全文获取类型
收费全文 | 2789篇 |
免费 | 48篇 |
国内免费 | 13篇 |
专业分类
化学 | 2070篇 |
晶体学 | 18篇 |
力学 | 57篇 |
数学 | 343篇 |
物理学 | 362篇 |
出版年
2020年 | 19篇 |
2019年 | 19篇 |
2018年 | 13篇 |
2017年 | 21篇 |
2016年 | 34篇 |
2015年 | 36篇 |
2014年 | 46篇 |
2013年 | 133篇 |
2012年 | 114篇 |
2011年 | 130篇 |
2010年 | 70篇 |
2009年 | 48篇 |
2008年 | 137篇 |
2007年 | 132篇 |
2006年 | 131篇 |
2005年 | 133篇 |
2004年 | 118篇 |
2003年 | 75篇 |
2002年 | 100篇 |
2001年 | 38篇 |
2000年 | 44篇 |
1999年 | 39篇 |
1998年 | 28篇 |
1997年 | 34篇 |
1996年 | 57篇 |
1995年 | 36篇 |
1994年 | 42篇 |
1993年 | 38篇 |
1992年 | 30篇 |
1991年 | 36篇 |
1990年 | 29篇 |
1989年 | 23篇 |
1988年 | 30篇 |
1987年 | 27篇 |
1986年 | 23篇 |
1985年 | 51篇 |
1984年 | 51篇 |
1983年 | 42篇 |
1982年 | 60篇 |
1981年 | 58篇 |
1980年 | 64篇 |
1979年 | 50篇 |
1978年 | 58篇 |
1977年 | 41篇 |
1976年 | 34篇 |
1975年 | 48篇 |
1974年 | 44篇 |
1973年 | 30篇 |
1972年 | 22篇 |
1968年 | 17篇 |
排序方式: 共有2850条查询结果,搜索用时 15 毫秒
31.
The products of the reaction of butadiene with diethyl malonate catalysed by a palladium complex and of butadiene with acetaldehyde catalysed by a nickel(0) complex have been used in two syntheses of 3,7-dimethylpentadecan-2-ol. 相似文献
32.
33.
Klapars A Waldman JH Campos KR Jensen MS McLaughlin M Chung JY Cvetovich RJ Chen CY 《The Journal of organic chemistry》2005,70(24):10186-10189
[reaction: see text] A mild and transition-metal-free method for the alpha-arylation of aliphatic nitriles with activated heteroaryl halides was developed using NaHMDS or KHMDS as base at ambient temperature. The key to the success of this method is generation of the nitrile anion in the presence of the heteroaryl halide. The method is applicable to both primary and secondary carbonitriles and a wide range of heteroaryl halides. Selective monoarylation was observed with primary carbonitriles. The operational simplicity and the mild reaction conditions add to the value of this method as a practical alternative to the preparation of alpha-heteroaryl carbonitriles. 相似文献
34.
