全文获取类型
收费全文 | 2315篇 |
免费 | 42篇 |
国内免费 | 13篇 |
专业分类
化学 | 1361篇 |
晶体学 | 24篇 |
力学 | 57篇 |
数学 | 303篇 |
物理学 | 625篇 |
出版年
2021年 | 16篇 |
2020年 | 18篇 |
2019年 | 16篇 |
2018年 | 17篇 |
2017年 | 13篇 |
2016年 | 32篇 |
2015年 | 24篇 |
2014年 | 33篇 |
2013年 | 104篇 |
2012年 | 124篇 |
2011年 | 129篇 |
2010年 | 58篇 |
2009年 | 41篇 |
2008年 | 118篇 |
2007年 | 107篇 |
2006年 | 85篇 |
2005年 | 94篇 |
2004年 | 89篇 |
2003年 | 71篇 |
2002年 | 66篇 |
2001年 | 41篇 |
2000年 | 46篇 |
1999年 | 28篇 |
1998年 | 20篇 |
1997年 | 23篇 |
1996年 | 38篇 |
1995年 | 33篇 |
1994年 | 32篇 |
1993年 | 37篇 |
1992年 | 39篇 |
1991年 | 20篇 |
1990年 | 29篇 |
1989年 | 32篇 |
1988年 | 37篇 |
1987年 | 24篇 |
1986年 | 16篇 |
1985年 | 43篇 |
1984年 | 42篇 |
1983年 | 28篇 |
1982年 | 39篇 |
1981年 | 28篇 |
1980年 | 36篇 |
1979年 | 30篇 |
1978年 | 39篇 |
1977年 | 31篇 |
1976年 | 34篇 |
1975年 | 20篇 |
1974年 | 29篇 |
1973年 | 30篇 |
1972年 | 15篇 |
排序方式: 共有2370条查询结果,搜索用时 15 毫秒
991.
Pontrello JK Allen MJ Underbakke ES Kiessling LL 《Journal of the American Chemical Society》2005,127(42):14536-14537
We report a general method for the solid-phase synthesis of polymers via the ring-opening metathesis polymerization (ROMP). The method involves polymerization in solution to form a block copolymer, immobilization of the polymer via reaction of one block with a resin-bound functional group, modification of the other block, and liberation of the polymer from the resin. We demonstrated the utility of this approach by generating a block copolymer with an N-hydroxysuccinimidyl ester-substituted block (for on-resin functionalization) and a maleimide-substituted block (for conjugation to the resin). We showed that the Diels-Alder reaction can be employed to immobilize the polymers and that amines of diverse structure can be used to modify the resin-bound polymers. The reversibility of the furan-maleimide Diels-Alder adduct was exploited to liberate the polymer from the support. Specifically, treatment of the resin with cyclopentadiene resulted in complete polymer release. The resulting polymers are functional: they were as potent in assays with the lectin concanavalin A as polymers generated by traditional solution routes. We anticipate that this method can be used for the rapid synthesis of diverse polymers via ROMP. 相似文献
992.
Polyphosphoric acid-catalysed cyclisation of derivatives of crotonophenone and chalcone is shown to be a general reaction which occurs without migration of aryl substituents evident with other Friedel-Crafts catalysts. Two other reactions, however, may intervene: hydrogenation and α-carbonyl cleavage. Thus 4-bromochalcone gives 3 - (p - bromophenyl) -1 - phenylpropan -1 - one, 3 - (p - bromophenyl)indan -1 - one and some benzole acid. A mechanism for the cyclisation is discussed in comparison with those reported for cyclisations of α,β-unsaturated esters and divinyl ketones. The formation of intermediate Wheland complexes is predicted to be conrotatory. 相似文献
993.
Zhu Naijue Harrison Allen Trudell Mark L. Klein-Stevens Cheryl L. 《Structural chemistry》1999,10(2):91-103
A QSAR and CoMFA study including 78 cocaine analogs has been completed. These analogs have varied functional groups on the 2 and 3 positions of the tropane ring and include various stereoisomers. The CoMFA program was used to calculate the steric and electrostatic interaction energies as a probe atom or probe charge interacts with the molecules. Shaded contour maps show regions of the cocaine analogs where an increase in bulky substituents is desirable for increased pharmacological activity. The maps also show that small electronegative substituents on the phenyl ring are favored for enhanced activity. The X-ray crystal structures of (–)-cocaine hydrochloride (1) and N-methyl-3-(p-fluorophenyl)tropane-2-carboxylic acid methyl ester (2) are reported. These molecules are mostly rigid except for some rotational flexibility in the orientation of the phenyl and benzoyl functional groups. Crystallographic data: (1) C17H21NO4·HCl, orthorhombic space group P212121, a = 7.622(1)Å, b = 10.285(1)Å, c = 21.428(3)Å, Z = 4, final R = 0.035 for 960 observed reflections (I>3(I)). (2) C16H20FNO2, monoclinic space group C2, a = 22.572(7)Å, b = 5.810(1)Å, c = 15.752(4)Å, = 133.65(2)°, Z = 4, final R = 0.059 for 1511 observed reflections (I>3(I)). 相似文献
994.
