全文获取类型
收费全文 | 4640篇 |
免费 | 136篇 |
国内免费 | 15篇 |
专业分类
化学 | 3355篇 |
晶体学 | 48篇 |
力学 | 121篇 |
数学 | 555篇 |
物理学 | 712篇 |
出版年
2023年 | 20篇 |
2021年 | 42篇 |
2020年 | 51篇 |
2019年 | 58篇 |
2018年 | 34篇 |
2017年 | 38篇 |
2016年 | 108篇 |
2015年 | 73篇 |
2014年 | 102篇 |
2013年 | 198篇 |
2012年 | 264篇 |
2011年 | 316篇 |
2010年 | 177篇 |
2009年 | 135篇 |
2008年 | 259篇 |
2007年 | 284篇 |
2006年 | 296篇 |
2005年 | 259篇 |
2004年 | 219篇 |
2003年 | 218篇 |
2002年 | 196篇 |
2001年 | 59篇 |
2000年 | 58篇 |
1999年 | 33篇 |
1998年 | 46篇 |
1997年 | 50篇 |
1996年 | 78篇 |
1995年 | 38篇 |
1994年 | 42篇 |
1993年 | 39篇 |
1992年 | 22篇 |
1991年 | 38篇 |
1990年 | 39篇 |
1989年 | 33篇 |
1988年 | 39篇 |
1987年 | 23篇 |
1986年 | 27篇 |
1985年 | 57篇 |
1984年 | 59篇 |
1983年 | 33篇 |
1982年 | 62篇 |
1981年 | 60篇 |
1980年 | 54篇 |
1979年 | 43篇 |
1978年 | 52篇 |
1977年 | 46篇 |
1976年 | 36篇 |
1975年 | 39篇 |
1974年 | 34篇 |
1973年 | 35篇 |
排序方式: 共有4791条查询结果,搜索用时 62 毫秒
151.
Thale Z Johnson T Tenney K Wenzel PJ Lobkovsky E Clardy J Media J Pietraszkiewicz H Valeriote FA Crews P 《The Journal of organic chemistry》2002,67(26):9384-9391
A reinvestigation of sponge natural products from additional Indo-Pacific collections of Xestospongiacf. carbonaria and X. cf. exigua has provided further insights on the structures, biological activities, and biosynthetic origin of bisannulated acridines. These alkaloids include one known pyridoacridine, neoamphimedine (2), and three new analogues, 5-methoxyneoamphimedine (4), neoamphimedine Y (5), and neoamphimedine Z (6). A completely new acridine, alpkinidine (7), was also isolated. A disk diffusion soft agar assay, using a panel of five cancer cell lines (solid tumors and leukemias) and two normal cells, was used to evaluate the differential cytotoxicity (solid tumor selectivity) of the sponge semipure extracts and selected compounds including amphimedine (1), 2, 4, and 7. While all four compounds were solid tumor selective, 1 and 2 were the most potent and 4 was the most selective. The rationale used to characterize the new structures is outlined along with the related biosynthetic pathways envisioned to generate 2 and 7. 相似文献
152.
An efficient strategy is described for the synthesis of enantiopure calystegine alkaloids. The key step employs a zinc-mediated fragmentation of benzyl-protected methyl 6-iodo-glycosides followed by in situ formation of the benzyl imine and Barbier-type allylation with zinc, magnesium, or indium metal. Stereochemistry in the pivotal allylation is controlled by the choice of the metal. The functionalized 1,8-nonadienes, thus formed, are converted into cycloheptenes by ring-closing olefin metathesis. Regioselective hydroboration and oxidation give the corresponding cycloheptanones, which are deprotected to afford the desired calystegines. Hereby, calystegine B(2), B(3), and B(4) are prepared from D-glucose, D-galactose, and D-mannose, respectively. This route constitutes the shortest synthesis of calystegine B(2) and gives rise to the first total syntheses of calystegine B(3) and B(4). 相似文献
153.
