全文获取类型
收费全文 | 2775篇 |
免费 | 147篇 |
国内免费 | 10篇 |
专业分类
化学 | 2331篇 |
晶体学 | 12篇 |
力学 | 25篇 |
数学 | 289篇 |
物理学 | 275篇 |
出版年
2023年 | 21篇 |
2022年 | 36篇 |
2021年 | 35篇 |
2020年 | 51篇 |
2019年 | 39篇 |
2018年 | 28篇 |
2017年 | 22篇 |
2016年 | 101篇 |
2015年 | 103篇 |
2014年 | 90篇 |
2013年 | 141篇 |
2012年 | 207篇 |
2011年 | 235篇 |
2010年 | 140篇 |
2009年 | 86篇 |
2008年 | 185篇 |
2007年 | 173篇 |
2006年 | 158篇 |
2005年 | 166篇 |
2004年 | 166篇 |
2003年 | 127篇 |
2002年 | 115篇 |
2001年 | 69篇 |
2000年 | 37篇 |
1999年 | 35篇 |
1998年 | 27篇 |
1997年 | 32篇 |
1996年 | 29篇 |
1995年 | 27篇 |
1994年 | 17篇 |
1993年 | 18篇 |
1992年 | 21篇 |
1991年 | 13篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1986年 | 7篇 |
1985年 | 14篇 |
1984年 | 18篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 13篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 14篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1969年 | 5篇 |
排序方式: 共有2932条查询结果,搜索用时 15 毫秒
71.
Monomeres [(CH3)3Si]2NBeN[Si(CH3)3]2 (I) entsteht in 68% Ausb. bei der Reaktion von BeCl2-Ätherat mit NaN[Si(CH3)3]2. Eigenschaften, IR-, Raman- und1H-NMR-Spektren sprechen für einen NBeN-Winkel von 180°. Die SiN- und BeN-Valenzkraftkonstanten werden zu 3,40 bzw. 2,96 mdyn/Å errechnet.Monomeric I has been prepared (yield 68%) by the reaction of BeCl2 with NaN[Si(CH3)3]2 in ether. Properties, infrared, raman and1H-NMR-spectra support a NBeN angle of 180°. SiN and BeN force constants were calculated to be 3,40 and 2,96 mdyn/Å, respectively.
9. Mitt. über silylsubstituierte Metallamide; 8. Mitt.:H. Bürger, W. Sawodny undU. Wannagat, J. Organometall. Chem.2, 113, (1965). 相似文献
9. Mitt. über silylsubstituierte Metallamide; 8. Mitt.:H. Bürger, W. Sawodny undU. Wannagat, J. Organometall. Chem.2, 113, (1965). 相似文献
72.
Thierry Boulanger Christine Evrard Daniel P. Vercauteren Guy Evrard François Durant 《Journal of chemical crystallography》1991,21(3):287-295
The crystal structures ofp-methoxyphenyl-3-triazolo [4,3-a] isoquinoline (SR95926),p-methoxyphenyl-3-triazolophtalazine (CMW1842), andp-methoxyphenyl-3-N-dimethoxyethylamino-6-triazolophtalazine (L16317) have been solved by direct methods from single-crystal X-ray diffraction data, and refined by full-matrix least squares. SR95926: monoclinic,P21/n,a=20.950(3),b=6.769(1),c=9.465(2) Å,=100.90(1)°. CMW1842: triclinic,P¯1,a=8.784(1),b=9.160(4),c=8.555(1) Å,=99.10(2),=93.90(1), =106.77(1)°. L16317: monoclinic,P21/n,a=20.124(3),b=9.586(1),c=10.788(1) Å,=91.91(1)°. FinalR factors are 0.034, 0.037, and 0.053, respectively. Experimental geometries were used to perform STO-3Gab initio molecular-orbital calculations. A relationship between the electronic pattern within the molecules and the affinity of the benzodiazepine receptor sites is pointed out. 相似文献
73.
