全文获取类型
收费全文 | 2521篇 |
免费 | 64篇 |
国内免费 | 8篇 |
专业分类
化学 | 1685篇 |
晶体学 | 42篇 |
力学 | 86篇 |
数学 | 568篇 |
物理学 | 212篇 |
出版年
2023年 | 10篇 |
2022年 | 34篇 |
2021年 | 62篇 |
2020年 | 41篇 |
2019年 | 45篇 |
2018年 | 30篇 |
2017年 | 21篇 |
2016年 | 62篇 |
2015年 | 66篇 |
2014年 | 56篇 |
2013年 | 119篇 |
2012年 | 109篇 |
2011年 | 154篇 |
2010年 | 94篇 |
2009年 | 82篇 |
2008年 | 131篇 |
2007年 | 112篇 |
2006年 | 139篇 |
2005年 | 136篇 |
2004年 | 113篇 |
2003年 | 91篇 |
2002年 | 82篇 |
2001年 | 35篇 |
2000年 | 19篇 |
1999年 | 27篇 |
1998年 | 27篇 |
1997年 | 20篇 |
1996年 | 48篇 |
1995年 | 28篇 |
1994年 | 31篇 |
1993年 | 26篇 |
1992年 | 21篇 |
1991年 | 21篇 |
1990年 | 21篇 |
1989年 | 12篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 8篇 |
1985年 | 26篇 |
1984年 | 15篇 |
1983年 | 22篇 |
1982年 | 24篇 |
1981年 | 21篇 |
1980年 | 32篇 |
1979年 | 26篇 |
1978年 | 20篇 |
1976年 | 16篇 |
1975年 | 13篇 |
1915年 | 9篇 |
1906年 | 7篇 |
排序方式: 共有2593条查询结果,搜索用时 718 毫秒
51.
Ehresmann JO Wang W Herreros B Luigi DP Venkatraman TN Song W Nicholas JB Haw JF 《Journal of the American Chemical Society》2002,124(36):10868-10874
Assessing the degree of proton transfer from a Br?nsted acid site to one or more adsorbed bases is central to arguments regarding the strength of zeolites and other solid acids. In this regard certain solid-state NMR measurements have been fruitful; for example, some (13)C, (15)N, or (31)P resonances of adsorbed bases are sensitive to protonation, and the (1)H chemical shift of the Br?nsted site itself reflects hydrogen bonding. We modeled theoretically the structures of adsorption complexes of several bases on zeolite HZSM-5, calculated the quadrupole coupling constants (Q(cc)) and asymmetry parameters (eta) for aluminum in these complexes and then in turn simulated the central transitions of their (27)Al MAS NMR spectra. The theoretical line width decreased monotonically with the degree of proton transfer, reflecting structural relaxation around aluminum as the proton was transferred to a base. We verified this experimentally for a series of adsorbed bases by way of single-pulse MAS and triple quantum MQMAS (27)Al NMR. The combined theoretical and experimental approach described here provides a strategy by which (27)Al data can be applied to resolve disputed interpretations of proton transfer based on other evidence. 相似文献
52.
Sergei V. Levchik Giovanni Camino Maria Paola Luda Luigi Costa Bruno Costes Yves Henry Emile Morel George Muller 《先进技术聚合物》1995,6(2):53-62
Tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) undergoes homopolymerization on heating. Intramolecular reactions which compete with crosslinking favor the formation of cyclic structures with increasing thermal and fire resistance of the resin, whereas physical mechanical properties tend to decrease. The mechanism of thermal decomposition of TGDDM is studied by thermogravimetry, differential scanning calorimetry and thermal volatilization analysis with characterization of volatiles evolved and residue left. Thermal degradation of poly-(TGDDM) starts at 260°C with elimination of water from secondary alcoholic groups which is a typical pathway for epoxy resin degradation. Resulting unsaturations weaken bonds in the β-position and provoke the first chain breaking at allyl–amine and allyl–either bonds. With increasing temperature, saturated alkyl–ether bonds and alkyl carbon–carbon bonds are broken first, followed by the most stable alkyl–aryl bonds at T>365°C. The combustion performance of TGDDM is discussed on the basis of the thermal degradation behavior. 相似文献
53.
