全文获取类型
收费全文 | 7979篇 |
免费 | 249篇 |
国内免费 | 25篇 |
专业分类
化学 | 5339篇 |
晶体学 | 52篇 |
力学 | 257篇 |
综合类 | 1篇 |
数学 | 1080篇 |
物理学 | 1524篇 |
出版年
2023年 | 59篇 |
2022年 | 91篇 |
2021年 | 261篇 |
2020年 | 166篇 |
2019年 | 171篇 |
2018年 | 156篇 |
2017年 | 146篇 |
2016年 | 329篇 |
2015年 | 246篇 |
2014年 | 270篇 |
2013年 | 498篇 |
2012年 | 550篇 |
2011年 | 572篇 |
2010年 | 431篇 |
2009年 | 411篇 |
2008年 | 515篇 |
2007年 | 500篇 |
2006年 | 390篇 |
2005年 | 340篇 |
2004年 | 291篇 |
2003年 | 267篇 |
2002年 | 208篇 |
2001年 | 153篇 |
2000年 | 125篇 |
1999年 | 89篇 |
1998年 | 87篇 |
1997年 | 66篇 |
1996年 | 78篇 |
1995年 | 64篇 |
1994年 | 57篇 |
1993年 | 56篇 |
1992年 | 57篇 |
1991年 | 49篇 |
1990年 | 37篇 |
1989年 | 30篇 |
1988年 | 36篇 |
1987年 | 24篇 |
1986年 | 30篇 |
1985年 | 48篇 |
1984年 | 35篇 |
1983年 | 24篇 |
1982年 | 23篇 |
1981年 | 22篇 |
1980年 | 30篇 |
1979年 | 18篇 |
1978年 | 26篇 |
1976年 | 19篇 |
1975年 | 18篇 |
1974年 | 14篇 |
1973年 | 12篇 |
排序方式: 共有8253条查询结果,搜索用时 15 毫秒
81.
Bergquist Peter L. Te’o V. S. Junior Gibbs Moreland D. Cziferszky Angela C. E. De Faria Fabricia P. Azevedo Maristela O. Nevalainen K. M. Helena 《Applied biochemistry and biotechnology》2002,98(1-9):165-176
Cost-effective production of enzymes for industrial processes makes the appropriate selection of the host-vector expression
system critical. We have developed two systems for the bulk production of bleaching enzymes from thermophiles. Kluyveromyces lactis has been developed as a secretion host employing expression vectors based on the 2μ-like plasmid pKD1 of Kluyveromyces drosophilarium. Our second system involves the filamentous fungus Trichoderma reesei. Fusion and nonfusion vectors have been constructed using the strong cellobiohydrolase 1 (cbh1) promoter. The KEX2 protease cleavage site and a 6 × HIS-tag have been incorporated to facilitate both cleavage and purification
of the mature foreign proteins. 相似文献
82.
Catellani M Mealli C Motti E Paoli P Perez-Carreño E Pregosin PS 《Journal of the American Chemical Society》2002,124(16):4336-4346
A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C16H19)PdCl]2*CH2Cl2 with ortho and para methyl substituents at the arene has been characterized by means of X-ray diffraction techniques. The C(ipso) atom of the arene lies almost at the fourth planar coordination site of the metal [Pd-C(ipso) = 2.22(1) A (average)], and due to the arene's tilting, the substituted C(ortho) atom is relatively close to the metal atom [2.54(1) A (average)]. The coordinated C(ipso)-C(ortho) linkage, in a seemingly dihapto coordination, is anti with respect to the CH2 bridge of the bicycloheptyl unit. Variable-temperature NMR experiments for the para-substituted dimer 9 reveal restricted rotation of the two aryl groups about the corresponding C-C(ipso) bonds (DeltaE < or =17 kcal x mol(-1)). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)I (4) and its related substituted derivatives. The essential results are as follows: (i) The potential energy surface for twisting the phenyl ring away from the symmetric eta(1) coordination in 4 is very flat (DeltaE < or = 1 kcal x mol(-1)) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd-C(ortho) bond in the seemingly eta2-type conformer. (ii) Complete rotation of the unsubstituted phenyl ring is not facile but feasible. A significant strain affects the transition-state structure featuring a Pd-HC(aryl) agostic-type bond. The calculated destabilization of 10.3 kcal x mol(-1), with respect to the ground state, can be compared to the experimental barrier of the dimer 9. (iii) Various methyl-substituted derivatives of 4 have been optimized, and their structural and energetic trends are discussed. An almost ideal eta1 coordination is shown by the anti conformer of the C(ortho)-substituted complex due steric effects. For all of the other cases, a slipped eta2 coordination may be described. As a general conclusion, the unsaturated metal center receives pi electron density of the arene mainly through its C(ipso) atom. The effect may be slightly improved if the C(ortho) atom also gets closer to the metal, but in no case, does the slipped eta2 coordination seem to be crucial for the stability of the system. 相似文献
83.