Thamyongkit P Speckbacher M Diers JR Kee HL Kirmaier C Holten D Bocian DF Lindsey JS 《The Journal of organic chemistry》2004,69(11):3700-3710
The incorporation of symmetrically branched tridecyl ("swallowtail") substituents at the meso positions of porphyrins results in highly soluble building blocks. Synthetic routes have been investigated to obtain porphyrin building blocks bearing 1-4 swallowtail groups. Porphyrin dyads have been synthesized in which the zinc or free base (Fb) porphyrins are joined by a 4,4'-diphenylethyne linker and bear swallowtail (or n-pentyl) groups at the nonlinking meso positions. The swallowtail-substituted Zn(2)- and ZnFb-dyads are readily soluble in common organic solvents. Static absorption and fluorescence spectra and electrochemical data show that the presence of the swallowtail groups slightly raises the energy level of the filled a(2u)(pi) HOMO. EPR studies of the pi-cation radicals of the swallowtail porphyrins indicate that the torsional angle between the proton on the alkyl carbon and p-orbital on the meso carbon of the porphyrin is different from that of a porphyrin bearing linear pentyl groups. Regardless, the swallowtail substituents do not significantly affect the photophysical properties of the porphyrins or the electronic interactions between the porphyrins in the dyads. In particular, time-resolved spectroscopic studies indicate that facile excited-state energy transfer occurs in the ZnFb dyad, and EPR studies of the monocation radical of the Zn(2)-dyad show that interporphyrin ground-state hole transfer is rapid. 相似文献
74.
Todd B. Vinzant Christine I. Ehrman William S. Adney Steven R. Thomas Michael E. Himmel 《Applied biochemistry and biotechnology》1997,62(1):99-104
A series of correlations was made between the performance of 15 wood species in simultaneous saccharification and fermentation (SSF) and their respective chemical compositions. A compelling inverse trend (p < 0.001) was demonstrated between the percent conversion of glucan to ethanol during SSF and the Klason lignin content of the wood samples before dilute acid pretreatment. No significant relationships were found between the glucan, xylan, and ash compositions of the native wood samples and ethanol yield. This observation is unique and provides a convenient predictor of biomass conversion efficiency. 相似文献
75.
The divalent lanthanide bis((cyclooctane-1,5-diyl)dihydroborate) complexes {K(THF)4}2{Ln{(mu-H)2BC8H14}4} (Ln = Eu, 3; Yb, 4) were prepared by a metathesis reaction between (THF)(x)LnCl2 and K[H2BC8H14] in THF in a 1:4 molar ratio. Although the reaction ratios were varied between 1:3 and 1:6, complexes 3 and 4 were the only lanthanide 9-BBN hydroborates produced. Because of disorder of THF in crystals of 3 and 4, good single-crystal X-ray structural data could not be obtained. However, when the potassium cation was replaced by the tetramethylammonium cation or when MeTHF (2-methyltetrahydrofuran) was employed in place of THF, good quality crystals were obtained. Complexes [NMe4]2[Ln{(mu-H)2BC8H14}4] (Ln = Eu, 5; Yb, 6) were afforded by metathesis reactions of NMe4Cl with 3 and 4 in situ. On the basis of the single-crystal X-ray diffraction analysis, the four 9-BBN tetrahydroborate ligands are tetrahedrally arranged around the lanthanide cation in 5 and 6. The two structures differ in that one alpha-C-H bond from each of the four {(mu-H)2BC8H14}4 units exhibits an agostic interaction with Eu(II) in 5 but, in complex 6, only two of the alpha-C-H bonds form agostic interactions with Yb(II). Complexes {K(MeTHF)3}2{Ln{(mu-H)2BC8H14}4} (Ln = Eu, 7; Yb, 8) were produced by employing MeTHF in place of THF. The structures of 7 and 8 display connectivity between the anion {Ln{(mu-H)2BC8H14}4}2- and the cation {K(MeTHF)3}+, in which the potassium not only interacts directly with the hydrogens of the Ln-H-B bridged bonds but is also involved in agostic interactions with alpha-C-H bonds. By systematically examining the structures of complexes 3-8 and taking into account the previously reported complexes (THF)4Ln{(mu-H)2BC8H14}2 (Ln = Eu, 1; Yb, 2), it is concluded that Eu(II) appears to have a better ability to form agostic interactions than Yb(II) because of its larger size, even though Yb(II) has a higher positive charge density. 相似文献
76.
