全文获取类型
收费全文 | 6626篇 |
免费 | 279篇 |
国内免费 | 19篇 |
专业分类
化学 | 4582篇 |
晶体学 | 20篇 |
力学 | 121篇 |
数学 | 1183篇 |
物理学 | 1018篇 |
出版年
2023年 | 59篇 |
2021年 | 94篇 |
2020年 | 118篇 |
2019年 | 124篇 |
2018年 | 81篇 |
2017年 | 59篇 |
2016年 | 193篇 |
2015年 | 155篇 |
2014年 | 187篇 |
2013年 | 284篇 |
2012年 | 306篇 |
2011年 | 332篇 |
2010年 | 204篇 |
2009年 | 143篇 |
2008年 | 232篇 |
2007年 | 232篇 |
2006年 | 233篇 |
2005年 | 229篇 |
2004年 | 195篇 |
2003年 | 138篇 |
2002年 | 142篇 |
2001年 | 60篇 |
2000年 | 110篇 |
1999年 | 68篇 |
1998年 | 68篇 |
1997年 | 72篇 |
1996年 | 94篇 |
1995年 | 80篇 |
1994年 | 81篇 |
1993年 | 108篇 |
1992年 | 84篇 |
1991年 | 81篇 |
1990年 | 58篇 |
1989年 | 59篇 |
1988年 | 61篇 |
1987年 | 76篇 |
1986年 | 71篇 |
1985年 | 106篇 |
1984年 | 74篇 |
1983年 | 71篇 |
1982年 | 78篇 |
1981年 | 62篇 |
1980年 | 91篇 |
1979年 | 75篇 |
1978年 | 84篇 |
1977年 | 63篇 |
1976年 | 80篇 |
1975年 | 55篇 |
1974年 | 67篇 |
1973年 | 47篇 |
排序方式: 共有6924条查询结果,搜索用时 15 毫秒
71.
Alfred Kreutzberger Elfriede Kreutzberger Hans-Joachim Saltler Hayriye Aml Aysel Gürsoy 《Journal of heterocyclic chemistry》1982,19(4):753-755
The three-component reaction comprising the interaction of 2-amino-4-antipyrinyl-5-ethylthiazole (1) with s-triazine (2) and pyrrolidine leads to 4-anitpyrinyl-5-ethyl-2-[(4-pyrrolidinyl)methyleneamino]thiazole (5). Structure 5 is supported by 1H- and 13C-NMR spectroscopy. 相似文献
72.
E.Gordon Muller Jeffrey L. Petersen Lawrence F. Dahl 《Journal of organometallic chemistry》1976,111(1):91-112
Our preparation of Ti(h5?C5H5)2 S5 by the reaction of elemental sulfur with Ti(h5?C5H5)2(CO)2 in hexane and of V(h5?C5H5)2S5? H2O by the reacti of V(h5?C5H5)2 Cl2 with Na2S5 in THF and structural analyses by single crystal X-ray diffraction (together with infrared, solution EPR, and temperaturedependent magnetic susceptibility measurements) represent an extension of our previous work on M(h5?C5H5)2 (SC6H5)2 (M = Ti, V). The crystallographic results provide further support of our previous conclusions that the Ballhausen—Dahl model is not valid for M(h5?C5H5)2L2 systems. The structuralfeatures of the chair-like titanium and vanadium pentasulfide molecules are compared to the corresponding phenylmercapto analogs and to the chair-like cyclohexasulfur molecule in rhombohedral sulfur. Ti(h5?C5H5)2S5 was isolated as a mixture of monoclinic and orthorhombic Crystalline phases-which were both characterized by preliminary X-ray data. A complete Structural determination and refinement of the monoclinic phase, which contains two independent molecules in a cell of dimensions a 22.843(2), b 7.958(1), c 14.465(1) Å, β 90.074(4)° and symmetry P21/c, yielded R1 5.3 % and R2 5.9 % for 2168 independent diffractometry-collected data with I≥ 2.5o(I). V(h5?C5H5)2S5- H2O contains four V(h5?C5H5)2S5 molecules and two water molecules of hydration (of crystallographic site symmetry C2-2) in onorthorhombic unit cell of symmetry P21212 and of dimensions a 13.491(1), b 12.748(1), c 7.715(1) Å. Least-squares refinement of 750 diffractometry data with I≥2.0σ(I) gave R1 2.4% and R2 3.0% Both of these compounds were independently synthesized and Spectroscopically characterized by Köpf and co-workers, and-a Complete X-ray diffraction study was performed by Epstein and Bernal on a different monoclinic phase of Ti(h5?C5H5)2S5 (isolated by-Köpf). An extraction of V(h5?C5H5)2S5 with re fluxing benzene under nitrogen atmosphere in a Soxhlet apparatus led to the formation of the previously reported [V2 (h5?C5H5)2S5]n compound which was characterized by physical measurements including a preliminary X-ray diffraction study. 相似文献
73.