Barros N. Feijóo S. Simoni A. Critter S. A. M. Airoldi C. 《Journal of Thermal Analysis and Calorimetry》2000,63(2):577-588
The microcalorimetric method was used to calculate the metabolic enthalpy change per mol of glucose degraded by soil microorganisms,
ΔH
met. This parameter has been calculated by microcalorimetry for many organic, inorganic and biochemical reactions, but there
is only some information about its quantification for microbial growth reactions in soils. Values of ΔH
met were calculated for different soil samples collected in Galicia (Spain) and Campinas (Săo Paolo, Brazil). Exponential microbial
growth was stimulated in all soil samples by the addition of glucose and power-time curves were recorded. Results showed changes
in the values of ΔH
met calculated for all the soil samples, suggesting a dependence of this value with the microbial growth rate constant, with
the percentage of growth, with the initial number of microorganisms of soil samples, with the quantity of glucose added and
with the strain of bacteria growing in soil.
The interpretation of variations of ΔH
met provides important qualitative and quantitative information. It reports data that allow to interpret from a qualitative point
of view, the increase in biomass as a consequence of the degradation of the organic matter in soil, to understand changes
in the percentages of soil organic matter and to know if the microbial population growing in differential soil samples is
homogeneous. Therefore, to report that value would be very important in ecological studies, but beforehand, it is necessary
to solve some problems that can appear in the experiments done to make the quantification
.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
84.
An amperometric flow-injection method for the determination of antithyroid drugs such as 6-methyl-2-thiouracil (MTU) using a carbon fiber cylindrical microelectrode-based detector is reported. A home-made flow-cell specially adapted for working with cylindrical microelectrodes ranging between 4 and 10 mm in length was used for this purpose. Methanol containing 0.05 mol l(-1) tetrabutylammonium perchlorate as the supporting electrolyte was employed, and a potential of +1.6 V was selected for the amperometric detection of MTU. No cleaning or electrode surface regeneration of the fiber was necessary during the whole working day. A limit of detection of 2.6x10(-7) mol l(-1) (37 mug MTU l(-1)) was achieved. The method was applied to the determination of MTU in spiked feed samples (peas and corn flour), at the 142 mug g(-1) level. The procedure involved the extraction of the drug in methanol, a clean-up step using Florisil cartridges and the use of the standard additions method. Recoveries of 81+/-6 and 85+/-8% were obtained for peas and corn flour, respectively. 相似文献
85.
M. Kabešová I. Vargová T. Šramko J. Gažo 《Journal of Thermal Analysis and Calorimetry》1978,13(1):65-70
The influence of the position of the CH3 group in picoline and lutidine ligands on the degree of chemical change of the NCS groups in coordination compounds of the type Cu(NCS)2L2 (whereL=2-, 3- and 4-picoline, and 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-lutidine) is dealt with. The most marked effect of the CH3 group is found to be exerted in position 4. This effect of the methyl group on the degree of chemical change points to the mutual influence of the ligands in coordination compounds of Cu(II).
Zusammenfassung Der Artikel befaßt sich mit dem Einfluß der Lage der CH3 Gruppe in Pikolinen und Lutidinen als Liganden auf den Grad der chemischen Änderungen der Gruppen NSC in Koordinationsverbindungen des Typs Cu(NCS)2L2 (L=2-, 3- und 4-Pikoline, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- und 3,5-Lutidine). Der ausgeprägteste Effekt der CH3 Gruppe wurde in der Position 4 beobachtet. Dieser Einfluß der Methylgruppe auf das Ausmaß der chemischen Änderungen deutet auch auf die gegenseitige Wirkung der Liganden in Koordinationsverbindungen von Cu(II).
