全文获取类型
收费全文 | 7054篇 |
免费 | 300篇 |
国内免费 | 10篇 |
专业分类
化学 | 4803篇 |
晶体学 | 50篇 |
力学 | 210篇 |
数学 | 974篇 |
物理学 | 1327篇 |
出版年
2023年 | 60篇 |
2022年 | 35篇 |
2021年 | 117篇 |
2020年 | 108篇 |
2019年 | 121篇 |
2018年 | 129篇 |
2017年 | 106篇 |
2016年 | 233篇 |
2015年 | 200篇 |
2014年 | 196篇 |
2013年 | 417篇 |
2012年 | 506篇 |
2011年 | 520篇 |
2010年 | 301篇 |
2009年 | 256篇 |
2008年 | 459篇 |
2007年 | 488篇 |
2006年 | 446篇 |
2005年 | 406篇 |
2004年 | 327篇 |
2003年 | 270篇 |
2002年 | 207篇 |
2001年 | 185篇 |
2000年 | 158篇 |
1999年 | 73篇 |
1998年 | 69篇 |
1997年 | 48篇 |
1996年 | 88篇 |
1995年 | 73篇 |
1994年 | 58篇 |
1993年 | 82篇 |
1992年 | 59篇 |
1991年 | 31篇 |
1990年 | 42篇 |
1989年 | 28篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 33篇 |
1985年 | 47篇 |
1984年 | 44篇 |
1983年 | 32篇 |
1982年 | 35篇 |
1981年 | 29篇 |
1980年 | 30篇 |
1979年 | 18篇 |
1978年 | 19篇 |
1977年 | 22篇 |
1976年 | 17篇 |
1975年 | 16篇 |
1973年 | 19篇 |
排序方式: 共有7364条查询结果,搜索用时 31 毫秒
81.
The ultraviolet photolysis dynamics of Kr-HBr(v=0) is investigated by means of wave packet calculations, focusing on the fragmentation pathway Kr-HBr+ variant Planck's over 2pi omega-->H+Kr-Br. Photolysis is simulated by starting from two different cluster initial states, namely the ground van der Waals (vdW) and an excited vdW bending state, associated with the Kr-H-Br and Kr-Br-H isomers, respectively. The results show that, for the two initial states of the cluster, the Kr-Br product yield is lower than that of Ar-Br radicals found in previous studies on Ar-HBr photolysis. Despite this decrease, the Kr-Br yield is found to be still rather high, in particular for the initial excited vdW state of Kr-HBr(v=0). In addition, the Kr-Br product state distributions exhibit a remarkably higher excitation (mainly rotational) than the corresponding Ar-Br distributions. The lower yield and higher excitation of Kr-Br as compared to Ar-Br, are attributed to a larger share of the energy available for the radical going to internal excitation in the case of the Kr-Br product. The different partition of the energy available for Kr-Br also causes significant deviations in the photolysis behavior of Kr-HBr when compared to that of Ar-HBr, in the case of the initial excited vdW state of both clusters. A common feature of the photodissociation of Kr-HBr and Ar-HBr is the manifestation of quantum interference effects in the Kr-Br and Ar-Br rotational state distributions, in the form of pronounced structures of supernumerary rotational rainbows. 相似文献
82.
In the literature it was found that titanium oxide clusters of a few metal atoms encapsulated inside the micropores of zeolite Y exhibit large blue shifts in the Ti-O ligand-to-metal charge-transfer band as compared to non-encapsulated bulk titanium dioxide particles. This blue shift of the Ti-O absorption band is believed to have a negative effect on the photocatalytic activity of zeolite-encapsulated TiO2. We report here on circumventing this problem and increasing visible-light absorption by means of a red shift of the absorption band caused by addition of some organic molecular modifiers containing acidic OH groups that can strongly bind with titanol groups TiOH. In the studied series of zeolite-encapsulated TiO2 samples, the red shift of the optical spectrum follows the order: catechol > 4-aminobenzoic acid > benzoic acid. Also N-doping of zeolite-encapsulated TiO2 clusters by thermal treatment with urea leads to a red shift of the TiO2 absorption band that depends on the annealing and hydration conditions. By comparison to the degradation of phenol in aqueous solution, we have demonstrated that these changes in the absorption spectrum on addition of the organic modifier are also reflected in the photocatalytic activity of the samples; a greater increase in photocatalytic activity (about 30%) was observed for the additive catechol. 相似文献
83.
