全文获取类型
收费全文 | 1982篇 |
免费 | 55篇 |
国内免费 | 12篇 |
专业分类
化学 | 1214篇 |
晶体学 | 8篇 |
力学 | 42篇 |
数学 | 259篇 |
物理学 | 526篇 |
出版年
2022年 | 17篇 |
2021年 | 14篇 |
2020年 | 14篇 |
2019年 | 23篇 |
2018年 | 14篇 |
2016年 | 34篇 |
2015年 | 22篇 |
2014年 | 39篇 |
2013年 | 56篇 |
2012年 | 101篇 |
2011年 | 133篇 |
2010年 | 63篇 |
2009年 | 60篇 |
2008年 | 85篇 |
2007年 | 113篇 |
2006年 | 110篇 |
2005年 | 103篇 |
2004年 | 71篇 |
2003年 | 76篇 |
2002年 | 81篇 |
2001年 | 51篇 |
2000年 | 51篇 |
1999年 | 32篇 |
1998年 | 20篇 |
1997年 | 16篇 |
1996年 | 38篇 |
1995年 | 27篇 |
1994年 | 14篇 |
1993年 | 29篇 |
1992年 | 25篇 |
1991年 | 20篇 |
1990年 | 30篇 |
1989年 | 16篇 |
1988年 | 19篇 |
1987年 | 15篇 |
1986年 | 28篇 |
1985年 | 23篇 |
1984年 | 22篇 |
1983年 | 22篇 |
1982年 | 18篇 |
1981年 | 20篇 |
1980年 | 15篇 |
1979年 | 17篇 |
1978年 | 28篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1975年 | 22篇 |
1974年 | 15篇 |
1973年 | 19篇 |
1969年 | 16篇 |
排序方式: 共有2049条查询结果,搜索用时 15 毫秒
151.
Uffe Anthoni Carsten Christophersen Michael Gajhede Per H. Nielsen 《Structural chemistry》1992,3(2):121-128
The structure of a trimethylamine oxide (TMAO)-urea 14 complex was investigated by x-ray crystallographic methods. C3H9NO, 4CH4N2O, Mr=315.33, monoclinic P21/n, a=11.004(2), b=9.8259, c=15.419(2),=106.88(4), V=1595.4(2) å3, Z=4, Dx=1.313(1) g cm–3, monochromatized Cu (=1.5418 å),=8.94 cm–1, F(OOO)=680, temperature = 110K, final R=0.067, for 1786 unique observed reflections. Based on these studies and comparison with related compounds a model of the interaction of urea and TMAO with proteins, especially enzymes, is proposed. This is important in the physiology of many organisms, especially elasmobranchs. The counteracting effect of TMAO on urea perturbation of protein structure and function is interpreted in terms of water structure as a balance between the structure breaking effect of urea and the structure-stabilizing effect of TMAO. 相似文献
152.
Lance Nielsen 《Acta Appl Math》2002,74(3):265-292
Stability properties of Feynman's operational calculus are addressed in the setting of exponential functions of noncommuting operators. Applications of some of the stability results are presented. In particular, the time-dependent perturbation theory of nonrelativistic quantum mechanics is presented in the setting of the operational calculus and application of the stability results of this paper to the perturbation theory are discussed. 相似文献
153.
Steen Honoré Hansen Per Helboe Mogens Thomsen 《Trends in analytical chemistry : TRAC》1985,4(9):233-237
A new approach to reversed-phase high-performance liquid chromatography has been developed and investigated in some detail. Bare silica is used as the column packing material with an aqueous eluent containing a buffer, an organic solvent as the modifier and a long-chain quaternary ammonium ion. The eluent exhibits a high affinity to the silica surface at pH values above 5, thus, dynamically, forming a nonpolar stationary phase. 相似文献
154.
Le Quement ST Flagstad T Mikkelsen RJ Hansen MR Givskov MC Nielsen TE 《Organic letters》2012,14(2):640-643
An application of readily available hydrazides in the Petasis 3-component coupling reaction is presented. An investigation of the substrate scope was performed to establish a general, synthetically useful protocol for the formation of hydrazido alcohols, which were selectively converted to oxazolidinone and oxadiazolone ring systems through triphosgene-mediated cyclization reactions. 相似文献
155.
Dr. Xin-Ming Hu Magnus H. Rønne Prof. Dr. Steen U. Pedersen Prof. Dr. Troels Skrydstrup Prof. Dr. Kim Daasbjerg 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(23):6568-6572
In a comparative study of the electrocatalytic CO2 reduction, cobalt meso-tetraphenylporphyrin (CoTPP) is used as a model molecular catalyst under both homogeneous and heterogeneous conditions. In the former case, employing N,N-dimethylformamide as solvent, CoTPP performs poorly as an electrocatalyst giving low product selectivity in a slow reaction at a high overpotential. However, upon straightforward immobilization of CoTPP onto carbon nanotubes, a remarkable enhancement of the electrocatalytic abilities is seen with CO2 becoming selectively reduced to CO (>90 %) at a low overpotential in aqueous medium. This effect is ascribed to the particular environment created by the aqueous medium at the catalytic site of the immobilized catalyst that facilitates the adsorption and further reaction of CO2. This work highlights the significance of assessing an immobilized molecular catalyst from more than homogeneous measurements alone. 相似文献
156.