Zech SG Olejniczak E Hajduk P Mack J McDermott AE 《Journal of the American Chemical Society》2004,126(43):13948-13953
A novel approach for detection of ligand binding to a protein in solid samples is described. Hydrated precipitates of the anti-apoptotic protein Bcl-xL show well-resolved (13)C-(13)C 2D solid-state NMR spectra that allow site-specific assignment of resonances for many residues in uniformly (13)C-enriched samples. Binding of a small peptide or drug-like organic molecule leads to changes in the chemical shift of resonances from multiple residues in the protein that can be monitored to characterize binding. Differential chemical shifts can be used to distinguish between direct protein-ligand contacts and small conformational changes of the protein induced by ligand binding. The agreement with prior solution-state NMR results indicates that the binding pocket in solid and liquid samples is similar for this protein. Advantages of different labeling schemes involving selective (13)C enrichment of methyl groups of Ala, Val, Leu, and Ile (Cdelta1) for characterizing protein-ligand interactions are also discussed. It is demonstrated that high-resolution solid-state NMR spectroscopy on uniformly or extensively (13)C-enriched samples has the potential to screen proteins of moderate size ( approximately 20 kDa) for ligand binding as hydrated solids. The results presented here suggest the possibility of using solid-state NMR to study ligand binding in proteins not amenable to solution NMR. 相似文献
154.
Light-induced metastable linkage isomers of trans-[Ru(NH(3))(4)Cl(SO(2))]Cl and trans-[Ru(NH(3))(4)(H(2)O)(SO(2))](C(6)H(5)SO(3))(2) have been identified for the first time using photocrystallographic methods. In both linkage isomers the SO(2) ligand is side bound, but the Ru-O and Ru-S distances are considerably longer and almost equal in the trans-H(2)O isomer. DFT calculations confirm that both isomers correspond to minima on the ground-state potential energy surface and also predict the existence of a second oxygen-bound isomer for both compounds. The decay of the light-induced species has been studied by both DSC and IR. Activation energies for the thermal back-reaction, as derived from the temperature-dependent disappearance of light-induced IR bands, are 50.0 and 58.4 kJ/mol for the two isomers, which is larger than the corresponding numbers for photoinduced side-bound nitrosyl linkage isomers. 相似文献
155.
Anthony B. Cornwell Philip G. Harrison John A. Richards 《Journal of organometallic chemistry》1977,140(3):273-287
The reaction of organotin chlorides with the lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) or hexaalkylditins with TCNQ yield stable organotin-substituted free radicals of the types R3SnTCNQ. (R = Me, n-Pr, n-Bu) and Me2Sn(TCNQ.)2. The reaction of hexaphenylditin with TCNQ yields a (σ → π) charge transfer complex of stoichiometry (Ph3SnSnPh3)·TCNQ, whilst [Me2SnCl(terpyridyl)+](TCNQ-·) was isolated from the reaction of [Me2SnCl(terpyridlyl)+][Me2SnCl3-] and LiTCNQ. The oxidation of hexaalkylditins by tetracyanoethylene (TCNE) yields stable free radicals of the type R3SnTCNE·, but treatment with 2,3,5,6-tetrachlorobenzoquinone yields either R3SnOC6Cl4O·-p (R = Me) or R3SnOC6Cl4OSnR3-p (R = n-Bu, Ph). Tin-119 Mössbauer spectroscopy shows that the derivatives R3SnTCNQ· and R3TCNE· have trigonally-bipyramidally coordinated tin with planar [SnC3] skeletons and bridging [TCNQ·] and [TCNE·] groups forming infinite one-dimensional chain structures. Me3SnOC6Cl4O·-p was inferred to possess a similar structure but with oxy bridges forming chains with a Sn---O---Sn---O backbone. Me2Sn(TCNQ·)2 has a structure intermediate between tetrahedral and octahedral with a non-linear MeSnMe unit and anisobidentate chelation by two TCNQ groups. The TCNQ derivatives were of two types: (i) “green” or “brown”, indicative of delocalisation of the Ione electron over the cyanoquinone ligand, and (ii) a “blue” form in which spin-pairing of the Ione electron between adjacent organic groups takes place. Me3SnTCNQ· may exist in both forms depending upon the mode of preparation. 相似文献
156.