Nordesferriferrithiocin, NDFFTH(2), is a derivative of the siderophore desferriferrithiocin, DFFTH(2), in which the methyl group is substituted by a hydrogen atom. Both compounds show high oral activity as possible drugs for the treatment of iron overload. While DFFTH(2) is significantly toxic, NDFFTH(2) exhibits a lower toxicity and offers a much better therapeutic window than other orally active iron chelators. In this study, complexes of DFFTH(2) and NDFFTH(2) with various trivalent metals have been synthesized and characterized. Five isomers (the maximum possible) have been observed in the case of [Co(DFFT)(2)](-) in solution, as proved by (1)H-NMR measurements. Although normally labile, complexes of Al(3+) ([Al(DFFT)(2)](-)) have been separated by HPLC. In general, DFFTH(2) forms kinetically inert complexes whereas complexes of NDFFTH(2) tend to isomerize quickly in solution, as indicated by CD spectroscopy of separated HPLC fractions of [Cr(NDFFT)(2)](-). The most stable isomers of the aluminum complexes of both ligands have been characterized by X-ray crystallography; K[Al(DFFT)(2)] crystallizes from methanol/diethyl ether in the orthorhombic space group P2(1)2(1)2 with a = 11.238(3) ?, b = 31.719(11) ?, c = 7.684(2) ?, V = 2739.2(24) ?(3), and Z = 4. This isomer has the mer-(N,O-Lambda)(S,S) configuration, while K[Al(NDFFT)(2)] crystallizes from methanol/diethyl ether in the space group P6(1) (a = 21.269(8) ?, c = 9.643(3) ?, V = 3777.8(42) ?(3), Z = 6) and has the same coordination geometry. The solution thermodynamics of the Al(3+), Ga(3+), and Fe(3+) complexes have been studied by spectrophotometric titration. The stability constants (log K) are 23.6(1), 29.2(3), and 31.04(3), respectively, for the DFFTH(2) complexes and 22.0(1), 27.8(2), and 29.09(3), respectively, for the NDFFTH(2) complexes. Cyclic voltammograms of both iron complexes have been recorded in water at a carbon disk working electrode and in DMF at a graphite working electrode. The reduction waves measured in DMF indicate no reversibility whereas in water a quasi-reversible reduction is observed. The reduction potentials (E(1/2)'s) in water are -166 mV for [Fe(DFFT)(2)](-) and -97 mV for [Fe(NDFFT)(2)](-) versus NHE. These potentials are well in the range for biological reductants, which makes possible an in vivo reduction mechanism for the iron removal from the siderophore. 相似文献
35.
3-(1-Methylethyl)-7-methyl2,6-octadienal, was prepared from 1,1,3-triethoxy-3,4-dimethylpentane and 3-methyl-2-buten-l-ol in a one-pot procedure utilising Claisen and Cope rearrangement of an intermediate dienol-allyl ether. 相似文献
36.
Crystal and Molecular Structure of Macrocyclic Musks. III. Muscone and its 2, 4-Diuitrophenylhydrazone Muscone (3-methylcyclopentadecanone C16H30O) forms orthorhombic plastic-crystals with Z = 2 (a = 5.560(4), b = 8.176(6), c = 17,168(20) Å). Single crystals are obtained by zone melting at 258 °K. The high degree of disorder and the ambiguity of space group have precluded a structural determination by X-ray diffraction methods. The crystal structure of muscone 2,4-dinitrophenylhydrazone (DNPHM; C22H34N4O4, triclinic, space group P¯, a = 8.015 (4), b = 8.235 (2), c = 17.409 (4) Å. α = 102.69 (4), β; = 93.86 (3), γ = 96.03 (3)° Z = 2) was solved by direct methods and refined to a final R of 0.155 (Rω = 0.102). The macrocycle shows static disorder, which could be partially resolved by analysis of the anisotropy of the atomic vibrational parameters. Eight possible conformers with reasonable geometries emerged from the subsequent refinements and were used as starting models in force-field calculations, which converged to four quinquangular conformations ([25233]; [34233]; [21423*4]; [3423*4]) with similar energies. In the molecular packing, the interactions between macrocycles and substituents are minimized. They are comparable to those described for the cis- and trans-civetone DNPH derivatives [1] [2]. 相似文献
37.
Raymond J. Abraham Lee Griffiths Philip Loftus 《Journal of computational chemistry》1982,3(3):407-416
Alternative methods of estimating atomic charges in haloalkanes are presented, derived from quantum mechanical and classical treatments. A scheme based on a breakdown of the transmission of charge by polar atoms into one-bond, two-bond, and three-bond additive contributions is given, in which the one-bond effect is proportional to the difference in the electronegativities of the bonded atoms, and the two- and three-bond effects functions of the atomic electronegativity and polarizability. Suitable developments of the basic scheme, including an iterative self-consistent process, give calculated dipole moments for a variety of haloalkanes in good agreement with the observed values. The atomic charges obtained by this scheme are compared with other estimates of these charges. They are similar to those derived from a simple LCAO –MO scheme but differ from those obtained by population analysis of more refined quantum mechanical calculations. 相似文献
38.
