全文获取类型
收费全文 | 78500篇 |
免费 | 8246篇 |
国内免费 | 7770篇 |
专业分类
化学 | 52543篇 |
晶体学 | 482篇 |
力学 | 2779篇 |
综合类 | 917篇 |
数学 | 10812篇 |
物理学 | 26983篇 |
出版年
2024年 | 119篇 |
2023年 | 748篇 |
2022年 | 1577篇 |
2021年 | 2547篇 |
2020年 | 2108篇 |
2019年 | 2142篇 |
2018年 | 1681篇 |
2017年 | 1951篇 |
2016年 | 2417篇 |
2015年 | 2582篇 |
2014年 | 3335篇 |
2013年 | 5616篇 |
2012年 | 4264篇 |
2011年 | 4682篇 |
2010年 | 4113篇 |
2009年 | 4923篇 |
2008年 | 5135篇 |
2007年 | 5243篇 |
2006年 | 4497篇 |
2005年 | 3524篇 |
2004年 | 3093篇 |
2003年 | 2818篇 |
2002年 | 4985篇 |
2001年 | 2333篇 |
2000年 | 1895篇 |
1999年 | 1642篇 |
1998年 | 1477篇 |
1997年 | 1223篇 |
1996年 | 1163篇 |
1995年 | 1042篇 |
1994年 | 970篇 |
1993年 | 930篇 |
1992年 | 873篇 |
1991年 | 651篇 |
1990年 | 515篇 |
1989年 | 433篇 |
1988年 | 438篇 |
1987年 | 338篇 |
1986年 | 305篇 |
1985年 | 415篇 |
1984年 | 316篇 |
1983年 | 190篇 |
1982年 | 373篇 |
1981年 | 522篇 |
1980年 | 470篇 |
1979年 | 506篇 |
1978年 | 385篇 |
1977年 | 302篇 |
1976年 | 265篇 |
1973年 | 171篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
971.
Two fluorophore-nitrilotriacetic acid conjugates, Pro-Q Sapphire 365 and Pro-Q Sapphire 488 oligohistidine gel stains, have been developed for the fluorescence detection of fusion proteins containing oligohistidine tags directly in sodium dodecyl sulfate polyacrylamide gels, without the requirement for electroblotting, reporter enzymes or secondary detection reagents. Pro-Q Sapphire 365 oligohistidine gel stain exhibits bright-blue fluorescence (emission maximum = 450 nm) when illuminated with UV-A or UV-B light from a standard ultraviolet transilluminator. Pro-Q Sapphire 488 oligohistidine gel stain exhibits bright-green fluorescence (emission maximum = 515 nm) when illuminated with visible light from a laser-based gel scanner equipped with a 470 nm second-harmonic generation (SHG) or 488 nm argon-ion laser source. Typically, 25-65 ng of oligohistidine-tagged fusion protein in whole cell lysates is detectable using either stain. After documenting the fluorescence signal from the Pro-Q Sapphire dyes, gels may be post-stained with the red-fluorescent SYPRO Ruby protein gel stain in order to reveal the total protein pattern. 相似文献
972.
The addition of the macrocyclic polyether 18-Crown-6 (18C6) increases the selectivity of oxidation of ethylbenzene to -phenylethylhydroperoxide (PEH) in the presence of Ni(acac)2. The initial oxidation rate, selectivity and degree of conversion of ethylbenzene to PEH are greater than those catalyzed by Ni(acac)2 only. The efficiency of the macrocyclic ligand as an activator of Ni(acac)2 exceeds that of monodentate donor ligands. The high selectivity of the process is due to both the primary Ni(acac)2 · 18C6 complexes and the products of their transformation in the course of oxidation. The mechanism of ethylbenzene oxidation catalyzed by Ni(acac)2 · 18C6 complexes is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1406–1411, August, 1994. 相似文献
973.
A. V. Belyakov A. V. Golubinskii L. V. Vilkov V. I. Shiryaev E. M. Styopina E. A. Kovalyova V. S. Nikitin 《Russian Chemical Bulletin》1993,42(2):346-349
The molecular structure of gaseous Br2Sn(CH2SiMe2)2O was studied by electron diffraction. The six-membered ring has a chair conformation whereas the entire molecule possessesC
s
symmetry. The existence of a boat conformer cannot be completely excluded. The results of theoretical calculations for a twisted-boat conformation are at variance with the experimental data. Steric strain caused by mutual repulsion of the two axial methyl groups is reduced to the tilt of the Me2Si fragments in opposite directions. This results in an increase (up to 26°C) in the angle formed by the bisector of the CM-Si-CM angle with the CcSiO plane. The main geometrical parameters are as follows:r
g (Å): Si-O 1.708(20); Si-CM 1.862(20); Si-Cc 1.882(9); Sn-C 2.108(26); Sn-Br 2.456(3); C-H 1.099(30); (degr.): C-Sn-C 105(2); Br-Sn-Br 107.9(1.2); Si-O-Si 129.6(3); CM-Si-CM 112; Si-C-H 113 (fixed value in accordance with experiment); Cc-Si-O 107(2); Sn-C-Si 109(2); torsion angles: (Si-C) 52(2); (Si-O) 62(1); (Cc-Sn) 54(1). The average amplitudes were fixed at the values calculated from the force field. Structural parameters of molecules with similar structures were analyzed and compared.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 384–387, February, 1993. 相似文献
974.
