全文获取类型
收费全文 | 2259篇 |
免费 | 60篇 |
国内免费 | 8篇 |
专业分类
化学 | 1603篇 |
晶体学 | 20篇 |
力学 | 51篇 |
数学 | 274篇 |
物理学 | 379篇 |
出版年
2022年 | 10篇 |
2021年 | 27篇 |
2020年 | 36篇 |
2019年 | 27篇 |
2018年 | 28篇 |
2017年 | 15篇 |
2016年 | 38篇 |
2015年 | 28篇 |
2014年 | 27篇 |
2013年 | 120篇 |
2012年 | 96篇 |
2011年 | 123篇 |
2010年 | 82篇 |
2009年 | 50篇 |
2008年 | 103篇 |
2007年 | 119篇 |
2006年 | 131篇 |
2005年 | 100篇 |
2004年 | 100篇 |
2003年 | 66篇 |
2002年 | 85篇 |
2001年 | 46篇 |
2000年 | 51篇 |
1999年 | 34篇 |
1998年 | 31篇 |
1997年 | 32篇 |
1996年 | 35篇 |
1995年 | 44篇 |
1994年 | 35篇 |
1993年 | 28篇 |
1992年 | 24篇 |
1991年 | 34篇 |
1990年 | 20篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 22篇 |
1986年 | 23篇 |
1985年 | 35篇 |
1984年 | 29篇 |
1983年 | 22篇 |
1982年 | 30篇 |
1981年 | 16篇 |
1980年 | 32篇 |
1979年 | 31篇 |
1978年 | 21篇 |
1977年 | 29篇 |
1976年 | 14篇 |
1975年 | 22篇 |
1974年 | 20篇 |
1973年 | 17篇 |
排序方式: 共有2327条查询结果,搜索用时 78 毫秒
81.
Alistair J. Sterling Alexander B. Dürr Russell C. Smith Edward A. Anderson Fernanda Duarte 《Chemical science》2020,11(19):4895
[1.1.1]Propellane is the ubiquitous precursor to bicyclo[1.1.1]pentanes (BCPs), motifs of high value in pharmaceutical and materials research. The classical Lewis representation of this molecule places an inter-bridgehead C–C bond along its central axis; ‘strain relief’-driven cleavage of this bond is commonly thought to enable reactions with nucleophiles, radicals and electrophiles. We propose that this broad reactivity profile instead derives from σ–π-delocalization of electron density in [1.1.1]propellane. Using ab initio and DFT calculations, we show that its reactions with anions and radicals are facilitated by increased delocalization of electron density over the propellane cage during addition, while reactions with cations involve charge transfer that relieves repulsion inside the cage. These results provide a unified framework to rationalize experimental observations of propellane reactivity, opening up opportunities for the exploration of new chemistry of [1.1.1]propellane and related strained systems that are useful building blocks in organic synthesis.A unified framework that explains the reactivity of [1.1.1]propellane through electron delocalization. 相似文献
82.
Russell P. Newton Jalal A. Khan Dipankar Ghosh James I. Langridge A. Gareth Brenton Frank M. Harris Terence J. Walton 《Journal of mass spectrometry : JMS》1991,26(5):447-453
The determination of cytidine 3′,5′-cyclic monophosphate-specific phosphodiesterase activity by means of fast-atom bombardment (FAB) mass Spectrometry with mass-analysed ion kinetic energy (MIKE) spectrum scanning is described. Initial efforts to determine the activity of the enzyme by this method were unsuccessful owing to the obfuscation of sample-related peaks by peaks emanating from the incubation buffer and cation adducts; dilution of buffer and a desalting procedure overcame these difficulties. In the resulting positive-ion FAB mass spectra, characteristic peaks of the enzyme substrate and product could be readily identified and the protonated molecular ions selected for MIKE scanning. By spiking enzyme incubates with known amounts of substrate and product, and measuring peak heights in the MIKE spectra of both spiked and unspiked samples, the substrate/product ratio at the end of a series of phosphodiesterase incubations was determined. From the data obtained, the Km and Vmax of the phosphodiesterase were calculated as 6.08 mM and 11 μmol min?1 mg?1, respectively, showing good agreement with the analogous values of 8.06 mM and 5.8 μmol?1 min?1 mg?1 obtained by radioactive assay. 相似文献
83.
Daniel L. Reger Russell P. Watson Mark D. Smith 《Journal of organometallic chemistry》2007,692(24):5414-5420
The new heterometallic complex {μ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br][Pt(p-tolyl)2]2 has been prepared by reaction of 1 equiv. of the dimer [Pt(p-tolyl)2(μ-SEt2)]2 with the monometallic rhenium precursor {1,3,5-[CH(pz)2]3C6H3}Re(CO)3Br, where 1,3,5-[CH(pz)2]3C6H3 is the tritopic, arene-linked bis(pyrazolyl)methane ligand 1,3,5-tris[bis(1-pyrazolyl)methyl]benzene. Similarly, the heterometallic complex {μ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]2[Pt(p-tolyl)2] has been made by the reaction of the dirhenium compound {μ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]2 and one-half of an equivalent of [Pt(p-tolyl)2(μ-SEt2)]2. X-ray crystallographic studies of the new compounds reveal significant noncovalent interactions in their molecular and supramolecular structures. 相似文献
84.
