全文获取类型
收费全文 | 5199篇 |
免费 | 149篇 |
国内免费 | 20篇 |
专业分类
化学 | 3635篇 |
晶体学 | 66篇 |
力学 | 68篇 |
数学 | 495篇 |
物理学 | 1104篇 |
出版年
2023年 | 52篇 |
2022年 | 144篇 |
2021年 | 135篇 |
2020年 | 121篇 |
2019年 | 135篇 |
2018年 | 98篇 |
2017年 | 108篇 |
2016年 | 194篇 |
2015年 | 142篇 |
2014年 | 151篇 |
2013年 | 309篇 |
2012年 | 331篇 |
2011年 | 429篇 |
2010年 | 259篇 |
2009年 | 243篇 |
2008年 | 344篇 |
2007年 | 320篇 |
2006年 | 275篇 |
2005年 | 259篇 |
2004年 | 245篇 |
2003年 | 182篇 |
2002年 | 138篇 |
2001年 | 94篇 |
2000年 | 77篇 |
1999年 | 52篇 |
1998年 | 38篇 |
1997年 | 35篇 |
1996年 | 50篇 |
1995年 | 34篇 |
1994年 | 25篇 |
1993年 | 30篇 |
1992年 | 43篇 |
1991年 | 22篇 |
1990年 | 14篇 |
1989年 | 22篇 |
1988年 | 10篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1974年 | 7篇 |
排序方式: 共有5368条查询结果,搜索用时 0 毫秒
61.
Thompson H Wasse JC Skipper NT Hayama S Bowron DT Soper AK 《Journal of the American Chemical Society》2003,125(9):2572-2581
The technique of hydrogen/deuterium isotopic substitution has been used to extract detailed information concerning the solvent structure in pure ammonia and metallic lithium-ammonia solutions. In pure ammonia we find evidence for approximately 2.0 hydrogen bonds around each central nitrogen atom, with an average N-H distance of 2.4 A. On addition of alkali metal, we observe directly significant disruption of this hydrogen bonding. At 8 mol % metal there remains only around 0.7 hydrogen bond per nitrogen atom. This value decreases to 0.0 for the saturated solution of 21 mol % metal, as all ammonia molecules have then become incorporated into the tetrahedral first solvation spheres of the lithium cations. In conjunction with a classical three-dimensional computer modeling technique, we are now able to identify a well-defined second cationic solvation shell. In this secondary shell the nitrogen atoms tend to reside above the faces and edges of the primary tetrahedral shell. Furthermore, the computer-generated models reveal that on addition of alkali metal the solvent molecules form voids of approximate radius 2.5-3.0 A. Our data therefore provide new insight into the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium-ammonia solutions. 相似文献
62.
Abstract— The quantum yields for the photohydration of dimethyluracil were determined for concentrations in the range 5 × 10--1--1 × 10--3M by use of 240–280 nm irradiation. The average quantum yield (0.0139 f 0.0005) was independent of both concentration and irradiation wavelength. 相似文献
63.
Santos E Schühle DT Jones H Schmickler W 《Langmuir : the ACS journal of surfaces and colloids》2005,21(14):6406-6421
The nonlinear optical properties of self-assembled monolayers obtained from bonding two different alpha-functionalized terthiophenes (alpha-T3) to (111) silver electrode surfaces have been investigated using second harmonic generation (SHG). The two (alpha-T3) compounds used were functionalized with alkane chains of different lengths (C8 and C4), and each was terminated with a thiol anchoring group. A nitrile group was attached to the terminal thiophene ring of the (alpha-T3) compound with the C4 chain. The orientation of the polarization of the incident beam was changed systematically and gradually between "p" and "s" orientation and the SH signal (isotropic and anisotropic contributions) analyzed in both directions ("P" and "S"). The symmetry of the system was reduced by the presence of the adlayers from C3v to C3. The dependence on the applied potential and the incident wavelength has also been studied. The relative magnitudes and phases of the various second-order tensor elements have been estimated and compared with the values for a bare surface. A resonance process in the (alpha-T3) pi moiety has been investigated, and from this, the effective "band-gap" energies of the organic semiconductor SAMs (i.e., the energy difference between the pi-pi bands) have been estimated. 相似文献
64.
