全文获取类型
收费全文 | 1751篇 |
免费 | 43篇 |
国内免费 | 5篇 |
专业分类
化学 | 1335篇 |
晶体学 | 21篇 |
力学 | 82篇 |
数学 | 181篇 |
物理学 | 180篇 |
出版年
2023年 | 11篇 |
2022年 | 38篇 |
2021年 | 33篇 |
2020年 | 17篇 |
2019年 | 25篇 |
2018年 | 13篇 |
2017年 | 18篇 |
2016年 | 43篇 |
2015年 | 48篇 |
2014年 | 43篇 |
2013年 | 74篇 |
2012年 | 95篇 |
2011年 | 97篇 |
2010年 | 68篇 |
2009年 | 53篇 |
2008年 | 134篇 |
2007年 | 123篇 |
2006年 | 126篇 |
2005年 | 113篇 |
2004年 | 105篇 |
2003年 | 80篇 |
2002年 | 72篇 |
2001年 | 38篇 |
2000年 | 22篇 |
1999年 | 18篇 |
1998年 | 24篇 |
1997年 | 17篇 |
1996年 | 33篇 |
1995年 | 21篇 |
1994年 | 16篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 12篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 15篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 10篇 |
1981年 | 18篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1971年 | 3篇 |
1929年 | 3篇 |
排序方式: 共有1799条查询结果,搜索用时 15 毫秒
61.
62.
Monica Bartolomei Maurizio Cignitti Marina Cotta Ramusino Gianfranco La Manna 《Journal of Molecular Structure》1995,330(1-3):431-435
7,8-dihydroquinoline-4,5 (1H,6H)-dione (1) and 7,8-dihydroquinoline-2,5-(1H,6H)-dione (2) in their tautomeric oxo and hydroxy forms have been studied by ab initio Hartree-Fock calculations; tautomerization energies predict a more stable hydroxy structure having an intramolecular hydrogen bond for compound 1, whereas the oxo form is slightly-preferred for compound 2. Fourier Transform-Infra Red (FT-IR) spectra in CHCl3 solution indicate that the predicted most stable tautomers in the vapour phase remain as such. 相似文献
63.
Maurizio Botta Francesco De Angelis Ingeborg Grgurina Mauro Marzi Rosario Nicoletti 《Journal of heterocyclic chemistry》1985,22(4):1001-1007
The reactivity of Δ3- and Δ2-3-bromomethylcephems toward carboxylate nucleophiles has been studied. The Δ3-bromomethylcephem 1 , less reactive than the Δ2-analogue 4 , is converted in high yields into 3-acyl-oxymethyl-3-cephems 2a-d , generally with no isomerization of the double bond, only within a narrow range of conditions. In particular, the Δ3-7-aminocephalosporanic acid (7-ACA) derivative 2a has been obtained as the only product in 91% yield by treatment of 1 with triethylammonium acetate in acetic acid. The Δ2-bromomethyl-cephem 4 is easily converted into the Δ2-acyloxymethyl-cepheras 5a-d without double bond isomerization, in very high yields. 相似文献
64.
Vincenzo Mirabello Maria Caporali Luca Gonsalvi Gabriele Manca Andrea Ienco Maurizio Peruzzini 《化学:亚洲杂志》2013,8(12):3177-3184
The selective functionalization of the polyphosphorus moiety Ph2PCH2PPh2PPPP present as a tetrahapto‐ligand in complex [Ir(dppm)(Ph2PCH2PPh2PPPP)]+ ( 1 , dppm=Ph2PCH2PPh2) was obtained by reaction of 1 with water under basic conditions at room temperature. The formation of the new triphosphaallyl moiety η3‐P3{P(O)H} was determined in solution by NMR spectroscopy, and confirmed in the solid state by a single‐crystal X‐ray structure of the stable product [Ir(κ2‐dppm)(κ1‐dppm)(η3‐P3{P(O)H})] ( 2 ). In solution, 2 has a fluxional behavior attributable to the four P atoms belonging to the tetraphosphorus moiety in 1 and exhibits a chemical exchange process involving the two PPh2 moieties of the same bidentate ligand, as determined by 1D and 2D NMR spectroscopy experiments carried out at variable temperature. The mechanism of the reaction was investigated at the DFT level, which suggested a selective attack of an in‐situ generated OH? anion on one of the non‐coordinated phosphorus atoms of the P4 moiety. The reaction then evolves through an acid‐assisted tautomerization, which leads to the final compound 2 . Bonding analysis pointed out that the new unsubstituted P3‐unit in the η3‐P3{P(O)H} moiety behaves as a triphosphallyl ligand. 相似文献
65.
66.
