全文获取类型
收费全文 | 2414篇 |
免费 | 82篇 |
国内免费 | 14篇 |
专业分类
化学 | 1742篇 |
晶体学 | 10篇 |
力学 | 40篇 |
数学 | 396篇 |
物理学 | 322篇 |
出版年
2022年 | 14篇 |
2021年 | 26篇 |
2020年 | 54篇 |
2019年 | 37篇 |
2018年 | 31篇 |
2017年 | 21篇 |
2016年 | 72篇 |
2015年 | 60篇 |
2014年 | 64篇 |
2013年 | 99篇 |
2012年 | 96篇 |
2011年 | 100篇 |
2010年 | 90篇 |
2009年 | 76篇 |
2008年 | 100篇 |
2007年 | 102篇 |
2006年 | 119篇 |
2005年 | 103篇 |
2004年 | 91篇 |
2003年 | 80篇 |
2002年 | 94篇 |
2001年 | 45篇 |
2000年 | 49篇 |
1999年 | 52篇 |
1998年 | 46篇 |
1997年 | 54篇 |
1996年 | 55篇 |
1995年 | 53篇 |
1994年 | 49篇 |
1993年 | 38篇 |
1992年 | 37篇 |
1991年 | 36篇 |
1990年 | 26篇 |
1989年 | 41篇 |
1988年 | 22篇 |
1987年 | 15篇 |
1986年 | 24篇 |
1985年 | 25篇 |
1984年 | 28篇 |
1983年 | 32篇 |
1981年 | 20篇 |
1980年 | 24篇 |
1979年 | 30篇 |
1978年 | 26篇 |
1977年 | 20篇 |
1976年 | 20篇 |
1975年 | 15篇 |
1974年 | 13篇 |
1973年 | 18篇 |
1971年 | 17篇 |
排序方式: 共有2510条查询结果,搜索用时 15 毫秒
991.
992.
2,4-Di-t-butyl-cyclopentadiene-1-carbaldehyde () and 2,4-di-t-butyl-cyclopentadienone oxime () are easily converted into the title compounds and , respectively. The spectroscopic and chemical properties of the new heterocyclic 10π-electron systems are described. 相似文献
993.
Schibli R Marti N Maurer P Spingler B Lehaire ML Gramlich V Barnes CL 《Inorganic chemistry》2005,44(3):683-690
This work describes new synthetic routes to produce mixed carbonyl-nitrosyl complexes of technetium(I) and rhenium(I) in aqueous media. NaNO2, NOHSO4, and NO2(g) have been used to produce in situ nitrous acid as the primary source of NO+. Starting from the organometallic precursor fac-[MX3(CO)3]+, 1 (M = 99Tc, Re; X = Cl, Br), the formation of mixed dicarbonyl-mononitrosyl complexes was observed in aqueous hydrochloric and hydrobromic acid. Time-dependent analyses of the reactions by means of HATR-IR and 99Tc NMR spectroscopy in solution revealed the almost quantitative substitution of one CO ligand by NO+ and, thus, the formation of complexes with facial arrangement of the three pi-acceptor ligands. In the case of technetium, the monomeric complex (NEt4)[TcCl3(CO)2NO] (3a) and the dimeric, chloride-bridged, neutral complex [TcCl(mu-Cl)(CO)2NO]2 (4a) were produced. In the case of rhenium, the monomeric species (NEt4)[ReBr2X(CO)2NO] (X = Br (3b), NO3 (5)) was solely isolated. The X-ray structure of complexes 4a and 5 are discussed. The crystallographic analyses revealed the coordination of the NO+ group trans to the terminal chloride (4a) or the bromide (5), respectively. Crystal data: complex 4a (C4Cl4N2O(6)Tc2), monoclinic, Cc, a = 18.82(3) A, b = 6.103(6) A, c = 12.15(2) A, alpha = 90 degrees , beta = 105.8(2) degrees , gamma = 90 degrees , V = 1343(3) A(3), Z = 4; complex 5 (C10H20N3O(6)Br2Re), orthorhombic, P2(1)2(1)2(1), a = 10.2054(5) A, b = 12.5317(7) A, c = 13.9781(7) A, V = 1787.67(16) A(3), Z = 4. The isolated complexes and their potential facial isomers have been further investigated by density functional theory (DFT) calculations. The energy differences of the isomers are relatively small; however, the calculated energies are consistent with the formation of the observed and isolated compounds. The calculated bond lengths and angles of complex 5 are in good agreement with the data determined by X-ray diffraction. Experiments on the no-carrier-added level starting from fac-[99mTc(H2O)3(CO)3]+ revealed the formation of the complex fac-[99mTcCl(H2O)2(CO)2NO]+ in reasonable good yields. This aqueous-based, synthetic approach will enable the future evaluation of this novel, low-valent metal precursor for potential use in radiopharmacy. 相似文献
994.