M. J. L. Santos A. F. Rubira R. M. Pontes E. A. Basso E. M. Girotto 《Journal of Solid State Electrochemistry》2006,10(2):117-122
Much effort has been made to synthesize novel compounds and enhance the optical properties of poly(terthiophenes). The electrochromic
properties of poly(4,4′′-dimetoxy-3′-methyl-2′-5′,2′′-terthiophene) (PDMMT) and poly(4,4′′-dipentoxy-3′-methyl-2′-5′,2′′-terthiophene)
(PDPMT) have been studied focusing on the differences in the alkoxy-group length. Theoretical calculations were employed to
elucidate the structural and thermodynamic stability of the monomers and dimmers. The results showed that in this type of
thiophenes large alkoxy chains assist positive charge dispersion through hyperconjugative effect. Thus, PDPMT is thermodynamically
more stable than PDMMT in the oxidized state, leading to better electrochromic stability and optical memory. 相似文献
65.
L. L. M. Santos J. A. Osso S. G. Araujo 《Journal of Radioanalytical and Nuclear Chemistry》1995,200(3):227-232
The development of an automated water target for the production of18F is described. The system was fully fested and shown to be reliable and secure. The chemical separation of18F was carried out using an anionexchange resin and K2CO3 as eluent. The18F production yields were, on average, (4.81±0.42) MBq/Ah and the specific activity was higher than 6.623×105 MBq/mmol. Heat transfer measurements and calculations were made. 相似文献
66.
The reaction between copper (II) and azide has been studied spectrophotometrically at four wavelengths, at 25 degrees , and ionic strength 4.00M (sodium perchlorate). The formation constants beta(2) and beta(3) found are 2.90 +/- 0.08 x 10(4) and 3.02 +/- 0.07 x 10(6) respectively. The results obtained from potentiometric measurements with a solid-state electrode disagree with those calculated from the spectrophotometric data. Causes of the discrepancy are discussed. 相似文献
67.
The aim of this work was the preparation of enantiomerically pure bis(pinene-bipyridine) ligands containing the ferrocenyl moiety. Several such ligands (1-3) were synthesized and completely characterized. These molecules can be diastereoselectively deprotonated at the acidic methylene group of the pinene moiety using a strong and sterically hindered base such as LDA. Subsequent reaction of the formed anion with alkyl halides yield the family of C(2)-symmetric enantiopure compounds (1a-c). Copper(I), silver(I), or zinc(II) complexes with several ligands (C1-C8) were prepared and structurally characterized in the solid state and in solution. Self-assembled helical species are formed in several cases. It became evident that the chiral groups present in the ligand do not completely determined the helical configuration of the assemblages. Diastereoselectivity is thus not complete with this type of ligands, contrary to other, similar ligands studied before. 相似文献
68.
69.
Blackwell HE 《Organic & biomolecular chemistry》2003,1(8):1251-1255
The application of microwave irradiation to expedite solid-phase organic reactions could be the tool that allows combinatorial chemistry to deliver on its promise--providing rapid access to large collections of diverse small molecules. Herein, several different approaches to microwave (MW)-assisted solid-phase reactions and library synthesis are introduced, including the use of solid-supported reagents, multicomponent coupling reactions, solvent-free parallel library synthesis, and spatially addressable library synthesis on planar solid supports. The future impact of MW-assisted organic reactions on solid-phase and combinatorial chemistry could prove to be immense, and methods for further improvement of this strategic combination of technologies are highlighted. 相似文献
70.
Cong Zhang Christian Ludin Marcel K. Eberle Helen Stoeckli-Evans Reinhart Keese 《Helvetica chimica acta》1998,81(1):174-181
(S)-5,5,5,5′,5′,5′-Hexafluoroleucine ((S)- 13 ) of 81 % ee is prepared from hexafluoroacetone ( l ) and ethyl bromopyruvate (= ethyl 2-oxopropanoate) in 7 steps with an overall yield of 18% (Schemes 1 and 2). Key step in this sequence is the highly enantioselective reduction of the carbonyl group in α-keto ester 4 either by bakers' yeast (91 % ee) or by ‘catecholborane’ 6 utilizing an oxazaborolidine catalyst, yielding hydroxy ester (R)- 5 with 99% ee. The absolute configuration was determined by X-ray analysis of the HCl adduct (S,R)- 9b of (2S)-N-[(R)- l-phenylethyl]-5,5,5,5′,5′,5′-hexafluoroleucine ethyl ester. 相似文献