[Reaction: see text] Transformation of enantiopure diastereoisomers (2R,1'S)- and (2S,1'S)-2-(1-aminoalkyl)epoxides into the corresponding 4-(1-aminoalkyl)-1,3-dioxolanes is achieved by reaction with different ketones in the presence of BF3.Et2O. The conversion takes place in very high yields, total selectivity, and without epimerization. A mechanism to explain this transformation is proposed. The obtained 1,3-dioxolanes can be deprotected, and (2R,3S)- and (2S,3S)-3-aminoalkano-1,2-diols were isolated. 相似文献
84.
Bernardi A Potenza D Capelli AM García-Herrero A Cañada FJ Jiménez-Barbero J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(20):4598-4612
As a step to delineate a strategy of ligand design for cholera toxin (CT), NMR studies were performed on several mimics of the GM1 ganglioside oligosaccharide. The conformation of these analogues was investigated first in solution and then upon binding to cholera toxin by transferred nuclear Overhauser effect (TR-NOE) measurements. It was demonstrated that CT selects a conformation similar to the global minima of the free saccharides from the ensemble of presented conformations. No evidence of major conformational distortions was obtained, but one or two of the available conformers of the hydroxyacid side chain appear to be selected in the bound state. The NMR data were interpreted with the aid of computer models, generated and analyzed by using a combination of different approaches (MacroModels' MC/EM and MC/SD, Autodock, and GRID). Analysis of the NMR data supported by computational studies allowed us to interpret the experimental observations and to derive workable models of the ligand:toxin complexes. These models suggest that the higher affinity of the (R)-lactic acid derivative 3 may stem from lipophilic interactions with a hydrophobic area in the toxin binding site located in the vicinity of the sialic acid side chain binding region of the CT:GM1 complex, and formed by the side chain of Ile-58 and Lys-34. Thus, the models obtained have allowed us to make useful design suggestions for the improvement of ligand affinity. 相似文献
85.
Alvaro M Formentín P García H Palomares E Sabater MJ 《The Journal of organic chemistry》2002,67(15):5184-5189
Three N-alkylpyridinium photosensitizers having chiral alkyl groups have been prepared by reacting 2,4,6-triphenylpyrylium tetrafluoroborate ion with (1R,2S)-(-)-norephedrine, (S)-(+)-2-(aminomethyl)pyrrolidine, and (R)-(-)-1-cyclohexylethylamine. Laser flash photolysis allows detection of the corresponding triplet excited states that are quenched by hydrogen atom donors and electron donors. Asymmetric quenching of the chiral triplet excited state was observed using enantiomerically pure 1,2-diamino cyclohexane as quencher. Low enantiomeric excess values (up to 7%) were measured for the photochemical cyclization of 5-methyl-4-hexenoic acid to its corresponding gamma-lactone using these chiral N-alkylpyridinium as photosensitizers. 相似文献
86.
Aránzazu Espina Enrique Jaimez Sergei A. Khainakov Camino Trobajo José R. García Julio Rodríguez 《Journal of inclusion phenomena and macrocyclic chemistry》1998,30(4):333-352
The intercalation of some amines (aniline, benzylamine, cyclohexylamine,piperidine, pyridine, pyrazine and piperazine) into -titaniumphosphate, Ti(HPO4)2×H2O,has been investigated by the batch method and/or by exposing the host to thevapour of the amines. The changes in the interlayer distance of the solidduring the intercalation process was followed by X-ray powder diffraction.The new intercalates were characterised by chemical and thermal analysis.Materials with a monolaminar and/or bilaminar arrangement of amine moleculesin the phosphate interlayer region are obtained depending on the nature ofthe amine. Due to steric hindrance, saturated phases are not obtained forall amines studied. The thermal decomposition of the intercalates (nitrogenatmosphere), takes place in three stages: dehydration, removal of amines andcondensation of the hydrogenphosphate to pyrophosphate groups. 相似文献
87.
