全文获取类型
收费全文 | 29014篇 |
免费 | 712篇 |
国内免费 | 233篇 |
专业分类
化学 | 19763篇 |
晶体学 | 130篇 |
力学 | 650篇 |
综合类 | 1篇 |
数学 | 5446篇 |
物理学 | 3969篇 |
出版年
2022年 | 194篇 |
2021年 | 281篇 |
2020年 | 357篇 |
2019年 | 321篇 |
2018年 | 285篇 |
2017年 | 298篇 |
2016年 | 613篇 |
2015年 | 628篇 |
2014年 | 678篇 |
2013年 | 1428篇 |
2012年 | 1438篇 |
2011年 | 1747篇 |
2010年 | 980篇 |
2009年 | 838篇 |
2008年 | 1505篇 |
2007年 | 1505篇 |
2006年 | 1593篇 |
2005年 | 1426篇 |
2004年 | 1347篇 |
2003年 | 1122篇 |
2002年 | 1071篇 |
2001年 | 463篇 |
2000年 | 419篇 |
1999年 | 404篇 |
1998年 | 395篇 |
1997年 | 442篇 |
1996年 | 411篇 |
1995年 | 369篇 |
1994年 | 396篇 |
1993年 | 348篇 |
1992年 | 327篇 |
1991年 | 324篇 |
1990年 | 282篇 |
1989年 | 284篇 |
1988年 | 266篇 |
1987年 | 229篇 |
1986年 | 255篇 |
1985年 | 367篇 |
1984年 | 376篇 |
1983年 | 321篇 |
1982年 | 350篇 |
1981年 | 325篇 |
1980年 | 340篇 |
1979年 | 270篇 |
1978年 | 302篇 |
1977年 | 281篇 |
1976年 | 223篇 |
1975年 | 192篇 |
1974年 | 196篇 |
1973年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Peter D. Dacre 《Theoretical chemistry accounts》1976,43(2):197-205
The direct CI method, which avoids explicit calculation of the Hamiltonian matrix, is presented in a new form. The method is linked with Davidson's algorithm for iterative evaluation of the ground state eigenvector. The viability of the method is indicated by the test calculations on water which are described. 相似文献
92.
Denis W. Clack Leon A. P. Kane-Maguire David H. Knight Peter A. Williams 《Transition Metal Chemistry》1980,5(1):376-378
Summary Molecular orbital calculations using-, the INDO method have been carried out fm the [FeL(CN)4]2 and [FeLH(CN)4] complexes, as well as for the tree ligands, with L 2 × pyridine, 1,10-phenanthroline.2,2-bipyridyl. 2,2-rimidine, 2, 3,3-bipyridazine and 4.4-bipyrimidine. Calculations of residual charge at carbon atoms in the ligand rings. correlating with relative nucleophilicity of the compounds, corresponds with observed differences of rate of reaction of the complex [FeI;]2 With nucleophiles. 相似文献
93.
Stang PJ 《The Journal of organic chemistry》2003,68(8):2997-3008
Tricoordinate, I(III), and pentacoordinate, I(V), polyvalent iodine compounds have been known for over a century. In the last twenty years, new polyvalent iodine reagents have been introduced along with synthetic methodologies, based on these and derived reagents, that play an ever increasing role in contemporary organic chemistry. In this Perspective, an overview of these developments is provided with emphasis on the chemistry and uses of aryl-, alkenyl-, and alkynyliodonium salts in preparative and synthetic organic chemistry. It is hoped that this brief overview, along with recent more comprehensive reviews of the field, will stimulate further developments and applications of this useful class of compounds across a broad spectrum of organic chemistry. 相似文献
94.
New experimental time-of-flight distributions are reported for Li+-N2 and Li2-CO at two center-of-mass energies of about 8 and 16 eV and large scattering angles θlab ? 120°. The Li+-N2 spectra show two widely spaced maxima, whereas the Li+-CO spectra show two and sometimes three maxima. The results are consistent with the model of rotational rainbows, and have also been analyzed in terms of an impulsive model involving collisions with the individual atoms of the molecules with energy-dependent masses. Classical trajectories for a simple model potential reveal only small contributions from vibrational excitation. 相似文献
95.
