全文获取类型
收费全文 | 64549篇 |
免费 | 5081篇 |
国内免费 | 10127篇 |
专业分类
化学 | 48253篇 |
晶体学 | 984篇 |
力学 | 1706篇 |
综合类 | 884篇 |
数学 | 11444篇 |
物理学 | 16486篇 |
出版年
2024年 | 126篇 |
2023年 | 665篇 |
2022年 | 1325篇 |
2021年 | 2318篇 |
2020年 | 1720篇 |
2019年 | 1784篇 |
2018年 | 1494篇 |
2017年 | 1773篇 |
2016年 | 2095篇 |
2015年 | 1858篇 |
2014年 | 2630篇 |
2013年 | 4444篇 |
2012年 | 4058篇 |
2011年 | 3672篇 |
2010年 | 3192篇 |
2009年 | 3986篇 |
2008年 | 4147篇 |
2007年 | 4366篇 |
2006年 | 3470篇 |
2005年 | 2787篇 |
2004年 | 2549篇 |
2003年 | 2259篇 |
2002年 | 4640篇 |
2001年 | 2108篇 |
2000年 | 1658篇 |
1999年 | 1417篇 |
1998年 | 1311篇 |
1997年 | 1018篇 |
1996年 | 1040篇 |
1995年 | 945篇 |
1994年 | 914篇 |
1993年 | 821篇 |
1992年 | 861篇 |
1991年 | 588篇 |
1990年 | 510篇 |
1989年 | 407篇 |
1988年 | 377篇 |
1987年 | 282篇 |
1986年 | 276篇 |
1985年 | 379篇 |
1984年 | 268篇 |
1983年 | 171篇 |
1982年 | 326篇 |
1981年 | 483篇 |
1980年 | 439篇 |
1979年 | 480篇 |
1978年 | 376篇 |
1977年 | 286篇 |
1976年 | 246篇 |
1973年 | 156篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
Highly enantioselective 1,4-addition of alkenylzirconocene chlorides to α,β-enones was found to be catalyzed by a chiral rhodium complex generated from [Rh(cod)(MeCN)2]BF4 and (S)-BINAP. The reaction can be applied to either cyclic or acyclic enones and the optical yield was up to 99% ee. The reaction mechanism would involve the transmetalation between the alkenylzirconocene chloride and the rhodium complex to give the alkenylrhodium species as a key intermediate. 相似文献
922.
The crystal of [Ni(dien)2]2[Mn(NCS)6]·H2O was synthesized and the structure of its single crystal was determined by X-ray diffraction. The crystal is monoclinic system, space group P21/c with a=16.544(3),b=15.137(2), c=17.334(3)?, β=99.90(1)°, V=4276.3(12)?3, Z=4, Dc=1.479g·cm-3, Mr=951.55, F(000)=1998, μ=1.489mm-1, R=0.0399, Rw=0.0958. IR was also determined. 相似文献
923.
邻甲氧基苯基锂和 6,6-二甲基富烯 (或 6,6-二乙基富烯 )反应得配体锂盐 ( Cp′ Li) ,Cp′ Li直接和 Cp Zr Cl3· DME反应生成 2个新的锆络合物 Cp′ Cp Zr Cl2 。 BBr3和 Li Br分别用来使 Cp′ Cp Zr Cl2 环化 ,发现在相同条件下 BBr3比 Li Br3更有效。Cp′ Cp Zr Cl2 通过和 BBr3反应得到 2个新的锆氧杂环络合物。 4个新的锆络合物经元素分析、1 H NMR和 MS确证 相似文献
924.
The novel complex [K(18-C-6)]2[Cd(mnt)2][18-C-6-18-crown-6,nmt=1,2-dicyanoethene-1,2-dithiolate,C2S2-(CN)2^2-] was synthesized and characterized by elemental analysis,IR spectrum and X-ray diffraction analysis.The complex displays two-dimensional network structure of [K(18-C-6)] complex segments and [Cd(nmt)2] complex segment bridged by S-K-S,S-K-N and N-K-N interactions between adjacent[K(18-C-6)] and [Cd(mnt)2]units. 相似文献
925.
