全文获取类型
收费全文 | 6953篇 |
免费 | 252篇 |
国内免费 | 34篇 |
专业分类
化学 | 5062篇 |
晶体学 | 50篇 |
力学 | 141篇 |
数学 | 915篇 |
物理学 | 1071篇 |
出版年
2023年 | 44篇 |
2022年 | 86篇 |
2021年 | 191篇 |
2020年 | 148篇 |
2019年 | 139篇 |
2018年 | 105篇 |
2017年 | 119篇 |
2016年 | 269篇 |
2015年 | 164篇 |
2014年 | 203篇 |
2013年 | 427篇 |
2012年 | 506篇 |
2011年 | 611篇 |
2010年 | 314篇 |
2009年 | 263篇 |
2008年 | 487篇 |
2007年 | 476篇 |
2006年 | 423篇 |
2005年 | 433篇 |
2004年 | 303篇 |
2003年 | 255篇 |
2002年 | 237篇 |
2001年 | 97篇 |
2000年 | 87篇 |
1999年 | 55篇 |
1998年 | 67篇 |
1997年 | 47篇 |
1996年 | 84篇 |
1995年 | 44篇 |
1994年 | 41篇 |
1993年 | 58篇 |
1992年 | 32篇 |
1991年 | 36篇 |
1990年 | 47篇 |
1989年 | 36篇 |
1988年 | 40篇 |
1987年 | 40篇 |
1986年 | 24篇 |
1985年 | 35篇 |
1984年 | 29篇 |
1983年 | 14篇 |
1982年 | 21篇 |
1981年 | 14篇 |
1980年 | 16篇 |
1979年 | 18篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1974年 | 10篇 |
1973年 | 5篇 |
1972年 | 4篇 |
排序方式: 共有7239条查询结果,搜索用时 31 毫秒
131.
Miguel Gascón-LópezMajid Motevalli George PaloumbisPeter Bladon Peter B Wyatt 《Tetrahedron》2003,59(47):9349-9360
Stabilised tributylphosphonium ylids Bu3PCHCH(EWG), where EWG is CO2Me, CO2tBu or CN, react with protected sugar lactones under mild conditions to give high yields of glycosylidene derivatives (4 and 5) with good Z/E selectivity. X-Ray crystallography shows that in the solid state the tetra-O-benzyl protected (Z)-glucosylideneacetonitrile (Z)-4c adopts a conformation intermediate between a boat and a twist-boat, whereas the isomeric galactose derivative (Z)-5c exists as a distorted chair. NMR data suggest that in solution chair-like conformations are again more favoured for galactosylidene derivatives than for their glucosylidene analogues. Solution phase NMR studies and molecular modelling show that the (E)-double bond geometry disfavours the chair-like geometry of the ring, even in the galactose series; this is consistent with the avoidance of allylic 1,3-strain. Reduction of the glycosylidene double bond to give stereoselective formation of β-C-glycoside derivatives may be achieved by using Et3SiH-CF3CO2H or Et3SiH-BF3·Et2O. 相似文献
132.
Castro C Karney WL Valencia MA Vu CM Pemberton RP 《Journal of the American Chemical Society》2005,127(27):9704-9705
Density functional and coupled cluster calculations show that facile thermal configuration change in [12]annulene occurs via a twist-coupled bond-shifting mechanism. The transition state for this process is highly aromatic with M?bius topology. At the CCSD(T)/cc-pVDZ//BH&HLYP/6-311+G** level, the isomerization of tri-trans-[12]annulene 1a (CTCTCT) to its di-trans isomer 2 (CCCTCT) via such a mechanism has a barrier of 18.0 kcal/mol, in good agreement with earlier experiments. Two other aromatic M?bius bond-shifting transition states were located that result in configuration change for other [12]annulene conformers. This mechanism contrasts sharply with diradical configuration change for acyclic polyenes and with planar bond-shifting mechanisms generally assumed for annulenes. This constitutes evidence that neutral M?bius aromatic annulenes play a role in the dynamic processes of neutral [4n]annulenes. 相似文献
133.
Esteso Ana Alemany M. M. E. Ortiz Angel Liu Shaofeng 《Central European Journal of Operations Research》2022,30(3):1101-1127
Central European Journal of Operations Research - Agri-food production must increase while food waste needs to be reduced for improving the position of farmers. To do so it is necessary to... 相似文献
134.
Graciela Díaz de Delgado Ramírez V. Belkis Miguel Delgado José Rosquete P. Carmelo 《Journal of chemical crystallography》1995,25(7):371-374
The title compound, formula C15H20O2, is orthorhombic, P212121 witha=8.747(2),b=12.025(3),c=12.554(3)Å,Z=4, andD
m
=1.32(2)g/ml. The structural analysis shows that the compound corresponds to eudesma-4(15),7(11)-dien-8,12-olide, a sesquiterpene lactone previously isolated fromAster umbellatus but whose crystal structure was unknown. 相似文献
135.
Angel Carocca 《Bulletin of the Brazilian Mathematical Society》1995,26(2):161-165
Given a finite groupG andp an odd prime number, we conclude thatO
p(G)G isp-nilpotent when for every subgroupH ofG of orderp there exists a subgroupK ofG such thatG=HK andH permutes with every subgroup ofK. 相似文献
136.
137.
J. Miguel Vadillo M. Esther Gonzalez Isabel Carretero J. Javier Laserna 《Mikrochimica acta》1995,118(3-4):273-282
Micellar liquid chromatography (MLC) and capillary zone electrophoresis (CZE) have been evaluated for the analysis of twelve banned drugs in sport including diuretics and -blockers. In MLC, a sodium dodecylsulphate aqueous solution has been used as mobile phase using an octadecylsilica column. In CZE, a pH 8 buffer solution and a silica capillary have been employed. Parameters of retention and efficiency have been compared. Limits of detection with UV detection at 254 nm and relative standard deviations for atenolol, furosemide, nadolol, spironolactone and triamterene were established and compared in both techniques. Examples of direct urine injection into the separation systems are presented. Drugs overlapping in MLC are well resolved in CZE, while the opposite is true for a limited number of drugs. Some interferences from urine may arise in CZE. The selectivity of analysis would be greatly enhanced by using both techniques, which require only filtration as pre-treatment. 相似文献
138.
139.
The photosensitized aquation of pentaammine(pyridine)ruthenium(II) by several dyes has been studied under conditions where only the sensitizers absorb light. The ratio of the quantum yields for ammine and pyridine substitution was the same as that for direct photoaquation. Sensitization was effective with singlet sensitizers Rhodamine-B (17 452 cm(-)(1)) and Safranine-T (17 690 cm(-)(1)), as well as the triplet sensitizer biacetyl (19 000 cm(-)(1)), but no reaction was observed with Neutral-Red (16 900 cm(-)(1)). The results indicate that the excited state precursor of the observed photosubstitution in the complex lies in the energy range between 17 000 and 17 700 cm(-)(1). 相似文献
140.