全文获取类型
收费全文 | 440篇 |
免费 | 24篇 |
专业分类
化学 | 358篇 |
晶体学 | 2篇 |
力学 | 1篇 |
数学 | 50篇 |
物理学 | 53篇 |
出版年
2023年 | 4篇 |
2022年 | 7篇 |
2021年 | 7篇 |
2020年 | 10篇 |
2019年 | 9篇 |
2018年 | 8篇 |
2017年 | 9篇 |
2016年 | 16篇 |
2015年 | 18篇 |
2014年 | 13篇 |
2013年 | 18篇 |
2012年 | 24篇 |
2011年 | 21篇 |
2010年 | 10篇 |
2009年 | 5篇 |
2008年 | 15篇 |
2007年 | 13篇 |
2006年 | 14篇 |
2005年 | 13篇 |
2004年 | 11篇 |
2003年 | 3篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1998年 | 3篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1972年 | 7篇 |
1970年 | 2篇 |
1960年 | 23篇 |
1925年 | 2篇 |
排序方式: 共有464条查询结果,搜索用时 390 毫秒
91.
The synthesis and anion recognition properties of four novel [2]rotaxane host architectures containing additional alkyl functionality integrated within macrocyclic and axle components to shield the binding cavity from the solvent are described. The rotaxane species containing a tetra(methyl)-functionalised macrocycle component is found to be a weaker anion complexant than the equivalent unfunctionalised receptor, which is likely due to steric hindrance restricting the anion's access to the interlocked cavity. Rotaxane molecules containing tetra(methyl)-functionalised axle components are also investigated, and the additional alkyl functionality serves to enhance anion binding affinity and selectivity when incorporated within the axle's flexible ethylene linkages. Moreover, the equivalent unfunctionalised rotaxane displays a rare preference for oxoanions over chloride guest species. 相似文献
92.
Alonso E Fuwa H Vale C Suga Y Goto T Konno Y Sasaki M LaFerla FM Vieytes MR Giménez-Llort L Botana LM 《Journal of the American Chemical Society》2012,134(17):7467-7479
Gambierol is a potent neurotoxin that belongs to the family of marine polycyclic ether natural products and primarily targets voltage-gated potassium channels (K(v) channels) in excitable membranes. Previous work in the chemistry of marine polycyclic ethers has suggested the critical importance of the full length of polycyclic ether skeleton for potent biological activity. Although we have previously investigated structure-activity relationships (SARs) of the peripheral functionalities of gambierol, it remained unclear whether the whole polycyclic ether skeleton is needed for its cellular activity. In this work, we designed and synthesized two truncated skeletal analogues of gambierol comprising the EFGH- and BCDEFGH-rings of the parent compound, both of which surprisingly showed similar potency to gambierol on voltage-gated potassium channels (K(v)) inhibition. Moreover, we examined the effect of these compounds in an in vitro model of Alzheimer's disease (AD) obtained from triple transgenic (3xTg-AD) mice, which expresses amyloid beta (Aβ) accumulation and tau hyperphosphorylation. In vitro preincubation of the cells with the compounds resulted in significant inhibition of K(+) currents, a reduction in the extra- and intracellular levels of Aβ, and a decrease in the levels of hyperphosphorylated tau. In addition, pretreatment with these compounds reduced the steady-state level of the N-methyl-D-aspartate (NMDA) receptor subunit 2A without affecting the 2B subunit. The involvement of glutamate receptors was further suggested by the blockage of the effect of gambierol on tau hyperphosphorylation by glutamate receptor antagonists. The present study constitutes the first discovery of skeletally simplified, designed polycyclic ethers with potent cellular activity and demonstrates the utility of gambierol and its synthetic analogues as chemical probes for understanding the function of K(v) channels as well as the molecular mechanism of Aβ metabolism modulated by NMDA receptors. 相似文献
93.
The atmospherically relevant reactions between dimethyl selenide (DMSe) and the molecular halogens (X(2) = Cl(2), Br(2), and I(2)) have been studied with ab initio calculations at the MP2/aug-cc-pVDZ level of theory. Geometry optimization calculations showed that the reactions proceed from the reagents to the products (CH(3)SeCH(2)X + HX) via three minima, a van der Waals adduct (DMSe:X(2)), a covalently bound intermediate (DMSeX(2)), and a product-like complex (CH(3)SeCH(2)X:HX). The computed potential energy surfaces are used to predict what molecular species are likely to be observed in spectroscopic experiments such as gas-phase photoelectron spectroscopy and infrared matrix isolation spectroscopy. It is concluded that, for the reactions of DMSe with Cl(2) and Br(2), the covalent intermediate should be seen in spectroscopic experiments, whereas, in the DMSe + I(2) reaction, the van der Waals adduct DMSe:I(2) should be observed. Comparison is made with previous related calculations and experiments on dimethyl sulfide (DMS) with molecular halogens. The relevance of the results to atmospheric chemistry is discussed. The DMSeX(2) and DMSe:X(2) intermediates are likely to be reservoirs of molecular halogens in the atmosphere which will lead on photolysis to ozone depletion. 相似文献
94.