Hans Beat Bürgi Hermann Gehrer Peter Strickler Fritz Karl Winkler 《Helvetica chimica acta》1976,59(7):2558-2565
Crystals of the title compound are triclinic, a = 27.87 Å, b = 10.77 Å, c = 12.94 Å, α 73.1°, β 116.1°, γ 120.0°, space group P1 . The structure consists of octanuclear ions: Eight Cd(II) ions are found at the corners of a distorted cube, the center of the cube is occupied by an iodide, the twelve thioglycolate sulfur atoms bridge the twelve edges of the cube thereby forming a distorted icosahedron. Cadmium ions are either five or seven coordinate. The phase problem for this structure was solved using a combination of very high and very low E-values. 相似文献
96.
J. Ty Redd Jerald S. Bradshaw Peter Huszthy Reed M. Izatt 《Journal of heterocyclic chemistry》1994,31(4):1047-1052
Eight new macrocyclic ligands containing the pyrimidine subcyclic unit ( 3-10 , Figure 1) have been prepared. Two of these new crown ethers are chiral. Pyrimidino-crowns 3-8 were prepared by treating the di-tosylate derivative of the appropriate oligoethylene glycol with 4-methoxy-5-raethyl-2,6-pyrimidinedimeth-anol in basic conditions. The yields were in the 30-50% range giving the crowns as viscous oils. Chiral dimethyl-substituted pyrimidino-crown 9 was prepared from 4-methoxy-5-methyl-2,6-pyrimidinedimethyl di-tosylate and chiral dimethyl-substituted tetraethylene glycol. Treatment of dimethyl 4-methoxy-5-methyl-2,6-pyrimidinedicarboxylate with the diamine derivative of chiral dibenzyl-substituted tetraethylene glycol gave the chiral dibenzyl-substituted pyrimidino-crown diamide 10. Starting 4-methoxy-5-methyl-2,6-pyrimidinedi-methanol was prepared by a six step process from acetamidine hydrochloride and diethyl oxalpropionate. 相似文献
97.
Rydberg spectra are shown to provide a spectral fingerprint that is sensitive to molecular structure in unique ways. The concepts are demonstrated using a set of isomeric fluorophenols and a sequence of aliphatic diamines. In the fluorophenols, the sensitivity extends to the placement of a single hydrogen atom and can be traced to the molecular charge distributions associated with the locations of atoms and functional groups with respect to the charge center. Experiments on tetramethyl diamines demonstrate that the structural sensitivity encompasses the extended molecular structure, including parts of the molecule that are remote from the ionization center. This global structure sensitivity makes Rydberg fingerprint spectroscopy uniquely suited to characterize structures of large-scale molecular systems. 相似文献
98.
Using ring closing metathesis (RCM) as the key operation, a rapid access to β-lactams fused to a sultam moiety of variable ring size was developed from low cost, commercially available starting materials. An efficient RCM of 4-vinyl-azetidin-2-ones to give 1-aza-bicyclo[4.2.0]oct-4-en-8-ones is also reported. 相似文献
99.
John H. Bowie Peter Y. White Thomas Blumenthal 《Journal of mass spectrometry : JMS》1987,22(8):541-544
Deprotonated 2-phenyl-1,3-dithiane undergoes competitive losses of H˙, C3H6 C2H4S, C3H4S, C7H6S, C10H10 and C10H10S upon collisional activation. The elimination of H occurs from the phenyl ring. The loss of C3H6 occurs by simple cleavage of the dithiane ring. All other processes involve specific proton transfer followed by either cleavage or internal nucleophilic displacement. 相似文献
100.
When the nucleation of a stable crystalline phase directly in a supersaturated old phase is greatly retarded, the crystal nuclei might nucleate within faster-forming particles of an intermediate phase. Here we present a theoretical investigation of the kinetics of this two-step nucleation of crystals and derive general expressions for the time dependence of the number of crystals nucleated within the particles of the intermediate phase. The results reveal that crystal nucleation can be strongly delayed by the slow growth of the particles and/or by the slow nucleation of the crystals in them. Furthermore, the linear part of the time dependence of the number of nucleated crystals is determined by the formation rate of the intermediate particles. This is in contrast with the one-step nucleation of crystals when this linear part is determined by the rate of crystal nucleation directly in the old phase. Criteria are proposed for distinction between the one- and two-step nucleation mechanisms, based on the supersaturation dependence of the delay time for nucleation. The application of the theoretical approach to the analysis of experimental data on the nucleation of crystals and other ordered aggregates of protein and other soluble materials is discussed. 相似文献