Thomas Marti Blake R. Peterson Andreas Fürer Tiziana Mordasini-Denti Jürgen Zarske Bernhard Jaun Franois Diederich Volker Gramlich 《Helvetica chimica acta》1998,81(1):109-144
Three double-decker cyclophane receptors, (±)- 2 , (±)- 3 , and (±)- 4 with 11–13-Å deep hydrophobic cavities were prepared and their steroid-binding properties investigated in aqueous and methanolic solutions. Pd°-Catalyzed cross-coupling reactions were key steps in the construction of these novel macrotricyclic structures. In the synthesis of D2-symmetrical (±)- 2 , the double-decker precursor (±)- 7 was obtained in 14% yield by fourfold Stille coupling of equimolar amounts of bis(tributylstannyl)acetylene with dibromocyclophane 5 (Scheme 1). For the preparation of the macrotricyclic precursor (±)- 15 of D2-symmetrical (±)- 3 , diiodocylophane 12 was dialkynylated with Me3SiC?CH to give 13 using the Sonogashira cross-coupling reaction; subsequent alkyne deprotection yielded the diethynylated cyclophane 14 , which was transformed in 42% yield into (±)- 15 by Glaser-Hay macrocyclization (Scheme 2). The synthesis of the C2-symmetrical conical receptor (±)- 4 was achieved via the macrotricyclic precursor (±)- 25 , which was prepared in 20% yield by the Hiyama cross-coupling reaction between the diiodo[6.1.6.1]paracyclophane 19 and the larger, dialkynylated cyclophane 17 (Scheme 4). Solid cholesterol was efficiently dissolved in water through complexation by (±)- 2 and (±)- 3 , and the association constants of the formed 1:1 inclusion complexes were determined by solid-liquid extraction as Ka = 1.1 × 106 and 1.5 × 105 l mol?1, respectively (295 K) (Table 1). The steroid-binding properties of the three receptors were analyzed in detail by 1H-NMR binding titrations in CD3OD. Observed steroid-binding selectivities between the two structurally closely related cylindrical receptors (±)- 2 and (±)- 3 (Table 2) were explained by differences in cavity width and depth, which were revealed by computer modeling (Fig. 4). Receptor (±)- 2 , with two ethynediyl tethers linking the two cyclophanes, possesses a shallower cavity and, therefore, is specific for flatter steroids with a C(5)?C(6) bond, such as cholesterol. In contrast, receptor (±)- 3 , constructed with longer buta-1,3-diynediyl linkers, has a deeper and wider hydrophobic cavity and prefers fully saturated steroids with an aliphatic side chain, such as 5α-cholestane (Fig. 7). In the 1:1 inclusion complexes formed by the conical receptor (±)- 4 (Table 3), testosterone or progesterone penetrate the binding site from the wider cavity side, and their flat A ring becomes incorporated into the narrower [6.1.6.1]paracyclophane moiety. In contrast, cholesterol penetrates (±)- 4 with its hydrophobic side chain from the wider rim of the binding side. This way, the side chain is included into the narrower cavity section shaped by the smaller [6.1.6.1]paracyclophane, While the A ring protrudes with the OH group at C(3) into the solvent on the wider cavity side (Fig. 8). The molecular-recognition studies with the synthetic receptors (±)- 2 , (±)- 3 , and (±)- 4 complement the X-ray investigations on biological steroid complexes in enhancing the understanding of the principles governing selective molecular recognition of steroids. 相似文献
995.
Synthesis and Crystal Structure of the Nonaselenide [Sr(15-crown-5)2]Se9 The title compound was prepared by the reaction of excess selenium with strontium diselenide in DMF in the presence of 15-crown-5. [Sr(15-crown-5)2]Se9 forms black crystals, which are soluble in DMF. They were characterized by FIR spectroscopy and by an X-ray structure determination. Space group P21/n, Z = 4, 2 381 observed unique reflections, R = 0.073. Lattice dimensions at 19°C: a = 1 228.7, b = 1 893.4, c = 1 575.7 pm, β = 99.15°. The compound consists of [Sr(15-crown-5)2]2+ ions in which the strontium ion is coordinated by the oxygen atoms of the crown ether molecules in a sandwich-like fashion, and of Se92? ions with a chain structure, which has a topolocical resemblance with the bicyclic ion Se. 相似文献
996.