Aline Maisse-François Laurine Azor Anne-Laure Schmitt Ariane Coquel Lydia Brelot Richard Welter Stéphane Bellemin-Laponnaz Samuel Dagorne 《Journal of organometallic chemistry》2012,696(26):4248-4256
The present contribution describes the synthesis and structural characterization of structurally diverse organoaluminum species supported by variously substituted aminophenolate-type ligands: these Al complexes are all derived from the reaction of AlMe3 with aminophenols 2-CH2NH(R)-C6H3OH (1a, R = mesityl (Mes); 1b, R = 2,6-di-isopropylphenyl (Diip)) and 2-CH2NH(R)-4,6-tBu2-C6H2OH (1c, R = Mes; 1d, R = Diip). The low temperature reaction of AlMe3 with 1a–b readily affords the corresponding Al dimeric species [μ-η1,η1-N,O-{2-CH2NH(R)-C6H4O}]2Al2Me4 (2a–b), consisting of twelve-membered ring aluminacycles with two μ-η1,η1-N,O-aminophenolate units, as determined by X-ray crystallographic studies. Heating a toluene solution of 2a (80 °C, 3 h) affords the quantitative and direct formation of the dinuclear aluminium complex Al[η2-N; μ,η2-O-{2-CH2N(Mes)-C6H4O}](AlMe2) (4a) while species 2b, under the aforementioned conditions, affords the formation of the Al dimeric species [η2-N,O-{2-CH2N(Dipp)-C6H4O}AlMe]2 (3b), as deduced from X-ray crystallography for both 3b and 4a. In contrast, the reaction of bulky aminophenol pro-ligands 1c–d with AlMe3 afford the corresponding monomeric Al aminophenolate chelate complexes η2-N,O-{2-CH2NH(R)-4,6-tBu2-C6H2O}AlMe2 (5c–d; R = Mes, Diip; Scheme 3) as confirmed by X-ray crystallographic analysis in the case of 5d. Subsequent heating of species 5c–d yields, via a methane elimination route, the corresponding Al-THF amido species η2-N,O-{2-CH2N(R)-4,6-tBu2-C6H2O}Al(Me)(THF) (6c–d; R = Mes, Diip). Compounds 6c–6d, which are of the type {X2}Al(R)(L) (L labile), may well be useful as novel well-defined Lewis acid species of potential use for various chemical transformations. Overall, the sterics of the aminophenol backbone and, to a lesser extent, the reaction conditions that are used for a given ligand/AlMe3 set essentially govern the rather diverse “structural” outcome in these reactions, with a preference toward the formation of mononuclear Al species (i.e. species 5c–d and 6c–d) as the steric demand of the chelating N,O-ligand increases. 相似文献
95.
Baxter PN Al Ouahabi A Gisselbrecht JP Brelot L Varnek A 《The Journal of organic chemistry》2012,77(1):126-142
An isomeric series of dehydro[m]pyrido[n]annulenes incorporating strained 1,4-buta-1,3-diyne units have been synthesized, where m = 2, n = 14 (1a-d); m = 2, n = 15 (2a,b); and m = 3, n = 15 (3). The number of pyridine rings and annulene ring π-electrons are denoted by m and n, respectively. The X-ray crystal structures of 1b and 1c confirmed their cyclic formulation. All macrocycles were found to be luminescent chromophores with differing isomer-dependent proton and metal ion-sensory emission responses, which appear collectively as analyte-specific color patterns. Within the series studied, 1a was singular in displaying the highest luminescence quantum yield and sharing the strongest emission energy and molar absorption changes upon protonation and Hg(II) binding. Spectroscopic and electrochemical results were supported by density functional theory calculations in showing 1a, 2a, and 3 to be low bandgap materials with lowest unoccupied molecular orbitals delocalized over the 1,4-di(pyridin-4-yl)buta-1,3-diyne bridges that provide a pathway for electronic communication between the nitrogens. Overall, the investigations suggest that 1a, 2a, and 3 would be excellent ligands for the construction of novel conjugated hybrid metallosupramolecular nanostructures, polymers, and ion-sensory systems. 相似文献
96.
