全文获取类型
收费全文 | 2634篇 |
免费 | 92篇 |
国内免费 | 5篇 |
专业分类
化学 | 2099篇 |
晶体学 | 16篇 |
力学 | 35篇 |
数学 | 222篇 |
物理学 | 359篇 |
出版年
2023年 | 13篇 |
2022年 | 42篇 |
2021年 | 79篇 |
2020年 | 37篇 |
2019年 | 57篇 |
2018年 | 36篇 |
2017年 | 37篇 |
2016年 | 80篇 |
2015年 | 76篇 |
2014年 | 86篇 |
2013年 | 126篇 |
2012年 | 165篇 |
2011年 | 161篇 |
2010年 | 120篇 |
2009年 | 97篇 |
2008年 | 173篇 |
2007年 | 163篇 |
2006年 | 136篇 |
2005年 | 180篇 |
2004年 | 128篇 |
2003年 | 115篇 |
2002年 | 98篇 |
2001年 | 46篇 |
2000年 | 33篇 |
1999年 | 38篇 |
1998年 | 22篇 |
1997年 | 38篇 |
1996年 | 29篇 |
1995年 | 20篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 19篇 |
1990年 | 18篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 21篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1972年 | 4篇 |
排序方式: 共有2731条查询结果,搜索用时 187 毫秒
81.
Rosa M. Gomila Antonio Frontera David Quiñonero Antonio Costa 《Tetrahedron letters》2004,45(51):9387-9391
Several porphyrin dimers have been newly designed and synthesized to construct assemblies with 1,4-diazabicyclo[2.2.2]octane (DABCO) as a bidentate binding ligand. Semi empirical (AM1) and ab initio calculations have been used to study the assemblies generated by the organization of dimers and DABCO, including the computation of 1H NMR complexation-induced chemical shifts using the ab initio/GIAO methodology. The diagnostic capacity of the theoretical method has been applied to explain experimental results and geometrical features of the complexes. 相似文献
82.
Erilov DA Bartucci R Guzzi R Shubin AA Maryasov AG Marsh D Dzuba SA Sportelli L 《The journal of physical chemistry. B》2005,109(24):12003-12013
Electron spin-echo envelope modulation (ESEEM) spectroscopy of phospholipids spin-labeled systematically down the sn-2 chain was used to detect the penetration of water (D2O) into bilayer membranes of dipalmitoyl phosphatidylcholine with and without 50 mol % cholesterol. Three-pulse stimulated echoes allow the resolution of two superimposed 2H-ESEEM spectral components of different widths, for spin labels located in the upper part of the lipid chains. Quantum chemical calculations (DFT) and ESEEM simulations assign the broad spectral component to one or two D2O molecules that are directly hydrogen bonded to the N-O group of the spin label. Classical ESEEM simulations establish that the narrow spectral component arises from nonbonded water (D2O) molecules that are free in the hydrocarbon chain region of the bilayer membrane. The amplitudes of the broad 2H-ESEEM spectral component correlate directly with those of the narrow component for spin labels at different positions down the lipid chain, reflecting the local H-bonding equilibria. The D2O-ESEEM amplitudes decrease with position down the chain toward the bilayer center, displaying a sigmoidal dependence on position that is characteristic of transmembrane polarity profiles established by other less direct spin-labeling methods. The midpoint of the sigmoidal profile is shifted toward the membrane center for membranes without cholesterol, relative to those with cholesterol, and the D2O-ESEEM amplitude in the outer regions of the chain is greater in the presence of cholesterol than in its absence. For both membrane types, the D2O amplitude is almost vanishingly small at the bilayer center. The water-penetration profiles reverse correlate with the lipid-chain packing density, as reflected by 1H-ESEEM intensities from protons of the membrane matrix. An analysis of the H-bonding equilibria provides essential information on the binding of water molecules to H-bond acceptors within the hydrophobic interior of membranes. For membranes containing cholesterol, approximately 40% of the nitroxides in the region adjacent to the lipid headgroups are H bonded to water, of which ca. 15% are doubly H bonded. Corresponding H-bonded populations in membranes without cholesterol are ca. 20%, of which ca. 6% are doubly bonded. 相似文献
83.