K. Grob 《Journal of separation science》1978,1(5):263-267
During one year continuous use of on-column injection, the typical advantages described in our first report have fully been confirmed. In addition the analysis of large sample volumes has proved promising. Only minor modifications have been applied to the on-column injector device. Broad evidence has been gathered showing that full separation efficiency of the capillary columns after on-column injection is attained only when cold trapping or the solvent effect, as band shortening mechanisms, are working- While, under the conditions of on-column injection, cold trapping is less efficient than with other injection techniques, the opposite holds true for the solvent effect. Compared with splitless injection, the danger of excessive solvent condensation on the column is increased. A working rule is presented for establishing the optimal chromatographic conditions for handling large sample volumes while ensuring full separation efficiency yet avoiding harm to the column. 相似文献
926.
The interaction of aryldiazonium ions with some Schiff-base complexes of cobalt and ruthenium have been studied. With cobalt, one-electron oxidation of [Co(II)Salen] occurred; with [Co(I)Salen] the corresponding Co(III)-aryl complexes were isolated. In the case of ruthenium oxidation also occurs, [Ru(Salen)(CO)py] gave the corresponding monocation. The results, especially for ruthenium, are in contrast to the stabilisation of both nitrosyl and aryldiazonium adducts in analogous porphyrin complexes. 相似文献
927.
Halina Y. Neujahr 《Applied biochemistry and biotechnology》1982,7(1-2):107-111
The enzyme phenol 2-hydroxylase was immobilized on Sepharose and used in conjunction with an O2 electrode for quantitating phenol. Similarly, catechol 1,2-oxygenase was used for quantitating catechol. A third probe was
prepared by immobilization ofTrichosporon cutaneum cells rather than purified phenol 2-hydroxylase for phenol quantitation. The whole cell system gave results comparable to
the immobilized enzyme system. 相似文献
928.
Barbara Dogan Hans-Dieter Beckhaus Hermann Birkhofer Christoph Rüchardt 《欧洲无机化学杂志》1990,123(6):1365-1368
Effects of Substituents on the Strength of C - C Bonds, 81. - Heats of Formation and Strain of 1,1,2,2-Tetraethylethylene Glycol Dimethyl Ether and D,L .-1,2-Dimethyl-l,2-diphenylethylene Glycol Dimethyl Ether The heats of combustion of the title compounds 1 and 2 were measured calorimetrically with the result (kcal mol -1, s. d. in parentheses) ΔH°c = − 1880.1 (± 0.6) and − 2373.3 (± 1.4). The heat of vaporisation of 1 ΔHv = 14.3 (± 0.3) and the heat of sublimation of 2 ΔHsub = 27.2 (± 0.5) were derived from their temperature dependance of the vapor pressure. The latter were determined between 30 and 80°C using a flow method. The resulting standard heats of formation ΔH°t(g) = −122.4 (± 0.7) and −43.8 (±1.5) for 1 and 2 correspond to a strain enthalpy (s) of 15.9 and 8.0 kcal mol-1, respectively. The steric strain of the dimethoxyethanes 1 and 2 is about one fourth lower than the strain of the corresponding dimethylethanes 3 and 4 bearing the same substituents. Thus, a methoxy group causes less steric stress than a methyl group. 相似文献
929.
A systematic extension of Ruedenberg's expansion formula is applied to evaluate two-electron integrals occurring in calculations on molecular structure. Minimum STO basis sets are used for all SCF-calculations within the framework of the MEDO-method (Multipole Expansion of Diatomic Overlap). The errors due to this approximation scheme are almost negligible compared to those introduced by the truncated basis set: LiH, Li2 and N2 are chosen as examples. 相似文献
930.
The exchange part of the usual Hartree-Fock potential in the unrestricted Hartree-Fock (UHF) theory is suitablyaveraged to construct an, average one-electron model Hamiltonian which generates a set of spin-restricted one-electron orbitals in
a self-consistent manner. These orbitals are then used to calculate the electronic energy of the open-shell system by using
the proper functional form for the energy which handles the exchange terms correctly. The eigenvalues ofF
av can be used for calculating either the spin-polarised or spin-averaged ionisation potentials of different orbitals at theKoopmans’ theorem level of approximation. Comparison ofE
ac with the UHF-energy shows thatE
UHF<E
ac in each case revealing some kind of an upper bound nature ofE
ac. An approximate variational argument is given. Relationship of our model with the hyper-Hartree-Fock theory of slater is
explored and the general problem of eliminating ‘self-interaction’ terms in average Fock-operator based theories is discussed. 相似文献