If robust spectroscopic techniques are to be developed for the detection and identification of pathogens, one must understand the relevant spectroscopic properties of the target molecules. In this paper, we employ density functional theory (DFT) to study the structural, electronic, vibrational, optical, and magnetic properties of dipicolinic acid (DPA) and its dianion DPA−2. Our full geometrical optimization and Mulliken charge analysis show that DFT does not lead to the significant discrepancies between charges on symmetric carbon, hydrogen, and oxygen atoms that are found in less accurate calculations based on the complete active space MCSCF method. Our calculated vibrational frequencies, Raman spectra, and infrared spectra for ground-state DPA and DPA−2 are in good agreement with experiment, and this is also true of the four calculated 13C NMR spectral lines (for α, β, γ, and carboxyl sites). Our time-dependent DFT study of the optical excitation and absorption of both DPA and DPA−2 provides the first interpretation of the observed near ultraviolet absorption and fluorescence spectra. Finally, we discuss for the first time the effect of a solvent on the spectral properties of DPA. 相似文献
995.
Wright RJ Phillips AD Allen TL Fink WH Power PP 《Journal of the American Chemical Society》2003,125(7):1694-1695
Reaction of Ar'MMAr' (M = Ga or In) with N3Ar' ' (Ar' = C6H3-2,6-Dipp2, Dipp = C6H3-2,6-Pri2, Ar' ' = C6H3-2,6(Xyl-4-But)2) afforded the first monomeric imides of heavier group 13 elements with two-coordinate metals. Planar, trans-bent structures with short M-N bond distances were observed, which are consistent with lone pair character at both M and N and a bond order less than the formally expected triple one. 相似文献
996.
A novel C-D...N interaction promotes an equilibrium isotope effect that causes a large chemical shift difference (43 ppb, 22 degrees C) for the diastereotopic methyl protons in the (N-CH2D)-(-)-isosparteinium cation. The observed chemical shift difference is in surprising agreement with a prediction of 44 ppb based upon an ab initio protocol. These calculations are also shown to reproduce the experimental shift difference in Anet's alpha-deutero-1,2-dimethylpiperidine (theory, 17 ppb; experiment, 14 ppb). The isosparteine-derived system described herein may provide an improved method for assigning the configuration and enantiomeric purity of stereogenic methyl groups (R-CHDT) by tritium NMR spectroscopy. 相似文献
997.
The kinetics of the solution polymerization of methyl methacrylate in THF, toluene and their mixtures were studied between 200 and 300 K using dilatometry (in the systems where it was valid), gravimetric determination and monitoring monomer and polymer concentrations by NMR spectrometry. The reaction followed zero order kinetics at 200 K, first order kinetics at 275 K and mixed order in between. At both the limits and intermediate range, the reaction followed an integrated rate equation consistent with terminationless propagation proceeding through a complex between monomer and the propagating species. Above 275 K, termination and side reactions were evident and the yields of high mol. wt polymer were small. Density-temperature calibrations for monomer in THF, toluene and mixtures were constructed for the range 190–283 K. However for polymerizations in toluene-rich mixtures, where very high mol. wt polymer forms, the contraction did not correlate linearly with conversion. 相似文献
998.
Burrage S Raynham T Williams G Essex JW Allen C Cardno M Swali V Bradley M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(8):1455-1466
The lantibiotics are a class of highly posttranslationally modified small peptide antibiotics containing numerous lanthionine and dehydroamino acid residues. We have prepared peptides containing multiple dehydroamino acids and cysteine residues in order to probe the biomimetic synthesis of the lantibiotics from their precursor peptides. A novel synthetic methodology was developed to allow the synthesis of multiple dehydroamino acid containing peptides. Cyclisations were rapid, quantitative and regiospecific. Remarkably the peptide sequences alone appear to contain sufficient information to direct a series of stereo- and regiospecific ring closures. Thus both the two linear peptides for the B and E-rings closed stereoselectively. In the case of the A-ring precursor peptide which contained two dehydroamino acids, cyclisation was again totally regioselective, although not totally stereoselective. 相似文献
999.
This paper reports our initial efforts to integrate phenylacetylene-based conjugate pi-electron systems into hybrid semiconductive coordination networks, as part of the larger scheme to fully synergize organic functionalities and electronic properties in crystalline solid-state materials. On the basis of a well-established Pd-catalyzed procedure, ligands of 3,3',4,4'-tetrakis(methylthio)tolan (L1) and 1,3,5-tris[[3,4-bis(methylthio)phenyl]ethynyl]benzene (L2) were efficiently synthesized in relatively simple procedures. Molecule L1 reacts with BiBr3 to form a 2D semiconductive coordination network (L1.2BiBr3), which consists of infinite chains of the BiBr3 component cross-linked by L1 through the chelation between the 1,2-bis(methylthio) groups and the Bi(III) centers. Molecule L2 reacts with BiBr3 to from a 1D semiconductive coordination network (L2.2BiBr3), which features discrete tetrameric Bi4Br12 units linked by the thioether groups from L2 [only two of the three 1,2-bis(methylthio) groups from each L2 molecule are bonded to the Bi(III) centers]. Diffuse reflectance spectra of both L1.2BiBr3 and L2.2BiBr3 feature strong optical absorptions at energy levels significantly lower than those of the corresponding molecular solids (L1 and L2) and BiBr3, indicating significant electronic interaction between the organic pi-electron systems and the BiBr3 components. Both L1.2BiBr3 and L2.2BiBr3 readily form in high yields and are stable to air, providing advantages for further studies as potentially applicable semiconductive materials. 相似文献
1000.