The 13C hyperfine splitting constants of the tetrachloro-o-benzosemiquinone radical anion are measured for naturally occurring 13C nuclei in all the possible positions in the molecule. Their assignment is based on the linewidth analysis of the single hyperfine components and this is discussed with reference to the reported spin density distribution in o-semiquinones. 相似文献
54.
Cacciapaglia R Casnati A Mandolini L Reinhoudt DN Salvio R Sartori A Ungaro R 《The Journal of organic chemistry》2005,70(2):624-630
The calix[4]arene scaffold, blocked in the cone conformation by proper alkylation of the lower rim hydroxyls, was used as a convenient molecular platform for the design of bi- and trimetallic Zn2+ catalysts. The catalytic activity of the Zn2+ complexes of calix[4]arenes decorated at the 1,2-, 1,3-, and 1,2,3-positions of the upper rim with 2,6-bis[(dimethylamino)methyl]pyridine units were investigated in the cleavage of ester 6 and of the RNA model compound HPNP. High rate enhancements, up to 4 orders of magnitude, were observed in a number of catalyst-substrate combinations. Interestingly the order of catalytic efficiency among regioisomeric dinuclear complexes in the cleavage of ester 6 is 1,2-vicinal > 1,3-distal, but it is reversed in the reaction of HPNP. The higher efficiency of trinuclear compared to dinuclear complexes provides an indication of the cooperation of three Zn2+ ions in the catalytic mechanism. 相似文献
55.
Luigi Fabbrizzi Piero Paoletti Markus C. Zobrist Gerold Schwarzenbach 《Helvetica chimica acta》1973,56(2):670-680
The thermodynamic functions of the proton transfer H2tn2++tn → 2 Htn+ (tn = 1,3-diaminopropane) have been determined in aqueous solutions containing different amounts of KCl (0.05 ? μ ? 3.01). The free energy (?ΔG) of the process decreases, whereas the enthalpy (-ΔH) increases with μ. There is reason to believe that the reaction is entirely controlled by the Coulomb forces between the two protonic charges. The electrostatic energy involved can be described in terms of a model incorporating an effective dielectric constant εe, such that δεe/δμ and δ2εe/δμδT are both positive. The polarisation of pure water is produced by orientation of hydrogen-bonded dipole molecules H2O, whereas the electrolyte solution is polarised in addition by dislocation of the ions K+ and Cl?. Our results demonstrate that the former type of polarisation is much more temperature dependent than the latter. 相似文献
56.
Favero G Campanella L Cavallo S D'Annibale A Perrella M Mattei E Ferri T 《Journal of the American Chemical Society》2005,127(22):8103-8111
The realization of a reliable receptor biosensor requires stable, long-lasting, reconstituted biomembranes able to supply a suitable biomimetic environment where the receptor can properly work after incorporation. To this end, we developed a new method for preparing stable biological membranes that couple the biomimetic properties of BLMs (bilayer lipid membranes) with the high stability of HBMs (hybrid bilayer membranes); this gives rise to an innovative assembly, named MHBLM (mixed hybrid bilayer lipid membrane). The present work deals with the characterization of biosensors achieved by embedding an ionotropic glutamate receptor (GluR) on MHBLM. Thanks to signal (transmembrane current) amplification, which is typical of natural receptors, the biosensor here produced detects glutamate at a level of nmol L(-1). The transmembrane current changes linearly vs glutamate up to 100 nmol L(-1), while the limit of detection is 1 nmol L(-1). In addition, the biosensor response can be modulated both by receptor agonists (glycine) and antagonists (Mg(2+)) as well, and by exploiting the biosensor response, the distribution of different kinds of ionotropic GluR present in the purified sample, and embedded in MHBLM, was also evaluated. Finally, one of the most important aspects of this investigation is represented by the high stability of the biomimetic system, which allows the use of biosensor under flowing conditions, where the solutions flow on both biomembrane faces. 相似文献
57.