M. M. Krayushkin M. A. Kalik L. G. Vorontsova E. Yu. Zvezdina M. G. Kurella 《Russian Chemical Bulletin》1993,42(7):1204-1208
9-Methyl-3,3a,4,5-tetrahydro-6H-thieno[2,3-b]thiocino[4,5-c]isoxazole and 9-hydroxyimino-2-methyl-7-methylene-6, 7,8,9-tetrahydro-5H-thieno[2,3-b]thiocine have been prepared for the first time by the oxidation of 2--alkenylthio-3-thiophenecarbaldoximes with NaOCl. X-ray structural investigations of thieno[2,3-b]thiocine and 2-homomethallylthio-5-methyl-3-thiophenecarbaldoxime have been carried out.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1262–1266, July, 1993. 相似文献
975.
The diagonal elements of the first and second order spinless density matrices have been calculated for the lowest excited1
P and3
P terms of Be, B+ and C++ using wavefunctions at different levels of approximations published in the literature. The analysis of these functions has
resulted in a new interpretation of Hund's first rule in terms of an anisotropic screening effect. 相似文献
976.
977.
978.
Zou JW Jiang YJ Guo M Hu GX Zhang B Liu HC Yu QS 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(2):740-751
Ab initio calculations have been performed on a series of complexes formed between halogen-containing molecules and ammonia to gain a deeper insight into the nature of halogen bonding. It appears that the dihalogen molecules form the strongest halogen-bonded complexes with ammonia, followed by HOX; the charge-transfer-type contribution has been demonstrated to dominate the halogen bonding in these complexes. For the complexes involving carbon-bound halogen molecules, our calculations clearly indicate that electrostatic interactions are mainly responsible for their binding energies. Whereas the halogen-bond strength is significantly enhanced by progressive fluorine substitution, the substitution of a hydrogen atom by a methyl group in the CH(3)X...NH(3) complex weakened the halogen bonding. Moreover, remote substituent effects have also been noted in the complexes of halobenzenes with different para substituents. The influence of the hybridization state of the carbon atom bonded to the halogen atom has also been examined and the results reveal that halogen-bond strengths decrease in the order HC triple bond CX > H(2)C=CHX approximately O=CHX approximately C(6)H(5)X > CH(3)X. In addition, several excellent linear correlations have been established between the interaction energies and both the amount of charge transfer and the electrostatic potentials corresponding to an electron density of 0.002 au along the R-X axis; these correlations provide good models with which to evaluate the electron-accepting abilities of the covalently bonded halogen atoms. Finally, some positively charged halogen-bonded systems have been investigated and the effect of the charge has been discussed. 相似文献
979.
I. S. Belostotskaya V. B. Vol'eva N. L. Komissarova M. O. Dekaprilevich V. N. Khrustalev A. Yu. Karmilov V. V. Ershov 《Russian Chemical Bulletin》1997,46(7):1272-1280
Oxidative transformations of 2-dialkylaminomethyl-4,6-di-tert-butylphenols depend on the nature of the oxidant, the character of the substituents at the nitrogen atom, and the medium.
A mechanism of the oxidation of these compounds is suggested. The molecular structure of the compound obtained as a result
of oxidative trimerization of 2-dimethylaminomethyl-4,6-di-tert-butylphenol was established by X-ray structural analysis.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1328–1335, July, 1997. 相似文献
980.
Zein films plasticized with oleic acid were formed by solution casting, by the stretching of moldable resins, and by blown film extrusion. The effects of the forming process on film structure were investigated by X-ray diffraction. Wide-angle X-ray scattering (WAXS) patterns showed d-spacings at 4.5 and 10 A, which were attributed to the zein alpha-helix backbone and inter-helix packing, respectively. The 4.5 A d-spacing remained stable under processing while the 10 A d-spacing varied with processing treatment. Small-angle X-ray scattering (SAXS) detected a long-range periodicity for the formed films but not for unprocessed zein, which suggests that the forming process-promoted film structure development is possibly aided by oleic acid. The SAXS d-spacing varied among the samples (130-238 A) according to zein origin and film-forming method. X-ray scattering data suggest that the zein molecular structure resists processing but the zein supramolecular arrangements in the formed films are dependent on processing methods. Structural model for a zein molecular aggregate (based on Matsushima et al.10). Rectangular prisms of individual zein molecules are hexagonally aligned parallel to each other. 相似文献