全文获取类型
收费全文 | 1672篇 |
免费 | 99篇 |
国内免费 | 4篇 |
专业分类
化学 | 1258篇 |
晶体学 | 4篇 |
力学 | 65篇 |
数学 | 148篇 |
物理学 | 300篇 |
出版年
2023年 | 18篇 |
2022年 | 32篇 |
2021年 | 43篇 |
2020年 | 70篇 |
2019年 | 53篇 |
2018年 | 27篇 |
2017年 | 16篇 |
2016年 | 72篇 |
2015年 | 71篇 |
2014年 | 82篇 |
2013年 | 95篇 |
2012年 | 136篇 |
2011年 | 175篇 |
2010年 | 85篇 |
2009年 | 80篇 |
2008年 | 110篇 |
2007年 | 133篇 |
2006年 | 99篇 |
2005年 | 81篇 |
2004年 | 62篇 |
2003年 | 45篇 |
2002年 | 36篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 11篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1985年 | 4篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1970年 | 2篇 |
1917年 | 2篇 |
1910年 | 2篇 |
1895年 | 2篇 |
排序方式: 共有1775条查询结果,搜索用时 0 毫秒
111.
Two tetranuclear manganese(II) complexes [Mn(II)4(thiaS)2] (1) and [Mn(II)4(thiaSO)2] (2) have been synthesized under solvothermal conditions in methanol with p-tert-butylthiacalix[4]arene (thiaS) and p-tert-butylsulfinylthiacalix[4]arene (thiaSO). For both complexes, the structure has been established from single-crystal X-ray diffraction. [Mn4(thiaS)2].H2O (1) crystallizes in the orthorhombic Immm (No. 71) space group with the following parameters: a = 18.213 (5) angstroms, b = 19.037 (5) angstroms, c = 29.159 (5) angstroms, V = 10110 (4) angstroms3, and Z = 4. [Mn4(thiaSO)2].H2O (2) crystallizes in the monoclinic C2/m (No. 12) space group with the following parameters: a = 33.046(1) angstroms, b = 19.5363 (8) angstroms, c = 15.7773 (9) angstroms, beta = 115.176 (2) degrees, V = 9218.3 (8) angstroms3, and Z = 4. The two complexes are neutral and are best described as manganese squares sandwiched between two thiacalixarene macrocycles. In both complexes, each manganese center is six-coordinated in a trigonal prismatic geometry with four phenoxo oxygen atoms plus two sulfur atoms for 1 or two oxygen atoms from SO groups for 2. The two tetranuclear complexes exhibit identical magnetic behaviors resulting from antiferromagnetic interactions between the four manganese centers. The simulation of the magnetic susceptibility was done considering a single exchange-coupling constant between the manganese(II) ions, J (H = -J(S1S2 + S2S3 + S3S4 + S1S4)). The best fits give the same result for the two complexes: g = 1.94 and J = -5.57 cm(-1). 相似文献
112.
David J. Aitken Dominique Guillaume Henri-Philippe Husson Angle Chiaroni Claude Riche 《Journal of heterocyclic chemistry》1991,28(3):705-709
The title compound, containing a new heterocyclic skeleton, was identified by X-ray crystallography as the product of condensation of (R)-phenylglycinol with an excess of formaldehyde. The molecule adopts a rigid double twist-chair conformation in both solid and solution states. 相似文献
113.
114.
Nicolas Lebouvier Typhanie Corbin Guillaume Le Baut Marc Le Borgne 《Tetrahedron letters》2006,47(36):6479-6483
New conazole antifungals, in the series of triazole alcohols 23a-d and 24a-e incorporating an indole moiety substituted at 5-position by halogens, a cyano or 4-methoxyphenyl group, have been synthesized by ring opening of corresponding oxiranes 15 and 16. These dihalogeno intermediates and their congeneers could be prepared in high yields by Corey-Chaykovsky reaction under microwave irradiation. 相似文献
115.
A classical electrostatic polarization scheme using the additive distribution procedure has been applied to determine the longitudinal polarizability of an all-trans hexatriene molecule in an infinite stretched fiber. The parameters have been derived from ab initio CPHF/6-31G calculations and the electrostatic scheme has been validated via comparison with ab initio results on small clusters. Upon packing the polarizability of all-trans hexatriene increases by 7%. This small increase results from the balance between the enhancement of the polarizability due to collinear packing and the reduction associated with lateral packing. 相似文献
116.
Direct Oxidative Coupling of N‐Acetyl Indoles and Phenols for the Synthesis of Benzofuroindolines Related to Phalarine 下载免费PDF全文
Terry Tomakinian Dr. Régis Guillot Prof. Dr. Cyrille Kouklovsky Dr. Guillaume Vincent 《Angewandte Chemie (International ed. in English)》2014,53(44):11881-11885
Inspired by the biogenetic synthesis of benzofuroindoline‐containing natural products, we designed an oxidative coupling between phenol and N‐acetyl indoles. This straightforward and direct radical process, mediated by 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone and FeCl3 allowed the regioselective synthesis of benzofuro[3,2‐b]indolines, whose structure is found in the natural product phalarine. 相似文献
117.