Luigi R. Nassimbeni Sevi Papanicolaou Madeleine H. Moore 《Journal of inclusion phenomena and macrocyclic chemistry》1986,4(1):31-42
The structures of Ni(NCS)2(4-PhPy)4·4DMSO and Ni(NCS)2(3-MePy)4·CHCl3 have been elucidated. Movement of guest molecules through channels in the host structure was simulated by potential energy calculations. Ni(NCS)2(3,5-diMePy)4 does not form inclusion compounds. An intra-molecular potential energy study shows that theortho-hydrogens on the 3,5-dimethylpyridine ligands control the conformation of the molecule. The same result is obtained with the 4-methylpyridine ligand, which suggests that the extent of rotation of substituted pyridines about the Ni–N bounds is not a factor governing clathrate formation.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82030 (43 pages). 相似文献
58.
An enzyme inhibition biosensor, developed in our laboratory and previously used for the analysis of compounds with anticholinesterase activity (e.g. physostigmine, neostigmine, pyridostigmine nicotine and organophosphorus compounds) has now been tested for the analysis of another recently synthesized cholinesterase inhibitor, i.e. eptastigmine. In addition nicotinic acid and nicotinamide, although displaying weaker inhibition properties, were also tested in pharmaceutical products using the same inhibition enzyme sensor. The biosensor consisted of a hydrogen peroxide amperometric electrode coupled to a functionalised nylon membrane chemically bonding both the enzymes butyrylcholinesterase and choline oxidase; a butyrylcholine standard solution in glycine buffer acted as substrate. The response of the system to all the inhibitors considered was characterised completely and the analysis of several pharmaceutical formulations containing nicotinamide or nicotinic acid was also performed. 相似文献
59.
Pier Luigi Ferrarini Claudio Mori Muwaffag Badawneh Clementina Manera Adriano Martinelli Federico Romagnoli Giuseppe Saccomanni Mauro Miceli 《Journal of heterocyclic chemistry》1997,34(5):1501-1510
Several 1,8-naphthyridine derivatives have been diazotizated to obtain the corresponding hydroxy derivatives or mixture of hydroxy and hydroxy nitro derivatives. The respective amounts of hydroxy and hydroxy nitro derivatives depends on the nature of the substituents, on their position on the naphthyridine nucleus, on the amount of sodium nitrite and on the reaction temperature. A study of the electronic density of some molecules suggests a possible explanation of the effects induced by the nature of the substituents and of their position. Some of the compounds were tested for their ability to inhibit human platelet aggregation in vitro induced by arachidonic acid. Only compound 26 showed interesting antiplatelet activity. 相似文献
60.
Raffaella Bergonzi Luigi Fabbrizzi Maurizio Licchelli Carlo Mangano 《Coordination chemistry reviews》1998,170(1):31-46
Three-component molecular systems (redox active subunit)-spacer-(light-emitting fragment) can operate as fluorescence switches, following the alternate addition of an oxidizing agent and a reducing agent (or the adjustment of the potential of the working electrode in an electrolysis experiment). The redox active subunit typically consists of a metal centred redox couple (M(n+1)+/Mn+), encircled by a macrocyclic receptor, and switching efficiency requires that one of the two oxidation states quenches the proximate fluorophore and the other does not. Four ON/OFF systems, based on either the CuII/CuI or NiIII/NiII couple, will be discussed. The nature of the quenching process responsible for the OFF state, either electron transfer or energy transfer, is related to the length and to the flexibility-rigidity of the spacer. 相似文献