Nielsen M Thomsen AH Cló E Kirpekar F Gothelf KV 《The Journal of organic chemistry》2004,69(7):2240-2250
Rigid linear and tripoidal organic modules based on the oligo(phenylene ethynylene) backbone having salicylaldehyde-derived termini are synthesized. A highly functionalized 5-iodosalicyl aldehyde was prepared and coupled to each ethynyl group of 1,4-diethynylbenzene or 1,3,5-triethynylbenzene in Sonogashira couplings. The two or three termini of the compounds are functionalized for incorporation in linear and branched oligonucleotide strands. For the linear module (LM), the two termini are equipped with amide spacers, and one of these was functionalized with a DMTr (dimethoxytrityl)-protected hydroxy group and the other with a phosphoramidite. One of the tripoidal modules is prepared with DMTr groups in two of its three termini. A tripoidal module is also synthesized with three different groups on its hydroxy termini: a phosphoramidite, a DMTr group, and an Fmoc group. Extended studies have shown that these rigid linear and tripoidal organic modules can be incorporated into short oligonucleotides. Several of these modules can be applied for DNA-directed assembly and covalent coupling into structures of predetermined connectivity. Such structures have potential application for molecular electronics and nanotechnology. 相似文献
157.
Nielsen IB Petersen MA Lammich L Nielsen MB Andersen LH 《The journal of physical chemistry. A》2006,110(46):12592-12596
The neutral retinal Schiff base is connected to opsin in UV sensing pigments and in the blue-shifted meta-II signaling state of the rhodopsin photocycle. We have designed and synthesized two model systems for this neutral chromophore and have measured their gas-phase absorption spectra in the electrostatic storage ring ELISA with a photofragmentation technique. By comparison to the absorption spectrum of the protonated retinal Schiff base in vacuo, we found that the blue shift caused by deprotonation of the Schiff base is more than 200 nm. The absorption properties of the UV absorbing proteins are thus largely determined by the intrinsic properties of the chromophore. The effect of approaching a positive charge to the Schiff base was also studied, as well as the susceptibility of the protonated and unprotonated chromophores to experience spectral shifts in different solvents. 相似文献
158.
Alan Hazell Christine J. McKenzie Jimmi Nielsen 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(7):859-860
The title compound, C9H9NS3, crystallizes with two molecules in the asymmetric unit. In both molecules, the dithiane‐2‐thione rings adopt a symmetric half‐boat conformation with the C atom opposite the C—Sthione bond out of the plane. The pyridine ring is in an equatorial position and is twisted out of the plane of the half‐boat by 82.7 (2) and 84.5 (2)° in the two molecules, so that the N atom is trans to the axial C—H bond in both cases. 相似文献
159.
Petersson B Nielsen BB Rasmussen H Larsen IK Gajhede M Nielsen PE Kastrup JS 《Journal of the American Chemical Society》2005,127(5):1424-1430
The X-ray structure of a partly self-complementary peptide nucleic acid (PNA) decamer (H-GTAGATCACT-l-Lys-NH(2)) to 2.60 A resolution is reported. The structure is mainly controlled by the canonical Watson-Crick base pairs formed by the self-complementary stretch of four bases in the middle of the decamer (G(4)A(5)T(6)C(7)). One right- and one left-handed Watson-Crick duplex are formed. The two PNA units C(9)T(10) change helical handedness, so that each PNA strand contains both a right- and a left-handed section. The changed handedness in C(9)T(10) allows formation of Hoogsteen hydrogen bonding between C(9)T(10) and G(4)A(5) of a PNA strand in an adjacent Watson-Crick double helix of the same handedness. Thereby, a PNA-PNA-PNA triplex is formed. The PNA unit A(3) forms a noncanonical base pair with A(8) in a symmetry-related strand of opposite handedness; the base pair is of the A-A reverse Hoogsteen type. The structural diversity of this PNA demonstrates how the PNA backbone is able to adapt to structures governed by the stacking and hydrogen-bonding interactions between the nucleobases. The crystal structure further shows how PNA oligomers containing limited sequence complementarity may form complex hydrogen-bonding networks. 相似文献
160.
Yasser M. Loksha Per T. Jrgensen Erik B. Pedersen Mahmoud A. El‐Badawi Ahmed A. El‐Barbary Claus Nielsen 《Journal of heterocyclic chemistry》2002,39(2):375-382
5‐Alkyl‐4‐benzyl‐1,3‐dihydroimidazol‐2‐ones ( 3a‐d ) and 5‐alkyl‐4‐benzyl‐1,3‐dihydroimidazole‐2‐thiones (7a‐d) were prepared via Dakin West reaction on DL‐phenylalanine with the appropriate aliphatic acid anhydrides followed by hydrolysis and reaction with potassium cyanate or potassium thiocyanate. Compounds 3a‐d were alkylated with ethoxymethyl chloride to give the alkylated imidazoles 5a‐d which were considered analogues of Emivirine with deletion of carbonyl group at the 4‐position. Alkylation of 7a‐d afforded the corresponding S‐alkylated derivatives 8a‐p which in a similar way were considered analogues of S‐DABO. However all the imidazole derivatives were devoid of activity against HIV. 相似文献