85.
86.
Singh SB Herath K Guan Z Zink DL Dombrowski AW Polishook JD Silverman KC Lingham RB Felock PJ Hazuda DJ 《Organic letters》2002,4(9):1431-1434
[structure: see text]. Integramides A and B are two novel 16-mer linear peptides rich in C(alpha)-methyl amino acids that were isolated from fungal extracts of Dendrodochium sp. by employing a bioassay-guided isolation procedure using recombinant HIV-1 integrase. The structure and stereochemistry were elucidated by a combination of 2D NMR and ESI- and FAB-MS including MS/MS studies and by Marfey's method. Integramides A and B inhibited the coupled reaction of HIV-1 integrase with IC50 values of 17 and 10 microM, respectively. 相似文献
87.
Scott L. Gibson Marwan K. Al-Shawi Alan E. Senior Russell Hilf 《Photochemistry and photobiology》1995,61(4):390-396
Abstract— The effectiveness of photodynamic therapy against P-glycoprotein ATPase activity in multidrug-resistant cells was studied. Chinese hamster ovary AUXB1 (drug-sensitive) and CR1R12 (multidrug-resistant) cell lines were compared with respect to uptake of 14 C-polyhematoporphyrin and porphyrin photosensitization. Phototoxicity of Photofrin® was similar in both cell lines, and no major differences in uptake or efflux of 14 C-polyhematoporphyrin were observed. Porphyrin photosensitization in vitro of CR1R12 cells or isolated plasma membranes from these cells caused inhibition of P-glycoprotein ATPase activity. Application of porphyrin photosensitization at a sublethal level to CR1R12 cells resulted in a small but significant increase in adriamycin-induced cytotoxicity. The hydrophobic "picket-fence" porphyrin, meso -tetrakis-( o -propionamidophenyl)porphyrin,α,α,α,β-isomer, was more inhibitory toward P-glycoprotein ATPase activity than the two less hydrophobic porphyrins tetraphenylporphine tetrasulfonate and Photofrin®. 相似文献
88.
James B. Clark Arthur L. Smirl Eric W. Van Stryland H.J. Mackey B.R. Russell 《Chemical physics letters》1981,78(3):456-460
The population kinetics and the rotational diffusion of the rhodamine B monomer and dimer were measured by using picosecond pulses from a mode-locked Nd : YAG laser to induce and time resolve the concentration-dependent transient absorption saturation of various aqueous solutions of this organic dye. 相似文献
89.
Russell J. Waugh John H. Bowie Roger N. Hayes 《Journal of mass spectrometry : JMS》1991,26(4):250-256
Deprotonated dipeptides, on collisional activation, fragment by the characteristic process NH2CH(R1) CONHCH(R2)CO2? → NH2?C(R1)CONHCH(R2)CO2H → ?NHCH(R2)CO2H + NH2C(R1)?C?O, when R1 and R2 = H or alkyl. However, when one of the constituent amino acids is either aspartic acid or glutamic acid, the standard cleavage becomes minor in comparison with fragmentation through the α-side-chain of Asp or Glu. For example, [Asp-Leu - H]? and [Leu-Asp - H]? both fragment principally by loss of water, a fragmentation not normally noted for peptides. In addition, [Leu-Asp - H]? loses CO2 and also forms HO2CCH?CHCO2?˙. These fragmentations establish that Asp is the C-terminal amino acid. In contrast, isomeric Glu dipeptides, e.g. [Glu-Ala - H]? and [Ala-Glu - H]? undergo similar fragmentation, both competitively losing H2O and CO2. Both spectra also contain a product ion at m/z 128, identified as the pyroglutamate anion. Product ion and deuterium-labelling studies have been used in an attempt to elucidate the complex fragmentation mechanisms in these systems. 相似文献
90.
Richard J. Wiener Philip W. Hammer Charles E. Swanson David C. Samuels Russell J. Donnelly 《Journal of statistical physics》1991,64(5-6):913-926
Taylor-Couette flow subject to a Coriolis force is studied experimentally and numerically. In the experiment, the Couette apparatus is mounted on a turntable with the axis of the cylinders orthogonal to the rotation vector of the turntable. The Coriolis force stabilizes the fluid against the onset of Taylor vortices and alters the velocity fields, both above and below the transition from the initial flow. At small dimensionless turntable frequencies, the transition yields time-independent Taylor vortices which are tilted with respect to the cylinder axis. At larger there is a direct transition to turbulence. We determine the first-order correction to the classical Couette initial flow, to account for the effects of the Coriolis force, by expanding in powers of. We present numerical results for the axial velocity (the only nonvanishing correction term to order) in the infinite-cylinder approximation. 相似文献