全文获取类型
收费全文 | 57134篇 |
免费 | 16020篇 |
国内免费 | 149篇 |
专业分类
化学 | 63867篇 |
晶体学 | 139篇 |
力学 | 1498篇 |
数学 | 3986篇 |
物理学 | 3813篇 |
出版年
2024年 | 427篇 |
2023年 | 4291篇 |
2022年 | 1517篇 |
2021年 | 2593篇 |
2020年 | 4914篇 |
2019年 | 2535篇 |
2018年 | 2552篇 |
2017年 | 805篇 |
2016年 | 5941篇 |
2015年 | 5868篇 |
2014年 | 5389篇 |
2013年 | 5863篇 |
2012年 | 4004篇 |
2011年 | 2130篇 |
2010年 | 3935篇 |
2009年 | 3786篇 |
2008年 | 1967篇 |
2007年 | 1637篇 |
2006年 | 1029篇 |
2005年 | 920篇 |
2004年 | 801篇 |
2003年 | 672篇 |
2002年 | 619篇 |
2001年 | 332篇 |
2000年 | 304篇 |
1999年 | 286篇 |
1998年 | 252篇 |
1997年 | 274篇 |
1996年 | 311篇 |
1995年 | 356篇 |
1994年 | 283篇 |
1993年 | 391篇 |
1992年 | 266篇 |
1991年 | 232篇 |
1990年 | 210篇 |
1989年 | 220篇 |
1988年 | 223篇 |
1985年 | 213篇 |
1984年 | 219篇 |
1982年 | 226篇 |
1981年 | 243篇 |
1980年 | 237篇 |
1979年 | 235篇 |
1978年 | 253篇 |
1977年 | 281篇 |
1976年 | 283篇 |
1975年 | 304篇 |
1974年 | 284篇 |
1972年 | 212篇 |
1970年 | 244篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
The orientational ordering of several liquid crystals containing a difluorosubstituted phenyl ring has been studied through the use of C-13 NMR. The fluorinated phenyl ring of these liquid crystals have Cs symmetry, so three order parameters are required to completely describe the ordering of this ring. All three of these order parameters have been calculated from carbon-fluorine dipolar coupling constants obtained from the carbon-fluorine splittings in the C-13 NMR spectra. Because of the complexity of the fluorine-coupled spectra, variable angle spinning (VAS) was used to resolve the carbon-fluorine splittings. In order to study the orientational ordering over wide ranges of temperature, we have developed an empirical correlation between the order parameter and the value of a carbon-fluorine dipolar coupling constant. This enabled us to study the change in the order parameter with temperature. The results of applying this method to several structurally similar liquid crystals containing the same type of difluorinated phenyl ring are presented. A comparison is made to a similar mono-fluorinated liquid crystal. 相似文献
22.
We give a self‐contained proof of the preservation theorem for proper countable support iterations known as “tools‐preservation”, “Case A” or “first preservation theorem” in the literature. We do not assume that the forcings add reals. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
23.
24.
R. Kleiss A. D. Martin W. J. Stirling 《Zeitschrift fur Physik C Particles and Fields》1988,39(3):393-398
We study means of identifying top quarks of mass in the range 70–120 GeV at the CERN and FNAL \(\bar pp\) colliders. We show that “W+dijet” production presents a serious background to the conventional “isolated electron+dijet” signature of top. We study an improved signature, calculate the expected event rates, and discuss the determination of the mass of the top quark. 相似文献
25.
Vladimir Rapić Prof. Dr. Karl Schlögl Brigitte Steinitz 《Monatshefte für Chemie / Chemical Monthly》1977,108(4):767-780
Starting from optically active methylferrocene-- and--carboxylic acids (1 a and1 b) of known absolute configuration and enantiomeric purity about 15 chiral ferrocenes (of each isomeric series— and ) were obtained by suitable ligand transformations. Thereby (almost) all possible chiral combinations of the ligands CH3, COOH (COO–), COOCH3, CN and NH2 (NH3
+) were accessible which are necessary for a potential test of approximations of chirality functions for compounds with basic symmetry C5v. The chiroptical properties of these disubstituted ferrocenes are recorded.Preliminary tests using a shortened Ansatz revealed large discrepancies between calculated () and found [M]D-values. Possible reasons for this failure are discussed.
61. Mitt. über Ferrocenderivate
39. Mitt.:A. Meyer, H. Neudeck undK. Schlögl, Chem. Ber.110, 1403 (1977).
60. Mitt.:V. Rapi, K. Schlögl undB. Steinitz, J. Organometal. Chem.94, 87 (1975). 相似文献
61. Mitt. über Ferrocenderivate
39. Mitt.:A. Meyer, H. Neudeck undK. Schlögl, Chem. Ber.110, 1403 (1977).
60. Mitt.:V. Rapi, K. Schlögl undB. Steinitz, J. Organometal. Chem.94, 87 (1975). 相似文献
26.
Martin Petrzilka 《Helvetica chimica acta》1978,61(8):3075-3078
Starting from 5-chloro-2-pentanone ( 1 ) the naturally occurring 10-membered lactone phoracantholide J ( 8a ) has been synthesized as its racemate in a sequence of six steps (Scheme 2). Salient features of the syntheis include an internal selenium assisted acetal formation ( 4→5 ) and a stereoselective Claisen rearrangement ( 6→7→8 ). This general synthetic strategy offers an alternative approach towards the construction of macrocyclic lactones. 相似文献
27.
The fragmentation patterns of 24 substituted phenyltrifluoromethanesulfonates has been determined by exact mass and metastable transition measurements. The influence of the ring substituent(s) on the abundance of the [ArO]+ ions has been investigated at low energies and a new standard parameter for plotting against Hammett's ó constants is proposed. The direct loss of CF3SO3 from the molecular ion of 3,5-(diisopropyl)-phenyl-, 3,5-(dicyclopropyl)phenyl- and 3,5-di(α-methylcyclopropyl)-phenyl-methanesulfonate, and further observations on the fragmentation processes of these compounds are consistent with M.O. calculations on the ‘through bond’ stabilization of the [Ar]+ σ state by meta electron donor substituents. 相似文献
28.
Enantioselective host-guest complexes between α-amino-alcohol salts and chiral tartrates can not be crystallised up to now. To study structural aspects of their enantioselectivity, crystal structures of the components were determined. Norephedrine was used as a reference guest α-amino-alcohol. (1R, 2S)-Norephedrine hydrochloride (monoclinic, space group P21 Z = 4, a = 8.455, b = 10.331, c = 12.570 Å, β = 107.45°) and (1R, 2R)-norpseudoephedrine hydrochloride (monoclinic, space group P21 Z = 2, a = 5.493, b = 8.052, c = 11.986 Å, β = 104.62°) both adopt M-synclinal conformations with respect to the ammonium and hydroxy groups. Rather short intramolecular N…?O distances indicate interaction between ammonium and hydroxy groups. 相似文献
29.
30.
The racemic cis-anti-trans-steroids 9 to 11 have been synthesized in a highly stereoselective manner starting from 4-methoxybenzocyclobutene carboxylic acid via the key step 8 → 9 . (cf. Scheme 2). 相似文献