全文获取类型
收费全文 | 686篇 |
免费 | 38篇 |
国内免费 | 2篇 |
专业分类
化学 | 591篇 |
晶体学 | 2篇 |
力学 | 7篇 |
数学 | 74篇 |
物理学 | 52篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 40篇 |
2021年 | 45篇 |
2020年 | 14篇 |
2019年 | 15篇 |
2018年 | 13篇 |
2017年 | 7篇 |
2016年 | 31篇 |
2015年 | 23篇 |
2014年 | 22篇 |
2013年 | 35篇 |
2012年 | 44篇 |
2011年 | 43篇 |
2010年 | 27篇 |
2009年 | 31篇 |
2008年 | 39篇 |
2007年 | 49篇 |
2006年 | 39篇 |
2005年 | 39篇 |
2004年 | 29篇 |
2003年 | 34篇 |
2002年 | 24篇 |
2001年 | 8篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 8篇 |
1997年 | 3篇 |
1996年 | 14篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有726条查询结果,搜索用时 31 毫秒
291.
The effect of irradiation, in the wavelength range of 310-800 nm, on aqueous solutions (pH = 7.4) of alginate in the presence of the photosensitizer riboflavin (RF) has been investigated with the aid of dynamic light scattering (DLS). Under aerobic conditions light irradiation of RF causes scission of the polymer chains which affects the polymer dynamics. The time correlation data obtained from DLS experiments showed at all conditions the existence of two relaxation modes: one single exponential at short times, followed by a stretched exponential at longer times. The slow relaxation time revealed, over the whole considered concentration range, lower values for the alginate/RF system, whereas no effect of photochemical degradation was observed for the fast relaxation time in the semidilute regime. The results suggest that the photochemically induced fragmentation of alginate affects the slow relaxation mode, associated with disengagement relaxation of individual chains or cluster relaxation, in a similar way as the zero-shear viscosity. These findings provide detailed insight into the dynamics of the polymer matrix, and this knowledge can be useful in the context of controlled-release delivery of drugs. The chemical units of alginate (M = mannuronic acid and G = guluronic acid). 相似文献
292.
293.
Orsatti L Pallaoro M Steinküler C Orru' S Bonelli F 《Rapid communications in mass spectrometry : RCM》2002,16(20):1919-1927
The present work reports a mass spectrometric investigation of the NS2/3 protein, a protease from hepatitis C virus (HCV). During routine protein manipulation, in the presence of 100 mM beta-mercaptoethanol and under denatured conditions, the protein was unexpectedly modified at its cysteine residues, and the increased molecular weight corresponded to one molecule of beta-mercaptoethanol bound. The modified protein, once refolded, was found to be less active than the unmodified one. The aim of this work was to investigate whether the reactivity of cysteines with beta-mercaptoethanol involves one specific, highly reactive residue of the sequence, or if the modification is a random process. Liquid chromatography (LC) coupled on-line with an electrospray ion trap mass spectrometer was used to identify the modification sites. It was found that five cysteines out of nine had reacted with beta-mercaptoethanol, none of them showing a significantly higher reactivity than the others. 95% of sequence coverage was obtained. 相似文献
294.
Giorgi G Salvini L Attanasi OA Filippone P Santeusanio S 《Journal of mass spectrometry : JMS》2002,37(7):709-715
Electron ionization mass spectrometry was used for the structural characterization of substituted 2-thiazolin-4-one derivatives in the gas phase. The compounds follow common fragmentation pathways, producing ions whose abundances are dependent on the chemical nature of the substituent at position 2. Collision-induced dissociation tandem mass spectrometric experiments, carried out on both molecular ions and fragment ions produced in the source, allowed the elucidation of gas-phase decompositions. The presence of tautomeric forms is suggested for some ionic species. Rapid identification of a primary or secondary amine moiety at position 2 of the thiazoline ring can be achieved by the detection of characteristic fragmentations occurring both in the ion source and under the collision-induced dissociation regime. 相似文献
295.
Stefania De LucaSaraj Ulhaq Mark J. DixonJonathan Essex Mark Bradley 《Tetrahedron letters》2003,44(15):3195-3197
A focused library of inhibitors of the enzyme trypanothione reductase was prepared using solid-phase synthesis. The inhibitors were based on a previously identified, non-competitive, lead compound comprising of two Pmc (2,2,5,7,8-pentamethylchroman-6-sulfonyl) side-chain protected, N-capped arginine residues linked by a spermidine bridge. In total six protecting groups and four capping groups were used to generate a 24-membered library. All compounds bearing the 5-methoxyindole-3-acetic acid capping group were found to have good activity. The most potent inhibitor was observed to contain the Mtr (4-methoxy-2,3,6-trimethylbenzenesulphonyl) protecting group on the arginine residue, terminated with tryptophan as the capping group. 相似文献
296.