Lamoureux M Patard L Hernandez B Couesnon T Santini GP Cognet JA Gouyette C Cordier C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,65(1):84-94
Successive investigations over the last decade have revealed and confirmed a stable loop closure in a family of d-[GTAC-5Pur6N7N-GTAC] hairpins, where 5Pur6N7N is a AAA, GAG and AXC loop (X being any nucleotide). The trinucleotide loop is characterized by a well defined 5Pur-7N mispairing mode, and by upfield chemical shifts for three sugar protons of the apical nucleotide 6N. The GTTC-ACA-GAAC DNA hairpin, of interest for its likely involvement in Vibrio cholerae genome mutations, has now been investigated. The GTAC-ACA-GTAC DNA hairpin has also been studied because it is intermediate between the other structures, as it contains the loop of the hairpin under consideration and the stem of the above family. The two hairpins with the ACA loop are stable. They show the same mispairing mode and similar upfield shifts as the previous family, but GTTC-ACA-GAAC seems to be slightly less compact than any other. GTTC-ACA-GAAC is remarkable in that it exhibits a B(II) character for the phosphate-ester conformation at 8Gp9A, together with a swing of the upper hairpin into the major groove that, in particular, brings 6CH1' roughly as close to 7AH2 as to 6CH6. These unexpected structural features are qualitatively deduced from (1)H and (31)P NMR spectra, and confirmed by Raman spectroscopy. This comparative study shows that not only the loop sequence but also the stem sequence may control hairpin structures. 相似文献
118.
van der Rest G Mourgues P Nedev H Audier HE 《Journal of the American Society for Mass Spectrometry》2000,11(8):705-710
The bimolecular reaction of the CH2CHOH.+ enol ion (m/z 44) with acetaldehyde gives a strongly dominant product,m/z 45, formed mainly by proton transfer from the ion to the molecule. The abundance of the product coming from a H. abstraction reaction from the neutral, albeit more exothermic, is negligible. In order to explain this result, the long lived [CH2CHOH.+, CH3CHO] solvated ion was generated by reaction of the CH2CHOH.+ enol ion with (CH3CHO) n in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. The structure of this solvated ion was clearly established. Labeling indicates that [CH2CHOH.+, CH3CHO], upon low energy collisions, reacts by H. abstraction more rapidly than by H+ transfer to the neutral moiety. This shows that the entropic factors are determinant when the enol ion reacts directly with acetaldehyde. 相似文献
119.
Bis(peroxo)vanadium(V) complexes are widely investigated as anticancer agents. They exert their antitumor and cyctotoxic effects through inhibition of tyrosine phosphatases and DNA cleavage, respectively. The latter process remains poorly understood. The mechanism of DNA cleavage by NH(4)[(phen)V(O)(eta(2)-O(2))(2)] (phen = 1,10-phenanthroline) was investigated. Kinetic studies on DNA cleavage revealed that the complex is a single-strand nicking agent with no specificity. EPR experiments using 2,2,6,6-tetramethyl-4-piperidone (TMP) and 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) as spin-traps for singlet oxygen and hydroxyl radical, respectively, implicated hydroxyl radical production upon photodecomposition of bis(peroxo)vanadium(V). This was corroborated by benzoate inhibition of DNA strand scission and stoichiometric oxidation of 2-propanol to acetone upon irradiation of bis(peroxo)vanadium(V) phenanthroline. High-resolution polyacrylamide gel analysis of the vanadium cleavage reaction and [Fe(II)EDTA](2)(-)/H(2)O(2) resulted in comigration of "ladder" pattern bands, which superimposed when both reactions were run on the same lane. These findings identify hydroxyl radical produced from the photooxidation of the peroxo ligand on vanadium as the active species in DNA cleavage. 相似文献
120.
Chevreux G Potier N Van Dorsselaer A Bahloul A Houdusse A Wells A Sweeney HL 《Journal of the American Society for Mass Spectrometry》2005,16(8):1367-1376
Among the myosin superfamily, myosin VI differs from all others by a reverse directionality and a particular motility. Little structural information is available for myosin VI. It is known that it binds one calmodulin (CaM) by means of a single "IQ motif" and that myosin VI contains a specific insert located at the junction between the motor domain (MD) and the lever arm, likely to play a critical role for the unusual motility previously observed. Electrospray ionization mass spectrometry (MS) was used to determine the CaM and Ca2+ stoichiometries in several myosin VI constructs. In particular, the experimental conditions required for the observation of multiprotein/Ca2+ noncovalent assemblies are detailed for two truncated MD constructs (less than 20 kDa) and for three full MD constructs (more than 90 KDa). The specificity of the detected stoichiometries is discussed for each construct and the resolving power of Time of Flight mass spectrometry is stressed, in particular for the detection of metal ions binding to high molecular weight complexes. MS reveals a new CaM binding site for myosin VI and highlights a different behavior for the five myosin VI constructs versus Ca2+ binding. In addition to these stoichiometry based experiments, gas-phase dissociation analyses on intact complexes are described. They reveal that Ca2+ transfer between protein partners occurs during the dissociation process for one construct with a full MD. Charge-transfer and dissociation behavior has allowed to draw structural assumptions for the interaction of the MD with the CaM N-terminal lobe. 相似文献