Alfred V. Willi 《Helvetica chimica acta》1973,56(6):2094-2098
It is suggested that the acid catalyzed hydrolyses of isobutene oxide and propene oxide proceed via rate-determining bimolecular reaction between a carbonium ion and solvent water (A2+ mechanism). The carbonium ion is formed by ring opening of the protonated epoxide, in a relatively fast preceding step. This mechanism is in agreement with all experimental facts known today. There is evidence for a similar mechanism in the acid catalyzed hydrolyses of cyclic acetals. 相似文献
74.
The products of pyrolysis, at 400°C in vacuo (1 Pa), of poly(ethyleneterephthalate) (PET) and poly(butyleneterephthalate) (PBT) were studied. The products were identified by gas chromatography/mass spectrometry technique. The highly volatile products of PET contained acetaldehyde, benzene, toluene, styrene and ethylbenzene and in the case of PBT, butadiene, tetrahydrofuran, vinylcyclohexene and ethylbenzene were the major products. A qualitative analysis of the products of low volatility revealed that the main components were benzoic acid, terephthalic acid, monovinyl esters of terephthalic acid and higher oligomers in the case of PET, whereas benzoic acid, monobutenyl esters of terephthalic acid and higher oligomers were the main products from PBT. The results obtained were in good agreement with those obtained from pyrolysis experiments, carried out directly in the mass spectrometer. Mechanisms to explain the occurrence of the different products are proposed. 相似文献
75.
Prefractionation of protein samples prior to two-dimensional electrophoresis (2-DE) has the potential to increase the dynamic detection range for proteomic analysis. We evaluated a membrane-based electrophoretic separation technique (Gradiflow) for its ability to fractionate an exoproteome sample from the filamentous fungus Trichoderma reesei. The sample was separated on the basis of size and charge. Buffer optimization was found to be necessary for successful size fractionation. Fractionation by charge was used to resolve the sample into four fractions that were subjected to analysis by two-dimensional electrophoresis (2-DE). Enhanced detection of low-abundance proteins with selective removal of high-abundance species was achieved. Fractionated and unfractionated samples were examined for differences in the ability to identify proteins following 2-DE using trypsin in-gel digestion followed by peptide mass fingerprinting using matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). Fractionated samples showed marked improvement in protein identification ability and sequence coverage. This study demonstrates the utility of the Gradiflow for fractionation, resulting in an enhancement of resolution and characterization of a moderately complex proteome. 相似文献
76.
[reaction: see text]. The riboflavin-catalyzed photooxidation of 2',3',5'-tri-O-acetyl-8-oxo-7,8-dihydroguanosine generates a radical intermediate that is competitively trapped by H(2)O, O2(-)(*), or O(2). The products of H(2)O trapping have been previously described as the spiroiminodihydantoin (pH >or= 7) and iminoallantoin/guanidinohydantoin (pH < 7) nucleosides. Trapping by O2(-)(*) leads to the oxaluric acid (pH or= 8.6) pathways (R' ', R' ' = H or 2,3,5-tri-O-Ac-ribofuranosyl). The pH-dependent role of superoxide was probed using Mn-SOD and compared to guanosine and 8-methoxyguanosine photooxidation. 相似文献
77.
