全文获取类型
收费全文 | 1808篇 |
免费 | 59篇 |
国内免费 | 5篇 |
专业分类
化学 | 1406篇 |
晶体学 | 25篇 |
力学 | 18篇 |
数学 | 146篇 |
物理学 | 277篇 |
出版年
2023年 | 6篇 |
2021年 | 8篇 |
2020年 | 11篇 |
2019年 | 23篇 |
2018年 | 23篇 |
2017年 | 12篇 |
2016年 | 40篇 |
2015年 | 37篇 |
2014年 | 51篇 |
2013年 | 98篇 |
2012年 | 79篇 |
2011年 | 95篇 |
2010年 | 64篇 |
2009年 | 49篇 |
2008年 | 96篇 |
2007年 | 76篇 |
2006年 | 93篇 |
2005年 | 81篇 |
2004年 | 79篇 |
2003年 | 100篇 |
2002年 | 84篇 |
2001年 | 39篇 |
2000年 | 58篇 |
1999年 | 29篇 |
1998年 | 26篇 |
1997年 | 30篇 |
1996年 | 27篇 |
1995年 | 34篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 17篇 |
1989年 | 9篇 |
1988年 | 15篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 19篇 |
1984年 | 23篇 |
1983年 | 10篇 |
1982年 | 23篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 32篇 |
1978年 | 25篇 |
1977年 | 16篇 |
1976年 | 15篇 |
1974年 | 8篇 |
1973年 | 8篇 |
1968年 | 5篇 |
排序方式: 共有1872条查询结果,搜索用时 359 毫秒
71.
The reaction potential map (RPM) method has been extended so that the molecular reactivity towards nucleophiles can be analyzed. An anion consisting of the nucleus with + 1 charge and a pair of electrons was adopted as the model nucleophile. From the interaction energy between a substrate and this model particle, RPM and its component maps were obtained. With respect to practical applications, the reactivities of acrylic acid and acrylonitrile towards nucleophiles were examined. In these molecules, the exchange interaction as well as the charge-transfer interaction were found to be very important in the elucidation of the observed site selectivity. 相似文献
72.
73.
74.
75.
76.
The light-emitting principle of the brittle star Ophiopsila californica has been isolated and purified. It was found to be a green-fluorescent photoprotein (molecular weight 45000) which emits green light (λmax 500 nm) when H2 O2 is added, independently of the presence or absence of O2 . The green fluorescence (emission maximum 500 nm, excitation maximum 440 nm) spectrally coincided with the H2 O2 -triggered luminescence, indicating that the green fluorescent chromophore is the light-emitter of the photoprotein luminescence. 相似文献
77.
Shigeru Ikuta 《Chemical physics letters》1981,77(2):369-372
The clustering energies of the reactions, H(M)++ M → H(M)+2 (M = CO and N2), were calculated by ab initio single configuration LCAO SCF MO and SCEP methods. The calculated energies are in good agreement with the experimental enthalpies. 相似文献
78.
Tachibana Y Otsuka K Terabe S Arai A Suzuki K Nakamura S 《Journal of chromatography. A》2003,1011(1-2):181-192
A robust and simple interface for microchip electrophoresis-mass spectrometry (MCE-MS) was developed using a spray nozzle connected to the exit of the separation channel of the microchip. The spray nozzle was attached to the microchip using a polyether ether ketone screw without adhesive, thus allowing easy replaced. Sample injection and electrophoretic separation was performed by control of the voltage only. The analysis of a few basic drugs was performed using the optimized MCE-MS system. The separation was improved by using a high-viscosity separation buffer and a spray nozzle with a small bore size. This system was also applied to the separation of peptides and protein-trypsin digests. Sample adsorption was minimized by adding acetonitrile to the separation buffer when using a quartz microchip. 相似文献
79.