Dickerson CA Fisher MJ Sykora RE Albrecht-Schmitt TE Cody JA 《Inorganic chemistry》2002,41(4):640-642
The compound K(3)PSe(4).2Se(6) was synthesized at 110 degrees C via solventothermal techniques from binary starting materials and Se in acetonitrile. The compound crystallizes in the space group Fd macro 3 of the cubic system with eight formula units in a cell with a dimension of a = 16.415(2) A at T = 193 K. The structure contains an unusual intermixing of ionic and uncharged species. The selenophosphate tetrahedral trianions PSe(4)(3-) are surrounded by potassium cations; other potassium cations in the structure are coordinated to 12 selenium atoms from four Se(6) rings in a tetrahedral arrangement. There are no short contacts between adjacent selenium rings. Heating the same reaction mixture to 160 degrees C results in the formation of only needles of trigonal selenium. 相似文献
77.
Nicole Frankenberg Christine Hager-Braun Ute Feiler Markus Fuhrmann Hans Rogl Nikolaus Schneebauer Nathan Nelson Günter Hauska 《Photochemistry and photobiology》1996,64(1):14-19
Membranes of Chlorobium tepidum contain about 35, 45 and2–10 molecules of menaquinone-7, chlorobium quinone (1′-oxo-menaquinone-7) and of the polar menaquinone (probably 1′-OH-menaquinone-7) per reaction center, respectively. None of these quinones was retained during the isolation of P840-reaction centers beyond the detection limit of about 0.2 quinones per reaction center, neither in the core complex nor in functionally intact reaction center preparations. The latter is shown to catalyze the formation of an electrochemical proton gradient in the presence of ascorbate and phenazinium methosulfate, when it is incorporated into lipid vesicles. 相似文献
78.
Tetrahydropyran derivatives
–
and
–
are formed in good yields by cyclisation of methyl-6-hydroxy-2-hexenoate
or 2-heptenoate
mediated by various electrophilic reagents (mCPBA, benzeneselenyl chloride, N-bromosuccinimide, iodine). Cyclisations of Z and E isomers are stereospecific. The diastereoselectivity of cyclisation of the secondary alcohol
varies with the nature of the electrophilic reagent. 相似文献
79.
Ruth-Maria Olk W. Dietzsch J. Mattusch J. Stach Christine Nieke E. Hoyer W. Meiler W. Robien 《无机化学与普通化学杂志》1987,544(1):199-208
Electrochemical Reduction of CSSe and CSe2 in Dimethylformamide: Heterocyclic 1,2-Dichalcogenolates and their Coordination Chemistry Starting from carbon diselenide or carbon selenidesulfide the electrochemical preparation (electrosynthesis) of heterocyclic dichalcogenolates C3X52? (X = Se: dsis; X = S/Se: C3SxSey2?) is outlined. The 1,2-dichalcogenolate compounds were isolated and characterized as dibenzoyl derivatives. Bis- or tris-chelates of general type Am[M(C3X5)n] (with A = Bu4N+, Ph4As+; M = ZnII, PtII, PdII, NiIII, CuIII, AuIII, InIII; X = Se, S/Se; m = 1, 2, 3; n = 2, 3, respectively) are available directly from methanolic solutions of the dibenzoylates after hydrolytic cleavage of the latter with sodium methanolate. In addition bis-chelates Bu4N[Ni(C3X5)2] (X = Se, S/Se) have been characterized by cyclovoltammetry and epr spectroscopy and compared with the corresponding all-sulfur ligand compound Bu4N[Ni(dmit)2] (X = S). Arguments are given for the fact that the allselenium ligand dsis (X = Se) yields the CuIII or NiIII chelate at once whereas with dmit using identical conditions the metal(II) compounds are formed. 相似文献
80.
Carl F. Brunius Ulrica Edlund Ann‐Christine Albertsson 《Journal of polymer science. Part A, Polymer chemistry》2002,40(21):3652-3661
This work is devoted to the design of a novel family of hydrosoluble biomaterials: poly(N‐vinyl‐2‐pyrrolidone) (PVP)‐based graft copolymers. A synthesis route has been elaborated in which ω‐functionalized PVP is prepared via chain‐transfer radical polymerization, end‐group modified, and subsequently grafted onto a polyhydroxylated backbone, typically dextran or poly(vinyl alcohol). The resulting graft copolymer biomaterials are designed for use in various biomedical applications, particularly as materials with a stronger potential for plasma expansion than already existing products have. The graft copolymers are potentially degradable because the PVP grafts are connected to the polyol backbone via a hydrolytically labile carbonate or ester linkage. The degradation of the graft copolymers was performed in vitro over a period of 6 weeks. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3652–3661, 2002 相似文献