Alfred Courtin 《Helvetica chimica acta》1981,64(6):1849-1853
Syntheses of Some Alkyl, Cycloalkyl and Aryl 3-Aminophenyl Sulfones Syntheses of alkyl ( 1a – 1i, 1m ), cycloalkyl ( 1j, 1k ) and aryl ( 1l ) 3-aminophenyl sulfones were achieved by ethanolic Béchamp-reduction of the appropriate 3-nitrophenyl sulfones ( 3a – 3m ). The alkyl ( 3a – 3i ) and cycloalkyl ( 3j, 3k ) 3-nitrophenyl sulfones were prepared via nitration of their respective sulfones ( 2a – 2k ). Methyl (3-nitrophenyl) sulfone ( 3a ) was also prepared by condensation of 3-nitrobenzenesulfinic acid ( 4 ) with bromoacetic acid to 3-nitrophenylsulfonyl-acetic acid ( 5 ) followed by decarboxylation. 相似文献
78.
One-dimensional magnetism and crystal structure ofcatena-di-bromobis(3,5-dimethylpyridine)copper(II)
Johannes A. C. van Ooijen Jan Reedijk Eduard J. Sonneveld Jan W. Visser 《Transition Metal Chemistry》1979,4(5):305-307
Summary Crystals ofcatena-di--bromobis(3,5-dimethylpyridine)copper(II) are monoclinic, space group P21/a. The unit cell constants area=13.900(2),b=14.416(2),c=4.097(1) Å,=93.49(2)°, V=819.4 Å3 and Z=2. The structure was determined from powder data using a Guinier-Johansson focussing powder camera. The structure was solved using a simplex method for function minimization to a conventional R-value of 0.13.The structure consists of infinite linear chains parallel toc in which the copper coordination is distorted elongated octahedral. Cu-Br distances were found to be 2.449(7) and 3.286(7) Å, whereas the Cu-N bond length is 2.02(2) Å. All distances are in the range usually observed for this type of compounds.The antiferromagnetic superexchange interactions between adjacent CuII ions (J = –21 cm–1) has been compared with those observed in structural similar CuBr2L2 compounds. The differences in observed J-values are discussed briefly, in relation to the structural variations. It appears that very small changes in structural parameters strongly affect the magnetic exchange. 相似文献
79.
Alfred Deege Heribert Husmann Eduard Hübinger Frank Kobor Gerhard Schomburg 《Journal of separation science》1993,16(10):587-593
Complex mixtures of partially alkylated cyclodextrins can be analyzed by both HPLC and high temperature capillary GC. Because of the limited efficiency of LC, suitable analytical and preparative separations can be achieved only with systems of carefully optimized selectivity. Using LC it has been possible to isolate and purify single cyclodextrin species from very complex mixtures of components which contain unreacted hydroxyl groups in addition to the alkoxy groups. Analysis of the reaction mixtures and of fractions taken from LC separations can be performed with advantage by high resolution capillary GC at high temperatures between 300 and 400 °C. The thermal stability of partially alkylated cyclodextrins in high temperature GC is considerably increased by trimethylsilylation of the free hydroxyl groups. Fast atom bombardment mass spectrometry and proton NMR were used to identify species isolated from the preparative LC separations. 相似文献
80.
By spectrophotometric measurements in the systemDMSO/H2O/Me
4NOH pK
a-values for several model compounds representing bile pigment partial structures were established. The acidic protons of pyrrole and lactame type nitrogen atoms are removed by bases governed by the electronic properties of the substituents on these ring systems. The pK
a-values for both types lie in the same region. In the pyrromethenones the lower one corresponds to the lactame type NH as was deduced by comparison with specifically methylated derivatives.The complexation of these ligands is determined by the possibility of removing an acidic proton and achieving a chelate structure by means of an adjacent pyrrolinone type nitrogen atom. Complexes are favoured in the series pyrromethenes > lactim ethers > pyrromethenones. With the latter there are two possibilities: one observed with BF2-chelation where the lactime form becomes stabilized, the other one with zink where both acidic centers are involved in the bonding.
19. Mitt.:H. Falk, A. Leodolter undG. Schade, Mh. Chem.109, 183 (1978). 相似文献
19. Mitt.:H. Falk, A. Leodolter undG. Schade, Mh. Chem.109, 183 (1978). 相似文献