全文获取类型
收费全文 | 496篇 |
免费 | 22篇 |
国内免费 | 12篇 |
专业分类
化学 | 404篇 |
晶体学 | 4篇 |
力学 | 7篇 |
综合类 | 1篇 |
数学 | 8篇 |
物理学 | 106篇 |
出版年
2022年 | 9篇 |
2021年 | 11篇 |
2020年 | 6篇 |
2019年 | 9篇 |
2018年 | 5篇 |
2017年 | 5篇 |
2016年 | 13篇 |
2015年 | 6篇 |
2014年 | 9篇 |
2013年 | 13篇 |
2012年 | 34篇 |
2011年 | 24篇 |
2010年 | 23篇 |
2009年 | 21篇 |
2008年 | 22篇 |
2007年 | 20篇 |
2006年 | 21篇 |
2005年 | 6篇 |
2004年 | 11篇 |
2002年 | 11篇 |
2000年 | 13篇 |
1999年 | 9篇 |
1997年 | 5篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 10篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1975年 | 12篇 |
1974年 | 5篇 |
1973年 | 8篇 |
1972年 | 9篇 |
1971年 | 4篇 |
1970年 | 4篇 |
1969年 | 10篇 |
1968年 | 5篇 |
1966年 | 7篇 |
排序方式: 共有530条查询结果,搜索用时 46 毫秒
101.
Trambitas AG Melcher D Hartenstein L Roesky PW Daniliuc C Jones PG Tamm M 《Inorganic chemistry》2012,51(12):6753-6761
Reaction of anhydrous rare earth metal halides MCl(3) with 2 equiv of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im(Dipp)NH) and 2 equiv of trimethylsilylmethyl lithium (Me(3)SiCH(2)Li) in THF furnished the complexes [(Im(Dipp)N)(2)MCl(THF)(n)] (M = Sc, Y, Lu). The molecular structures of all three compounds were established by single-crystal X-ray diffraction analyses. The coordination spheres around the pentacoordinate metal atoms are best described as trigonal bipyramids. Reaction of YbI(2) with 2 equiv of LiCH(2)SiMe(3) and 2 equiv of the imino ligand Im(Dipp)NH in tetrahydrofuran did not result in a divalent complex, but instead the Yb(III) complex [(Im(Dipp)N)(2)YbI(THF)(2)] was obtained and structurally characterized. Treatment of [(Im(Dipp)N)(2)MCl(THF)(n)] with 1 equiv of LiCH(2)SiMe(3) resulted in the formation of [(Im(Dipp)N)(2)M(CH(2)SiMe(3))(THF)(n)]. The coordination arrangement of these compounds in the solid state at the metal atoms is similar to that found for the starting materials, although the introduction of the neosilyl ligand induces a significantly greater distortion from the ideal trigonal-bipyramidal geometry. [(Im(Dipp)N)(2)Y(CH(2)SiMe(3))(THF)(2)] was used as precatalyst in the intramolecular hydroamination/cyclization reaction of various terminal aminoalkenes and of one aminoalkyne. The complex showed high catalytic activity and selectivity. A comparison with the previously reported dialkyl yttrium complex [(Im(Dipp)N)Y(CH(2)SiMe(3))(2)(THF)(3)] showed no clear tendency in terms of activity. 相似文献
102.
The reaction of the cycloheptatrienylzirconium half-sandwich complex [(η(7)-C(7)H(7))ZrCl(tmeda)] (1) (tmeda = N,N,N',N'-tetramethylethylenediamine) with Li(Im(Dipp)N), generated from bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im(Dipp)NH) with methyllithium, yields the imidazolin-2-iminato complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(tmeda)] (2). The corresponding tmeda-free complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)] (5) can be synthesized via the 1,3-bis(trimethylsilyl)allyl complex [(η(7)-C(7)H(7))Zr{η(3)-C(3)H(3)(TMS)(2)}(THF)] (3; TMS = SiMe(3)), which undergoes an acid-base reaction with Im(Dipp)NH to form 5 and 1,3-bis(trimethylsilyl)propene. 5 exhibits an unusual one-legged piano stool ("pogo stick") geometry with a particularly short Zr-N bond of 1.997(2) ?. Addition of 2,6-dimethylphenyl or tert-butyl isocyanide affords the complexes [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(CNR)] (R = o-Xy, 6; R = t-Bu, 7), while the reaction with 2,6-dimethylphenyl isocyanate results in a [2 + 2] cycloaddition to form the ureato(1-) complex [(η(7)-C(7)H(7))Zr{Im(Dipp)N(C═O)N-o-Xy}] (8). 5 can also act as an initiator for the ring-opening polymerization of ε-caprolactone. These reactivity patterns together with density functional theory calculations reveal a marked similarity of the bonding in imidazolin-2-iminato and conventional imido transition-metal complexes. 相似文献
103.
The temperature dependence of hydrogen overvoltage on mechanically and chemically polished nickel electrodes, on iron armco,
and iron of high purity in 0.25 M H2SO4 are studied in the interval of temperatures 298–278 K. It is established that the influence of temperature on the slope of
Tafel line depends essentially on the type of electrodes. The apparent energy of activationE and pre-exponential factorB are determined. The results show an important role of the electrode surface treatment and purity of the metal in the determination
of energetic parameters of the hydrogen evolution reaction. The most striking result is that in the case of nickel electrodes
with mechanically polished surfaceE increases andB decreases with the increase of overvoltage. Some aspects of nonclassical behavior of electrodes studied are discussed.
Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth. 相似文献
104.