The Floating Point Systems, Inc. Model 164 Attached Processor (FPS-164) is a high-speed, pipelined, parallel processor designed for large-scale scientific computation. Benchmark studies of operations common in quantum chemistry codes are discussed and the performance of the FPS-164 is compared with other commonly available computers. A complete system of electronic structure codes has been implemented on the FPS-164 using the Fortran-77 cross-compiler and calls to optimized vector and matrix routines. The conversion of a generalized valence bond (GVB ) code illustrates the strategy adopted to adapt Fortran codes to the FPS-164. A typical production example, a large scale (GVB ) and configuration interaction calculation on the vinyl radical, shows a net throughput equivalent to nearly nine VAX 11/780 computers. 相似文献
39.
Tongqian Chen Eric Voelk Matthew S. Platz Raymond P. Goodrich † 《Photochemistry and photobiology》1996,64(4):622-631
Abstract— The photochemistry and photophysics of 3-amino-6-io-doacridine (Acr-I) was studied. Photolysis (350 nm) of Acr-I (free base) generates products consistent with a free radical intermediate in methanol, benzene and carbon tetrachloride. The Acr-I hydrochloride is shown to bind to calf thymus DNA and to the self-complementary dinucleotide cytidylyl-(3′-5′)-guanosine (CpG) minidu-plex in a manner similar to that of proflavine (Acr-NH2), a known DNA intercalator. The Acr-I is shown to more efficiently nick supercoiled plasmid DNA pBR322 upon 350 nm or 420 nm photolysis than Acr-NH2. The efficiency of Acr-I-sensitized DNA nicking is not oxygen dependent. Photolysis of the Acr-I/(CpG)2 complex leads to cleavage of the dinucleotide and to cytidine base release by selective damage to a specific ribose moiety. Dinucleotide cleavage occurs equally well in the presence or absence of oxygen, thereby eliminating a singlet oxygen- or peroxyl radical-mediated process. Photolysis of Acr-I in the presence of a mononucleotide (GMP) or a non-self-complementary dinucleotide (uridylyl-[3′-5′]-cytidine– UpC) does not lead to fragmentation and base release. Similarly, photolysis of the Acr-NH2/(CpG)2 complex does not lead to fragmentation and base release. The data indicate that photolysis of an iodinated intercalator bound to CpG or plasmid DNA generates an intercalated aryl radical and that the reactive intermediate initiates a sequence of reactions that efficiently nick nucleic acids. The inactivation of Λ phage sensitized by Acr-I with UV (350 nm) light is oxygen independent but with visible (420 nm) light is strongly oxygen dependent. The Acr-I fluoresces more intensely when excited at 446 than at 376 nm. Thus, UV photolysis may lead to C-I bond homolysis and free radical formation, a process that is not energetically feasible with visible light. The results demonstrate the difficulty of extrapolating model studies involving simple molecules and DNA to understanding the mechanism of viral inactivation with a particular sensitizer. 相似文献
40.
Hirotaka Kudo Marvin L. Tedjamulia Raymond N. Castle Milton L. Lee 《Journal of heterocyclic chemistry》1984,21(6):1833-1839
The synthesis of phenanthro[1,2-c]dibenzothiophene (6) , phenanthro[4,3-c]dibenzothiophene (10) , phenanthro[2,1-a]dibenzothiophene (14) , phenanthro[3,4-a]dibenzothiophene (16) , phenanthro[1,2-a]dibenzothiophene (19) , phenanthro[2,1-b]dibenzothiophene (20) , 8-methylphenanthro[3,2-a]dibenzothiophene (24) , 7-methylphenanthro[1,2-a]dibenzothiophene (25) , phenanthro[3,4-a]dibenzothiophene (27) , phenanthro[4,3-a]-dibenzothiophene (28) , 6-methylphenanthro[2,3-a]dibenzothiophene (31) , and 5-methylphenanthro[4,3-a]dibenzothiophene (32) is described. 相似文献