Robin D. Rogers Philip D. Richards 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(5):631-638
The title complex was crystallized from a saturated solution of 18-crown-6 in nitromethane at 5°C and cooled to –150°C prior to X-ray diffraction data collection. At –150° C 18-crown-6·2(CH3NO2) is monoclinic,P21/n witha=9.290(2),b=7.864(6),c=13.627(8) Å, =1000.84(4)° andD
calc=1.31 g cm–3 for Z=2. Leastsquares refinement using 1521 independent observed reflections [F
o5(F
o)] led to a final conventionalR value of 0.041. The complex at –150°C is isostructural with its room temperature structure with the exception of the orientation of the methyl hydrogen atoms and their crown ether oxygen interactions. The methyl group hydrogen atoms were fully refined isotropically. The crown ether resides around a center of inversion and hasD
3d symmetry. There is one methyl hydrogen...crown interaction at 2.35(3) Å, one apparently bifurcated hydrogen bond utilizing a second methyl hydrogen atom (2.55(3), 2.65(3) Å) and the third hydrogen atom is actually directed away from the crown ring (closest H...O contact=2.67(3) Å).
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82048 (5 pages).For part 2, see reference [24]. 相似文献
157.
The total synthesis of tetrahydroisoquinoline alkaloids (+/-)-renieramycin G (4) and a lemonomycinone analogue (7) is described. A general strategy to synthesize both the mono- and bistetrahydroisoquinoline alkaloids from a common advanced intermediate, 17, is presented. 相似文献
158.
Yurii N. Panchenko Sergei V. Krasnoshchiokov Philip George Charles W. Bock 《Structural chemistry》1992,3(1):15-26
The results of complete geometry optimizations of the high energy stable gauche,Trans,trans- and gauche,Cis,trans- rotamers of hexa-1,3,5-trienes are reported at the RHF/6-31G//RHF/B-31G level. The angles of rotation around one of the single C-C bonds are found to be 33.7° and 45.5°, respectively. The corresponding harmonic force fields of these molecules are also reported at this level and corrected using scale factors transferred from buta 1,3-diene. Aspecial scale factor was used for the central C=C double bond stretching coordinate to take into account vibronic coupling. The theoretical vibrational frequencies, calculated with the scaled quantum mechanical (SQM) force fields, allow a complete interpretation of the experimental vibrational spectra of these molecules.Preliminary results were reported at the Austin XII Symposium on Molecular Structure, Austin, TX, February 28 through March 3, 1988, S 18, p. 111 (USA) and at the XIXth European Congress on Molecular Spectroscopy, Dresden, September 4 through September 8, 1989, p. 226 (GDR). 相似文献
159.
Relative intensities of the Stokes and anti-Stokes Raman lines associated with the I-I stretching mode of I2 and symmetric stretching mode of MnO?4 are presented. The data indicate that the maxima in the excitation profile of the anti-Stokes scattering are shifted from those of the Stokes scattering. The experimental Stokes/anti-Stokes intensity ratios agree with the theoretical values obtained with parameters from the electronic absorption spectra. 相似文献
160.
The bis[1-(9-anthracene)phosphirane]dithiolatoplatinum(II) complexes, Pt[1-(9-anthracene)phosphirane](2)(dithiolate), where dithiolate = 1,1-dimethoxycarbonyl-ethylene-2,2-dithiolate (dmdt) (2), 1,1-diethoxycarbonyl-ethylene-2,2-dithiolate (dedt) (3), 1-ethoxycarbonyl-1-cyano-ethylene-2, 2-dithiolate (ecdt) (4), and 1,1-dicyano-ethylene-2,2-dithiolate (dcdt) (5), were prepared from cis-dichlorobis[1-(9-anthracene)phosphirane]platinum(II) (1). Complexes 3 and 5 were characterized by X-ray crystallography and were found to have vastly different crystal and molecular structures. The crystal and molecular structure of 3 is dominated by intramolecular pi-stacking between the anthracene rings of the cis-bis(anthracene)phosphiranes with a ring...ring separation of 3.48(6) A. The molecular structure of 5 does not exhibit an intramolecular interaction between the anthracene rings. Instead, the crystal structure of 5shows significant intermolecular pi-stacking between the anthracene rings of the phosphirane ligands of adjacent molecules packed in the crystal lattice. The intermolecular stacking interaction results in a ring...ring separation of 3.33(4) A. Complexes 2-5 were found to emit at 530 nm at low temperatures in the solid state. Complex 5 emits strongly in fluid THF or benzene solution at 430 nm. 相似文献