Pedro Molina Angeles Lorenzo Rosa Ma Claramunt José Elguero 《Tetrahedron letters》1984,25(47):5427-5428
A number of mesoionic compounds derivatives of the bicyclic system 1,2,4-triazolo(4,3-b)-1,2,4-triazole have been prepared from 4-amino-1-methyl-3,5-bis(methylthio)-1,2,4-triazolium iodide and aryl isothiocyanates. 相似文献
84.
Water adsorption and absorption on crystalline polyvinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE, 70:30), was examined by thermal desorption spectroscopy. Two distinctly different water adsorption sites are identified: one adsorbed species that resembles ice and another species that interacts more strongly with the polymer thin film. The existence of the latter species is consistent with X-ray diffraction studies of water absorbed into the bulk of copolymers of polyvinylidene fluoride with trifluoroethylene crystalline thin films. There are strong steric effects observed in the angle-resolved thermal desorption that may be a result of the large polymer thin film surface dipoles. 相似文献
85.
The capacity for anaerobic decolorization of a sulfonated azo dye, Congo Red, by a strain of a sulfate-reducing bacterium
was evaluated. After optimizing the growth rate of the bacteria on a simple carbon source and terminal electron acceptor pair,
lactate and sulfate, respectively, the effect of the dye concentration on their growth rate was analyzed. The decolorization
rate was affected by the dye concentration in the growth medium. The azo-bond cleavage mechanism of reductive decolorization
with the formation of benzidine was consistent with the results, as this metabolite was identified by high-performance liquid
chromatography. Several fractions of the culture medium, including lysed cell extracts, were examined for the capacity to
reduce the azo dye. This reduction capacity was found in the culture medium in which the cells had previously grown. The results
showed that the mechanism of reductive decolorization of this sulfonated azo dye was extracellular and nonenzymatic, consistent
with the production of sulfide anion by the microorganisms while growing on lactate and sulfate. The sulfide anions were the
cause of the reduction leading to the disappearance of color in the medium. To increase the rate of decolorization, the presence
of ferrous ion was also necessary together with the lactate and sulfate substrates. 相似文献
86.
Liquid chromatographic/electrospray ionization tandem mass spectrometric study of the phenolic composition of cocoa (Theobroma cacao) 总被引:2,自引:0,他引:2
Sánchez-Rabaneda F Jáuregui O Casals I Andrés-Lacueva C Izquierdo-Pulido M Lamuela-Raventós RM 《Journal of mass spectrometry : JMS》2003,38(1):35-42
Liquid chromatography coupled with ionspray mass spectrometry in the tandem mode (LC/MS/MS) with negative ion detection was used for the identification of a variety of phenolic compounds in a cocoa sample. Gradient elution with water and acetonitrile, both containing 0.1% HCOOH, was used. Standard solutions of 31 phenolic compounds, including benzoic and cinnamic acids and flavonoid compounds, were studied in the negative ion mode using MS/MS product ion scans. At low collisional activation, the deprotonated molecule [M - H](-) was observed for all the compounds studied. For cinnamic and benzoic acids, losses of CO(2) or formation of [M - CH(3)](-*) in the case of methoxylated compounds were observed. However, for flavonol and flavone glycosides, the spectra present both the deprotonated molecule [M - H](-) of the glycoside and the ion corresponding to the deprotonated aglycone [A - H](-). The latter ion is formed by loss of the rhamnose, glucose, galactose or arabinose residue from the glycosides. Different fragmentation patterns were observed in MS/MS experiments for flavone-C-glycosides which showed fragmentation in the sugar part. Fragmentation of aglycones provided characteristic ions for each family of flavonoids. The optimum LC/MS/MS conditions were applied to the characterization of a cocoa sample that had been subjected to an extraction/clean-up procedure which involved chromatography on Sephadex LH20 and thin-layer chromatographic monitoring. In addition to compounds described in the literature, such as epicatechin and catechin, quercetin, isoquercitrin (quercetin-3-O-glucoside) and quercetin-3-O-arabinose, other compounds were identified for the first time in cocoa samples, such as hyperoside (quercetin-3-O-galactoside), naringenin, luteolin, apigenin and some O-glucosides and C-glucosides of these compounds. 相似文献
87.