Luigi Fabbrizzi Maurizio Licchelli Piersandro Pallavicini Luisa Parodi 《Angewandte Chemie (International ed. in English)》1998,37(6):800-802
Simply by changing the pH value, the side chain of complex 1 can be reversibly moved between two positions. Coordination to the metal center through the nitrogen atom of the side chain at moderate pH values is accompanied by a decrease in fluorescence intensity (from IF=100% to IF=60%). A further decrease is observed upon deprotonation of the bound water molecule at higher pH (IF≤2%). Therefore, 1 can be seen as a molecular three-position switch. 相似文献
67.
The nu(C=O) Raman band frequencies of acetone have been analyzed to separate the contributions of the environmental effect and the vibrational coupling to the gas-to-liquid frequency shifts of this band and to elucidate the changes in these two contributions upon dilution in DMSO. We have measured the frequencies of the nu((12)C=O) band in acetone/DMSO binary mixtures, the nu((13)C=O) band of the acetone-(13)C=O present as a natural abundance isotopic impurity in these mixtures, and both the nu((12)C=O) and nu((13)C=O) bands in the acetone-(12)C=O/acetone-(13)C=O isotopic mixtures at infinite dilution. These frequencies are compared with those of the nu((12)C=O) band in the acetone/CCl(4) binary mixtures measured previously. We have found the following three points: (i) The negative environmental contribution for the nu((12)C=O) oscillator of acetone completely surrounded by DMSO is reduced in magnitude by +5.5 cm(-1) and +7.8 cm(-1) upon the complete substitution of DMSO with acetone and CCl(4) molecules, respectively, indicating the progressive reduction of the attractive forces exerted by the environment on the nu((12)C=O) mode of acetone. (ii) In DMSO and other solvents, the contribution of the vibrational coupling to the frequency of the isotropic Raman nu((12)C=O) band of acetone becomes progressively more negative with increasing acetone concentration up to a value of -5.5 cm(-1), while the contribution to the frequency of the anisotropic Raman band remains approximately unchanged. The only difference resides in the curvatures of the concentration dependencies of these contributions which depend on the relative solute/solvent polarity. (iii) The noncoincidence effect (separation between the anisotropic and isotropic Raman band frequencies) of the nu(C=O) mode in the acetone/DMSO mixtures exhibits a downward (concave) curvature, in contrast to that in the acetone/CCl(4) mixtures, which shows an upward (convex) curvature. This result is supported by MD simulations and by theoretical predictions and is interpreted as arising from the reduction and enhancement of the short-range orientational order of acetone in the acetone/DMSO and acetone/CCl(4) mixtures, respectively. 相似文献
68.
The solvent effect on 17O isotropic shielding was computed by different methods: the polarizable continuum model and a mixed approach, including a few real solvent molecules treated as the solute. The experimental data show that the behavior of protic and aprotic solvents is markedly different: we found that the continuum approach describes well the observed shielding at various dielectric constants for aprotic solvents, while the mixed procedure is needed when hydrogen bonds to the magnetic centre are present.Contribution to the Jacopo Tomasi Honorary Issue 相似文献
69.
Boiocchi M Del Boca L Gómez DE Fabbrizzi L Licchelli M Monzani E 《Journal of the American Chemical Society》2004,126(50):16507-16514
1,3-bis(4-nitrophenyl)urea (1) interacts through hydrogen bonding with a variety of oxoanions in an MeCN solution to give bright yellow 1:1 complexes, whose stability decreases with the decreasing basicity of the anion (CH3COO- > C6H5COO- > H2PO4- > NO2- > HSO4- > NO3-). The [Bu4N][1.CH3COO] complex salt has been isolated as a crystalline solid and its molecular structure determined, showing the formation of a discrete adduct held together by two N-H...O hydrogen bonds of moderate strength. On the other hand, the F- ion first establishes a hydrogen-bonding interaction with 1 to give the most stable 1:1 complex, and then on addition of a second equivalent, induces urea deprotonation, due to the formation of HF2-. The orange-red deprotonated urea solution uptakes carbon dioxide from air to give the tetrabutylammonium salt of the hydrogencarbonate H-bond complex, [Bu4N][1.HCO3], whose crystal and molecular structures have been determined. 相似文献
70.
Guldi DM Zilbermann I Anderson G Kotov NA Tagmatarchis N Prato M 《Journal of the American Chemical Society》2004,126(44):14340-14341
Novel organic (positively charged fullerene)-inorganic (negatively charged CdTe nanoparticle) nanoensembles were devised through electrostatic interactions and probed as versatile donor-acceptor hybrids. Photoirradiation of their homogeneous solutions, containing the electrostatically packed components, let to very long-lived (1.3 ms) charge separated states. 相似文献