Bottamauro M Casellato U Scalco C Tamburini S Tomasin P Vigato PA Aime S Barge A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(17):3917-3926
Heterodinuclear lanthanide(III)-sodium(I) complexes [LnNa(L)(Cl)(2)(CH(3)OH)] (Ln=La[bond]Nd, Sm[bond]Lu), where H(2)L is a [1+1] asymmetric compartmental macrocyclic ligand containing a N(3)O(2) Schiff base and a O(3)O(2) crown-ether-like coordination site, have been prepared and characterized by IR, (1)H, (13)C, and (23)Na NMR spectroscopy, mass spectrometry, and electron microscopy. In the solid state, the lanthanide(III) ions coordinate the Schiff-base N(3)O(2) site, and the sodium ion occupies the O(3)O(2) crownlike cavity, as shown by the X-ray crystal structures of the Nd, Eu, Gd, and Yb derivatives. In these complexes, the lanthanide(III) ion is coordinated by two chlorine atoms in the trans position and by three nitrogen and two negatively charged phenol oxygen atoms of the Schiff base, and the ion is heptacoordinated with a pentagonal bipyramidal geometry. The sodium ion is coordinated by three etheric oxygen atoms and the two phenolic oxygens that act as a bridge. A methanol molecule is also coordinated in the apical position of the resulting pentagonal pyramidal polyhedron. A detailed (1)H and (13)C NMR study was carried out in CD(3)OD for both diamagnetic and paramagnetic heterodinuclear complexes [LnNa(L)(Cl)(2)(CH(3)OH)]. The complexes are also isostructural in solution, and their structures parallel those found in the solid state. Moreover, some significative distances determined in the solid state and in solution are comparable. Finally, the potential use of these complexes as molecular probes for the selective recognition of specific metal ions has been tested. In particular, their ability to act as shift reagents and the selectivity of the O(3)O(2) site towards Li(+), Ca(2+), and K(+) were investigated by (23)Na NMR spectroscopy. 相似文献
88.
O Ladrón de Guevara C Cortinas de Nava P Padilla J Espinosa M Cebrian L García 《Journal of chromatography. A》1990,528(1):35-41
For the determination of beta-aminoisobutyric acid (BAIBA) in urine samples in which the beta-alanine concentrations are higher than those of BAIBA, the resolution between these two amino acids, separated by reversed-phase liquid chromatography on an octadecylsilane column, was optimized. The chromatographic analysis included precolumn derivatization of amino acids with o-phthalaldehyde, followed by a 15-min isocratic elution and detection at 340 nm. Because of its simplicity, this method should be useful for monitoring urinary excretion of BAIBA. 相似文献
89.
The application of a program written in BASIC for an IBM/PC interfaced with a commercial spectrofluorimeter enables variable-angle scanning fluorescence spectra to be obtained, without modification of the instrument. The technique is effective for determinations of the components of mixtures of carbaryl, fuberidazol and warfarin, all of which are pesticides with intrinsic fluorescence and closely overlapping profiles. The proposed method permitted simultaneous determinations with RSD of less than 2.5% and recoveries of 99–110%. 相似文献
90.
The covalent conjugation of a 20-mer peptide belonging to the VP3 capsid protein of hepatitis A virus to the surface of preformed liposomes was investigated. Three different bonds (disulfide, thioether and amide) were established between the peptide sequence and liposomes bearing at their surface appropriate reactive groups. The effect of the relative concentration of the N-[4-(p-maleimidophenyl)butyryl]dipalmitoylphosphatidylethanolamine anchor in liposomes on stability during coupling of the peptide sequence was studied. The interaction of the three liposomal preparations with phospholipids in a biomembrane model system, monolayers at the air-water interface, is also reported. The results showed that although the peptides associate with liposomes in similar yields for the three strategies studied, differences can be observed when their interaction with phospholipid monolayers composed of dipalmitoylphosphatidylcholine is analysed. 相似文献