Karin Cabrera Martin Jung Christoph Kempter Volker Schurig 《Analytical and bioanalytical chemistry》1995,352(7-8):676-678
Chemically bonded β-cyclodextrin (ChiraDex®) has been packed into 0.25 mm i.d. capillaries and used for the enantiomer separation of several pharmaceuticals by capillary HPLC with UV and electrospray ionization (ESI) MS detection. While having several advantages over conventional HPLC, capillary HPLC yielded comparable results, e.g. with respect to resolution and sensitivity. The good compatibility to ESI-MS was demonstrated. 相似文献
997.
Syntheses and characteristics of the heterobimetalorganics of the silicon with the 2‐(dimethylaminomethyl)ferrocenyl ligand FcN (η5‐C5H5)Fe[η5‐C5H3(CH2NMe2)]— The heterobimetallic lithiumorganyl [2‐(dimethylaminomethyl)ferrocenyl] lithium, LiFcN, reacts with silicon(IV)‐chlorid, SiCl4, under the formation of heterobimetallic silicon(IV) organyl [(FcN)3SiCl] ( 1 ). The heterobimetallic organosilanol [(FcN)3SiOH] ( 2 ) is formed at hydrolysis of 1 . A detailed characterization of the defined compounds 1 and 2 was carried out by NMR‐ rsp. mass‐spectrometry and by crystal X‐ray analysis of 2 . 相似文献
998.
999.
Barbara Winter-Werner Franois Diederich Volker Gramlich 《Helvetica chimica acta》1996,79(5):1338-1360
The Cinchona alkaloid analogs (+)- and (?)- 5 with a quinuclidine-2-methanol residue attached to C(2) of a 9,9′-spirobifluorene moiety were prepared as a racemic mixture by reacting lithiated 2-bromo-9,9′-spirobifluorene 7 with (2-ethoxycarbonyl)quinuclidine (±)- 6 to give ketone (±)- 8 , followed by diastereoselective reduction with diisobutylaluminum hydride (DIBAL-H). The absolute configuration at C(9) and C(8), i.e., at the methanol bridge and the adjacent quinuclidine C-atom, in the two enantiomers of 5 is identical to the configuration at the corresponding centers in (?)-quinine ( 1 ) and (+)-quinidine ( 2 ), respectively. For the optical resolution of (±)- 5 , a chiral stationary phase for HPLC was prepared by covalently bonding quinine via a thiol spacer to a silica-gel surface. The enantiomer separation was accomplished at an α value of 1.61 with (±)- 5 being eluted last, in agreement with 1H-NMR studies in CDCl3 which showed that (+)- 5 underwent a more stable host-guest association with quinine than (?)- 5 . 1H{1H} Nuclear Overhauser effect (NOE) difference spectroscopical analysis of the host-guest associations with quinine in CDCl3, combined with computer-model examinations, allowed the assignment of the absolute configurations as (+)-(8R,9S)- 5 and (?)-(8S,9R)- 5 . A detailed conformational analysis displayed excellent agreement between the results of computational methods (Monte Carlo multiple minimum simulations, analyses of the total energy as a function of the flexible dihedral angles in the molecule) and 1H{1H}-NOE difference spectroscopical data. It was found that (?)- 5 and (+)- 5 differ significantly in their conformational preference from their natural counterparts quinine ( 1 ) and quinidine ( 2 ). Whereas the natural alkaloids prefer the ‘open’ conformation, with the quinuclidine N-atom pointing away from the quinoline ring, analog (±)- 5 adopts preferentially (by ca. 4 kcal mol?1) a ‘closed’ conformation, in which the quinuclidine N-atom points into the cleft of the 9,9′-spirobifluorene moiety. Since the basic quinuclidine N-atom in the ‘closed’ conformation is sterically shielded from forming strong H-bonds, the new Cinchona alkaloid analogs form less stable host-guest associations via H-bonding than quinine or quinidine. 相似文献
1000.
Volker Böhmer Diethard Stotz Klaus Beismann Wilhelm Niemann 《Monatshefte für Chemie / Chemical Monthly》1983,114(4):411-423
The bromination of 15 dinuclear phenolic compounds (dihydroxydiphenylmethanes, methylene bisphenols) by molecular bromine in acetic acid was studied kinetically at 22°C. In all compounds the electrophilic substitution occurred inortho-position to the phenolic hydroxy group of the methyl phenol unit while the non reacting neighboring unit was differently substituted by H, CH3,t-Bu and NO2. A decrease in the reaction rate was observed in 2,2′-dihydroxydiphenylmethanes, where the +M-effect of the hydroxy group is diminished by an intramolecular hydrogen bond. The strength of this hydrogen bond may be influenced mainly by steric factors. Strong electron withdrawing substituents like NO2 show a rate decreasing influence on the reactivity of the neighboring unit also in 2,4′- and 4,4′-dihydroxydiphenylmethanes. 相似文献