105.
106.
In adaptive optics (AO) system, the phase compensation capability is limited greatly by the actuator number of the deformable mirror (DM). 相似文献
107.
The formerly discribed [1] unknown substance CHT 208, which was obtained as side product when 3β-benzoyloxy-Δ1-cholestene ( 3 ) was treated with ‘neutral’ activated alumina in an unpolar solvent, has been recognized as consisting of ca. 67% 3β-hydroxy-Δ1-cholestene ( 6 ) and of ca. 33% 3β-hydroxy-Δ1-cholestene ( 4 ) (molecular compound 2:1). Analogous treatment of 3β-chloro-Δ1-cholestene ( 5 ) with alumina yielded the same mixture of the allylic alcohols 4 and 6 as substitution products and of Δ1,3-cholestadiene ( 2 ) as elimination product in equal amounts. 相似文献
108.
From cultures of a Phoma species (strain S 298) (Fungi imperfecti) two new antibiotics, phomine ( 1 ) and 5-dehydrophomine ( 4 ) have been isolated. Phomine and 5-dehydrophomine represent a novel type of macrolide antibiotics, the large lactone ring being fused to a highly substituted octahydro-isoindole system. Both show cytostatic activity. 相似文献
109.
Pentafluoroethyl derivatives of [60]fullerene C60(C2F5)n (n = 6, 8, and 10) were synthesized by the reaction of C60 with C2F5I in glass ampoules at 380–440 °C. Isomers of composition C60(C2F5)6 (one isomer), C60(C2F5)8 (five isomers), and C60(C2F5)10 (two isomers) were isolated by chromatographic separation. Their molecular structures were established by X-ray diffraction.
The relative stabilities of isomers were compared by density functional theory calculations.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 881–887, May, 2007. 相似文献
110.
Athanassios Tzikas Christoph Tamm Arthur Boller Andor Fürst 《Helvetica chimica acta》1976,59(5):1850-1866
20, 21-Aziridine Steroids: Reaction of Derivatives of the Oximes of 5-Pregnen-20-one, 9β, 10α-5-Pregnen-20-one and 9β, 10α-5,7-Pregnadiene-20-one with Lithium Aluminium Hydride, and of 3β-Hydroxy-5-pregnen-20-one Oxime with Grignard Reagents. Reduction of 3β-hydroxy-5-pregnen-20-one oxime ( 2 ) with LiAlH4 in tetrahydrofuran yielded 20α-amino-5-pregnen-3β-ol ( 1 ), 20β-amino-5-pregnen-3β-ol ( 3 ), 20β, 21-imino-5-pregnen-3β-ol ( 6 ) and 20β, 21-imino-5-pregnen-3β-ol ( 9 ). The aziridines 6 and 9 were separated via the acetyl derivatives 7 and 10 . The reaction of 6 and 9 with CS2 gave 5-(3β-hydroxy-5-androsten-17β-yl)-thiazolidine-2-thione ( 8 ). Treatment of the 20-oximes 12 and 15 of the corresponding 9β,10α(retro)-pregnane derivatives with LiAlH4 gave the aziridines 13 and 16 , respectively. Their deamination led to the diene 14 and triene 17 , respectively. Reduction of isobutyl methyl ketone-oxime with LiAlH4 in tetrahydrofuran yielded 2-amino-4-methyl-pentane ( 19 ) as main product, 1, 2-imino-4-methyl-pentane ( 22 ) as second product and the epimeric 2,3-imino-4-methyl-pentanes 20 and 21 as minor products. – 3β-Hydroxy-5-pregnen-20-one oxime ( 2 ) was transformed by methylmagnesium iodide in toluene to 20α, 21-imino-20-methyl-5-pregnen-3β-ol ( 23 ) and 20β, 21-imino-20-methyl-5-pregnen-3β-ol ( 26 ). Acetylation of these aziridines was accompanied by elimination reactions leading to 3β-acetoxy-20-methylidene-21-N-acetylamino-5-pregnene ( 30 ) and 3β-acetoxy-20-methyl-21-N-acetylamino-5,17-pregnadiene ( 32 ). The reaction of oxime 2 with ethylmagnesium bromide in toluene gave 20α, 21-imino-20-ethyl-5-pregnen-3β-ol ( 24 ) and 20α,21-imino-20-ethyl-5-pregnen-3β-ol ( 27 ). Acetylation of 24 and 27 led to 3β-acetoxy-20-ethylidene-21-N-acetylamino-5-pregnene ( 31 ), 3β-acetoxy-20-ethyl-21-N-acetylamino-5,17-pregnadiene 33 and 3β, 20-diacetoxy-20-ethyl-21-N-acetylamino-5-pregnene ( 37 ). With phenylmagnesium bromide in toluene the oxime 2 was transformed to 20β, 21-imino-20-phenyl-5-pregnen-3β-ol ( 25 ) and 20β,21-imino-20-phenyl-5-pregnen-3β-ol ( 28 ). Acetylation of 25 and 28 yielded 3β-acetoxy-20-phenyl-21-N-acetylamino-5, 17-pregnadiene ( 34 ) and 3β,20-diacetoxy-20-phenyl-21-N-acetylamino-5-pregnene ( 39 ). LiAlH4-reduction of 39 gave 3β, 20-dihydroxy-20-phenyl-21-N-ethylamino-5-pregnene ( 41 ). – The 20, 21-aziridines are stable to LiAlH4. Consequently they are no intermediates in the formation of the 20-amino derivatives obtained from the oxime 2 . 相似文献