Résumé L'article a trait à l'influence de la position du groupe CH2 dans les picolines et lutidines, en tant que ligands, sur le degré des changements chimiques des groupes SCN dans les composés de coordination du type Cu(SCN)2L2 (L=2-, 3 et 4-picoline, 2,3-, 2,4-,2,5-, 2,6-, 3,4- et 3,6-lutidine). L'effet le plus prononcé du groupe CH3 s'observe en position 4. Cette influence du groupe méthyle sur le degré des changements chimiques indique aussi l'influence mutuelle des ligands dans les composés de coordination du Cu(II).
- , , NCS Cu(NCS)2,L2, L=2-, 3- 4- , 2.3-, 2.4-, 2.5-, 2.6-, 3.4- 3.5-. , 4. Cu(II).相似文献
86.
Mafra L Paz FA Shi FN Rocha J Trindade T Fernandez C Makal A Wozniak K Klinowski J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,12(2):363-375
A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry). 相似文献
87.
Joaquim Branco Catarina de Jesus Dias Antnio Pereira Gonalves Teresa Almeida Gasche Antnio Pires de Matos 《Thermochimica Acta》2004,420(1-2):169-173
In our laboratories we have been studying the synthesis and reactivity of binary actinide and lanthanide intermetallic compounds. In this work, the air-oxidation of ThCu2 and AnNi2 (An = Th, U) was followed by thermogravimetry (TG) and the products were characterized by X-ray powder diffraction (XRD). The heterobimetallic oxides obtained are described by the formulas 2MO·ThO2 (M = Cu, Ni) and 2NiO·UO3. The thermogravimetric analysis under hydrogen of these heterobimetallic oxides show one mass loss for 2MO·ThO2 and two mass losses for 2NiO·UO3 over a wide range of temperature (293–1273 K). The characterization by XRD shows that the reduction products are 2M·ThO2 (M = Cu, Ni) and 2Ni·UO2, with all the actinides in the 4+ oxidation state. The actinide heterobimetallic oxides were described as copper or nickel supported catalysts. 相似文献
88.
We calculate energy barriers of atom- and proton-transfer reactions in hydrogen-bonded complexes in the gas phase. Our calculations do not involve adjustable parameters and are based on bond-dissociation energies, ionization potentials, electron affinities, bond lengths, and vibration frequencies of the reactive bonds. The calculated barriers are in agreement with experimental data and high-level ab initio calculations. We relate the height of the barrier with the molecular properties of the reactants and complexes. The structure of complexes with strong hydrogen bonds approaches that of the transition state, and substantially reduces the barrier height. We calculate the hydrogen-abstraction rates in H-bonded systems using the transition-state theory with the semiclassical correction for tunneling, and show that they are in excellent agreement with the experimental data. H-bonding leads to an increase in tunneling corrections at room temperature. 相似文献
89.
Berezkin Victor G. Mardanov Robert G. Moiseew Aleksey A. Malinowska Irena Różyło Jan K. 《平面色谱法杂志一现代薄层色谱法》2002,15(5):377-379
JPC – Journal of Planar Chromatography – Modern TLC - 相似文献
90.
Aguado GP Moglioni AG García-Expósito E Branchadell V Ortuño RM 《The Journal of organic chemistry》2004,69(23):7971-7978
The hydrogenation reactions of several cyclobutyl enamides derived from (-)-alpha-pinene or (-)-verbenone have been investigated by using different catalysts. The chiralities of both the substrate and the catalyst as well as the Z/E stereochemistry of the double bond have been considered, and the observed diastereoselectivity has been rationalized. For enamides with the double bond separated from the cyclobutane by a methylene, the Wilkinson catalyst did not induce any diastereoselection, but excellent diastereoselectivity was observed when using Et-DuPHOS-Rh and ChiraPHOS-Rh. The configuration of the new stereogenic center was catalyst-dependent and can be rationalized according to the Halpern mechanism. For (Z)-enamides with the double bond directly linked to the cyclobutane ring, the chirality of the substrate governed the diastereoselection and the Halpern mechanism seemed not to be operative in the hydrogenation with ChiraPHOS, with the configuration of the new stereogenic center being determined by steric effects. On the contrary, the chirality of the catalyst was the factor determining the stereochemistry of the major products with alkyl-DuPHOS-Rh. Z/E stereochemistry influenced the stereodifferentiation, and a different behavior for each Z or E stereoisomer was found. For both (Z)- and (E)-enamides, some instances of match/mismatch between the chirality of the substrate and that of the catalyst were observed. As a result of all of these studies, a series of new cyclobutyl alpha-amino acids has been synthesized. These products are interesting to incorporate into conformationally constrained peptides. 相似文献