 | 91篇 |
2013年 | 125篇 |
2012年 | 139篇 |
2011年 | 185篇 |
2010年 | 93篇 |
2009年 | 79篇 |
2008年 | 139篇 |
2007年 | 156篇 |
2006年 | 140篇 |
2005年 | 146篇 |
2004年 | 139篇 |
2003年 | 94篇 |
2002年 | 95篇 |
2001年 | 60篇 |
2000年 | 50篇 |
1999年 | 58篇 |
1998年 | 44篇 |
1997年 | 62篇 |
1996年 | 56篇 |
1995年 | 35篇 |
1994年 | 52篇 |
1993年 | 47篇 |
1992年 | 27篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 31篇 |
1988年 | 30篇 |
1987年 | 31篇 |
1986年 | 42篇 |
1985年 | 47篇 |
1984年 | 50篇 |
1983年 | 40篇 |
1982年 | 46篇 |
1981年 | 26篇 |
1980年 | 37篇 |
1979年 | 37篇 |
1978年 | 29篇 |
1977年 | 34篇 |
1976年 | 24篇 |
1973年 | 16篇 |
1972年 | 19篇 |
1971年 | 16篇 |
排序方式: 共有3137条查询结果,搜索用时 31 毫秒
981.
In this paper, we describe the usage of a soluble high-loading polyglycerol support for functionalized boronic acids without further linker design. The quantitatively formed polyglycerol boron esters were subsequently employed in homogeneous Suzuki cross-coupling reactions to give high yields (84-91%) of functional biaryls with minimal amounts of the Pd catalyst (0.2 mol %). In situ precipitation and ultrafiltration were used as simple and effective purification protocols. Furthermore, the reaction conditions were optimized by the choice of the solvent and the catalyst. 相似文献
982.
The 3-aza-Cope rearrangements of 3-azoniahexa-1,5-diene (1), 3-azoniahex-1-ene-5-yne (3), and 3-azahex-1-ene-5-yne (5) were investigated up to the coupled-cluster level, CCSD(T), by using a valence triple-zeta basis set. Activation barriers and geometrical parameters of the transition states are provided. Conformational studies were performed for all reactants and products of the reactions. Solvent effects were estimated from self-consistent reaction field calculations. In contrast to the other two species, the Cope rearrangement of 5 was found to proceed by a stepwise mechanism. 相似文献
983.
984.
Dipole parallel-alignment of organic molecular crystals of azines has been achieved with a design that was based on the hypothesis that the azine bridge is a conjugation stopper. This hypothesis has now been tested in detail, and (1)H and (13)C NMR spectroscopic data of symmetric and asymmetric acetophenone azines are presented in support of this design concept. Previous structural, ab initio, and electrochemical studies have shown that the azine bridge largely inhibits through-conjugation in molecules with the general structure DPhC(Me) [double bond] N [bond] N [double bond] C(Me)PhA, where D is a donor group and A is an acceptor group. NMR spectroscopy is an excellent tool to probe the degree of conjugation through the azine bridge. The NMR results reported here for nine symmetrical and 18 asymmetrical azines show in a compelling fashion that the hypothesis holds. Varying the donor group does not change the chemical shifts of the aromatic hydrogen and carbon atoms on the acceptor-substituted phenyl ring. Likewise, varying the acceptor group does not change the chemical shifts of the atoms in the donor-substituted phenyl ring. 相似文献
985.
986.
Propyne iminium triflates R1C(N+R3R4)CCR2 CF3SO readily react with Ph3PNPh to form 1:1 adducts which formally result from a metathetical addition of the phosphorane imine across the triple bond of the alkyne. These adducts are best described as enamino‐phosphonium salts or iminio‐substituted phosphorus ylides. The configuration of these salts has been determined from NMR data and an X‐ray crystal structure analysis of salt 3h . The base‐induced elimination of the PPh3 substituent from enamino‐phosphonium salt 3a was studied. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:437–446, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20131 相似文献
987.
Rainer Verch 《Communications in Mathematical Physics》2001,223(2):261-288
A model-independent, locally generally covariant formulation of quantum field theory over four-dimensional, globally hyperbolic
spacetimes will be given which generalizes similar, previous approaches. Here, a generally covariant quantum field theory
is an assignment of quantum fields to globally hyperbolic spacetimes with spin-structure where each quantum field propagates
on the spacetime to which it is assigned. Imposing very natural conditions such as local general covariance, existence of
a causal dynamical law, fixed spinor- or tensor type for all quantum fields of the theory, and that the quantum field on Minkowski
spacetime satisfies the usual conditions, it will be shown that a spin-statistics theorem holds: If for some of the spacetimes
the corresponding quantum field obeys the “wrong” connection between spin and statistics, then all quantum fields of the theory,
on each spacetime, are trivial.
Received: 1 March 2001 / Accepted: 28 May 2001 相似文献
988.
989.
Eva‐Luise Hobl Bernd Jilma Josef Ebner Rainer W. Schmid 《Biomedical chromatography : BMC》2013,27(6):695-698
A selective, sensitive and rapid high‐performance liquid chromatography method with post‐column hydrolysis and fluorescence detection was developed for the simultaneous quantification of acetylsalicylic acid and its metabolite salicylic acid in human plasma. Following the addition of 2‐hydroxy‐3‐methoxybenzoic acid as internal standard and simple protein precipitation with acetonitrile, the analytes were separated on a ProntoSIL 120 C18 ace‐EPS column (150 × 2 mm, 3 µm) protected by a C8 guard column (5 µm). The mobile phase, 10 mm formic acid in water (pH 2.9) and acetonitrile (70:30, v/v), was used at a flow rate of 0.35 mL/min. After on‐line post‐column hydrolysis of acetylsalicylic acid (ASA) to salicylic acid (SA) by addition of alkaline solution, the analytes were measured at 290 nm (λex) and 400 nm (λem). The method was linear in the concentration ranges between 0.05 and 20 ng/μL for both ASA and SA with a lower limit of quantification of 25 pg/μL for SA and 50 pg/μL for ASA. The limit of detection was 15 pg/μL for SA and 32.5 pg/μL for ASA. The analysis of ASA and SA can be carried out within 8 min; therefore this method is suitable for measuring plasma concentrations of salicylates in clinical routine. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
990.