Leibold C Alvarez RM Cutin EH Della Vedova CO Oberhammer H 《Inorganic chemistry》2003,42(13):4071-4075
The IR (gas) and Raman (liquid) spectra of FC(O)NSCl(2) demonstrate the presence of a conformational mixture in both phases. According to a gas electron diffraction study, the main conformer (94(8)%) possesses a syn-syn structure (C(O)F group synperiplanar with respect to the SCl(2) bisector and the C=O bond synperiplanar to the N=S bond). Quantum chemical calculations (HF, B3LYP and MP2 with 6-31G basis set, and MP2/6-311(2df)) predict a syn-anti structure for the second conformer. Analysis of the IR (gas) spectrum results in a contribution of 5(1)% of the minor form, corresponding to a Gibbs free energy difference DeltaG degrees = G degrees (syn-anti) - G degrees (syn-syn) = 1.75(15) kcal/mol. This value is reproduced very well by quantum chemical calculations, which include electron correlation effects (DeltaG degrees = 1.28-1.56 kcal/mol). The HF approximation overestimates this energy difference (DeltaG degrees = 3.24 kcal/mol). 相似文献
88.
Fandos R Hernández C Otero A Rodríguez A Ruiz MJ Terreros P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(3):671-677
Titanium complexes with chelating alkoxide ligands [TiCp*(O(2)Bz)(OBzOH)] (1) and [TiCp*(Me)((OCH(2))(2)Py)] (2) were synthesised by reaction of [TiCp*Me(3)] (Cp*=eta(5)-C(5)Me(5)) with 2-hydroxybenzyl alcohol ((HO)(2)Bz) and 2,6-pyridinedimethanol ((HOCH(2))(2)Py), respectively. Complex 1 reacts with [(M(mu-OH)(cod))(2)] (M=Rh, Ir) to yield the early-late heterobimetallic complexes [TiCp*(O(2)Bz)(2)M(cod)] [M=Rh (3), Ir (4)]. Carbon monoxide readily replaces the COD ligand in 3 to give the rhodium dicarbonyl derivative [TiCp*(O(2)Bz)(2)Rh(CO)(2)] (5). Compound 2 reacts with [(M(mu-OH)(cod))(2)] (M=Rh, Ir) with protonolysis of a Tibond;Me bond to give [TiCp*((OCH(2))(2)Py)(mu-O)M(cod)] [M=Rh (6), Ir (7)]. The molecular structures of complexes 3, 5 and 7 were established by single-crystal X-ray diffraction studies. 相似文献
89.
Jose G. Trujillo‐Ferrara Rosa Luisa Santilln‐Baca Norberto Farfn‐García Itzia I. Padilla‐Martínez Efrn V. García‐Bez 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(3):o125-o128
The molecular and supramolecular structures of 2‐(1,3‐dioxo‐2,3,3a,4,7,7a‐hexahydro‐1H‐isoindol‐2‐yl)phenyl acetate, C16‐H15NO4, (I), and its para isomer, 4‐(1,3‐dioxo‐2,3,3a,4,7,7a‐hexahydro‐1H‐isoindol‐2‐yl)phenyl acetate, (II), are reported. The torsion angle between the succinimide and benzene rings depends on the position of the acetoxy substitution [89.7 (1) and 61.9 (1)° for (I) and (II), respectively]. The twist of the acetoxy group relative to the mean plane of the benzene ring is almost independent of the acetoxy position [66.0 (1) and 70.0 (1)°]. Packing interactions for both compounds include soft C—H⋯X (X = O and Ph) interactions, forming chains of centrosymmetric dimers and interlinked chains for (I) and (II), respectively. In addition, three perpendicular dipole C=O⋯C=O interactions contribute to the supramolecular structure of (II). 相似文献
90.
We describe a density functional theory approach to solvation in molecular solvents. The solvation free energy of a complex solute can be obtained by direct minimization of a density functional, instead of the thermodynamic integration scheme necessary when using atomistic simulations. In the homogeneous reference fluid approximation, the expression of the free-energy functional relies on the knowledge of the direct correlation function of the pure solvent. After discussing general molecular solvents, we present a generic density functional describing a dipolar solvent and we show how it can be reduced to the conventional implicit solvent models when the solvent microscopic structure is neglected. With respect to those models, the functional includes additional effects such as the microscopic structure of the solvent, the dipolar saturation effect, and the nonlocal character of the dielectric constant. We also show how this functional can be minimized numerically on a three-dimensional grid around a solute of complex shape to provide, in a single shot, both the average solvent structure and the absolute solvation free energy. 相似文献