Rhodium-catalyzed reactions of 2-aryl-substituted 2H-chromenes with α-diazo esters prepared from dimethyl and tert-butyl methyl malonates were investigated, and the results were compared with reactions carried out with phenyliodonium ylides prepared from the same esters. The phenyliodonium ylide prepared from dimethyl malonate was found to give superior yields of cyclopropane products compared to the corresponding α-diazo equivalent. However, this result was reversed with tert-butyl methyl malonate when Rh2(S-TBSP)4 was used to decompose the diazo compound. All reactions gave 1,1-cyclopropane diesters as single diastereomers. 相似文献
97.
Fast-atom bombardment mass spectrometry has been used to obtain spectra of conjugated benzo(a)pyrene (bap) metabolites using a 1:1, glycerol + thioglycerol matrix, bap Glucuronides give positive- and negative-ion spectra with peaks due to [M + H]+ and [M - H]- ions and a major fragment peak (base peak) at [bap-OH]+ and [bap-O]-. bap Sulfates (sodium salts) give similar negative-ion spectra with [M - Na]- and [bap-O]- peaks, but the positive-ion spectra are dominated by sodium and glycerol adducts of the bap sulfates. 相似文献
98.
Alfredo Carlos Alder Peter Rüedi Roland Prewo Jost H. Bieri Conrad Hans Eugster 《Helvetica chimica acta》1986,69(6):1395-1417
Novel Diterpenoids from Leaf-Glands of a Plectranthus sp. from Rwanda More than 20 highly modified abietanoid compounds belonging to 6,7-dioxocoleons, serveral kinds of royleanones, spirocoleons, extended quinones, quinone methides, and 1,4-phenanthraquinones have been isolated from a Plectranthus sp. from the borders of Lake Kiwu (Rwanda). The structure of plectranthon A ( 1 ), a 5,7,8-trimethyl-1,4-phenanthraquinone, has been confirmed by an X-ray analysis of its p-bromobenzoate. It is twisted around the C(4)–C(5) moiety; this fact renders the molecule chiral. Notable features of the hydroxylation pattern in all of the isolated compounds are: (a) a predominant hydroxylation at C(16) (abietan numbering) leading to the (15S)-configuration of the 2-hydroxy-l-methylethyl side-chain and to the (13S, 15R)-configuration of the derived spirocyclopropanes and to the derived (S)-2-hydroxypropyl side-chain; (b) the 6β,7α-dihydroxylation which, together with further hydroxylations at C(3) and/or C(2), are considered as a prerequisite for the formation of the conjugated system extending over rings A, B and C (e.g. in the coleons E and F), hence allowing a complete aromatization of the basic ring skeleton. The reasons for these extensive dehydrogenation and oxygenation reactions that cost the plant a great many of oxidation equivalents are not clear. Presumably, these modifying reactions preceed the degradation of the ring system. 相似文献
99.
R. Nemetschek R. Hackl M. Opel R. Philipp M.T. Béal-Monod J.B. Bieri K. Maki A. Erb E. Walker 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):495-503
After briefly recalling the d
+
s model valid for some anisotropic high T
c superconductors, we present a theory of electronic Raman spectra in that model and then compare it with new experimental
data obtained for an overdoped Y123 single crystal. The d
+
s model appears to describe satisfactorily the experimental results, indicating a possible doping dependence of the mixing
ratio. We note that the Raman spectrum of the overdoped Bi2212 could also be accounted for by the d
+
s superconductivity model. The case of Hg1212 (or Hg1223) is reexamined. It appears that the spontaneous breakdown of d-wave symmetry may be rather universal in high T
c cuprates.
Received: 3 March 1998 / Accepted: 30 April 1998 相似文献
100.
Ion‐Pair SN2 Reaction of OH− and CH3Cl: Activation Strain Analyses of Counterion and Solvent Effects
下载免费PDF全文
![点击此处可从《化学:亚洲杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jalal Z. A. Laloo Dr. Lydia Rhyman Dr. Olatz Larrañaga Prof. Dr. Ponnadurai Ramasami Prof. Dr. F. Matthias Bickelhaupt Dr. Abel de Cózar 《化学:亚洲杂志》2018,13(9):1138-1147
We have theoretically studied the non‐identity SN2 reactions of MnOH(n?1)+CH3Cl (M+=Li+, Na+, K+, and MgCl+; n=0, 1) in the gas phase and in THF solution at the OLYP/6‐31++G(d,p) level using polarizable continuum model (PCM) implicit solvation. We want to explore and understand the effect of the metal counterion M+ and solvation on the reaction profile and the stereoselectivity of these processes. To this end, we have explored the potential energy surfaces of the backside (SN2‐b) and frontside (SN2‐f) pathways. To explain the computed trends, we have carried out analyses with an extended activation strain model (ASM) of chemical reactivity that includes the treatment of solvation effects. 相似文献