Yamashita M Yamamoto Y Akiba KY Hashizume D Iwasaki F Takagi N Nagase S 《Journal of the American Chemical Society》2005,127(12):4354-4371
Pentacoordinate and tetracoordinate carbon and boron compounds (27, 38, 50-52, 56-61) bearing an anthracene skeleton with two oxygen or nitrogen atoms at the 1,8-positions were synthesized by the use of four newly synthesized tridentate ligand precursors. Several carbon and boron compounds were characterized by X-ray crystallographic analysis, showing that compounds 27, 56-59 bearing an oxygen-donating anthracene skeleton had a trigonal bipyramidal (TBP) pentacoordinate structure with relatively long apical distances (ca. 2.38-2.46 A). Despite the relatively long apical distances, DFT calculation of carbon species 27 and boron species 56 and experimental accurate X-ray electron density distribution analysis of 56 supported the existence of the apical hypervalent bond even though the nature of the hypervalent interaction between the central carbon (or boron) and the donating oxygen atom was relatively weak and ionic. On the other hand, X-ray analysis of compounds 50-52 bearing a nitrogen-donating anthracene skeleton showed unsymmetrical tetracoordinate carbon or boron atom with coordination by only one of the two nitrogen-donating groups. It is interesting to note that, with an oxygen-donating skeleton, the compound 61 having two chlorine atoms on the central boron atom showed a tetracoordinate structure, although the corresponding compound 60 with two fluorine atoms showed a pentacoordinate structure. The B-O distances (av 2.29 A) in 60 were relatively short in comparison with those (av 2.44 A) in 59 having two methoxy groups on the central boron atom, indicating that the B-O interaction became stronger due to the electron-withdrawing nature of the fluorine atoms. 相似文献
80.
Kobayashi K Ishii K Sakamoto S Shirasaka T Yamaguchi K 《Journal of the American Chemical Society》2003,125(35):10615-10624
The guest- or solvent-induced assembly of a tetracarboxyl-cavitand 1 and a tetra(3-pyridyl)-cavitand 2 into a heterodimeric capsule 1.2 in a rim-to-rim fashion via four intermolecular CO(2)H.N hydrogen bonds has been investigated both in solution and in the solid state. In the (1)H NMR study, a 1:1 mixture of1a and 2a (R = (CH(2))(6)CH(3)) in CDCl(3) gave a mixture of various complicated aggregates, whereas this mixture in CDCl(2)CDCl(2) or p-xylene-d(10) exclusively produced the heterodimeric capsule 1a.2a. It was found that an appropriate 1,4-disubstituted-benzene is a suitable guest for inducing the exclusive formation of 1a.2a in CDCl(3). The ability of a guest to induce the formation of guest-encapsulating heterodimeric capsule, guest@(1a.2a), increased in the order p-ethyltoluene < 1-ethyl-4-methoxybenzene < or = 1-ethyl-4-iodobenzene < or = 1,4-dibromobenzene < 1-iodo-4-methoxybenzene < or= 1,4-dimethoxybenzene < or = 1,4-diiodobenzene. The (1)H NMR study revealed that a CH-halogen interaction between the inner protons of the methylene-bridge rims (-O-H(out)CH(in)-O-) of the 1a and 2a units and the halogen atoms of 1,4-dihalobenzenes and a CH-pi interaction between the methoxy protons of 1,4-dimethoxybenzene and the aromatic cavities of the 1a and 2a units play important roles in the formation of 1,4-dihalobenzene@(1a.2a) and 1,4-dimethoxybenzene@(1a.2a), respectively. A preliminary single-crystal X-ray diffraction analysis of guest@(1b.2b) (R = (CH(2))(2)Ph; guest = 1-iodo-4-methoxybenzene or p-xylene) confirmed that the guest encapsulated in 1b.2b is oriented with the long axis of the guest along the long axis of 1b.2b and that the iodo and the methoxy groups of the encapsulated 1-iodo-4-methoxybenzene are specifically oriented with respect to the cavities of the 2b and 1b units, respectively. 相似文献