Tessore F Locatelli D Righetto S Roberto D Ugo R Mussini P 《Inorganic chemistry》2005,44(7):2437-2442
To evidentiate the role of the nature of sulfonate ancillary ligands on the value of the quadratic hyperpolarizability of Zn(II) complexes with stilbazole-like ligands, the second-order nonlinear optical (NLO) properties of [ZnY(2)(4,4'-trans-NC5H4CH=CHC6H4NMe2)2] complexes (Y = CF3SO3, CH3SO3, or p-CH3C6H4SO3) are investigated. By working at relatively high concentrations (>3 x 10(-4) M), the positive effect of the triflate ligand remains unique while, with nonfluorinated sulfonate ligands, the second-order NLO response is comparable to that of the related complexes with acetate or trifluoroacetate as ancillary ligands. However, at dilutions higher than 10(-4) M, all of the sulfonate complexes reach huge quadratic hyperpolarizabilities because of solvolysis with the formation of cationic species such as [ZnY(4,4'-trans-NC5H4CH=CHC6H4NMe2)2]+, characterized by a large second-order NLO response. This view is supported by careful conductivity measurements. The same behavior occurs if 4,4'-trans-NC5H4CH=CHC6H4NMe2 is substituted by 4,4'-trans,trans-NC5H4(CH=CH)2C6H4NMe2. 相似文献
297.
Huub Kooijman Stefania Tanase Elisabeth Bouwman Jan Reedijk Anthony L. Spek 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(11):m510-m512
The title compound, [Fe(C12H8N3O2)2]ClO4·2C2H3N, contains FeIII in a distorted octahedral coordination environment, with the Fe—N(pyridine) bonds significantly longer than the Fe—N(amine) bonds. The crystal packing involves a bifurcated C—H⋯(O,O) contact that is also found in all other [M(C12H8N3O2)2] complexes reported previously. 相似文献
298.
Duati M Tasca S Lynch FC Bohlen H Vos JG Stagni S Ward MD 《Inorganic chemistry》2003,42(25):8377-8384
The synthesis and characterization of mixed ligand 2,2';6',2' '-terpyridine (tpy) ruthenium complexes with 2,6-bis([1,2,4]triazol-3-yl)pyridine, 2,6-bis(5-phenyl-[1,2,4]triazol-3-yl)pyridine, and 2,6-bis([1,2,3,4]tetrazol-5-yl)pyridine are reported. The species are characterized by HPLC, 1H NMR, UV/vis, and emission spectroscopy. The photophysical properties of the complexes are investigated as a function of temperature over the range 80-320 K. The emission lifetime observed for the fully deprotonated compounds at room temperature is about 80 ns. This increase by 2 orders of magnitude with respect to the parent "[Ru(tpy)2](2+)" complex is rationalized by an increase in the energy of the metal based dsigma orbital, rather than by manipulation of the pi* orbitals on the ligands. The acid-base and electrochemical properties of the compounds are reported also. 相似文献
299.
Alexandru C. Razus Liviu Birzan Stefania Nae Mirela Nina Surugiu Valentin Cimpeanu 《Journal of heterocyclic chemistry》2003,40(6):995-1004
Several derivatives belonging to a new compound class, namely azulene‐1‐azo‐2′‐thiazoles, were prepared by the diazotization of 2‐aminothiazoles in the presence of HNO3/H3PO4 followed by the coupling of diazonium salts with azulenes in buffered medium. The reactions proved to be general for this class, the yields are, however, considerably influenced by the substituents at thiazole moiety. For the first time a N‐oxide provided from an amino substituted five‐member nitrogenous heterocycle was diazotized and coupled. The structure of the obtained compounds was assigned and their physico‐chemical properties were discussed. The new azulene azo derivatives exhibit a strong bathochromic shift in UV‐Vis due to the intense push‐pull effect of aromatic system and to the intrinsic properties of thiazole moiety. 相似文献
300.
Our aim in this paper is to study, in term of finite dimensional exponential attractors, the Willmore regularization, (depending on a small regularization parameter β > 0), of two phase‐field equations, namely, the Allen–Cahn and the Cahn–Hilliard equations. In both cases, we construct robust families of exponential attractors, that is, attractors that are continuous with respect to the perturbation parameter. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献