全文获取类型
收费全文 | 789篇 |
免费 | 22篇 |
国内免费 | 2篇 |
专业分类
化学 | 601篇 |
晶体学 | 14篇 |
力学 | 4篇 |
数学 | 76篇 |
物理学 | 118篇 |
出版年
2023年 | 4篇 |
2022年 | 8篇 |
2021年 | 22篇 |
2020年 | 30篇 |
2019年 | 18篇 |
2018年 | 10篇 |
2017年 | 6篇 |
2016年 | 17篇 |
2015年 | 16篇 |
2014年 | 8篇 |
2013年 | 41篇 |
2012年 | 39篇 |
2011年 | 53篇 |
2010年 | 35篇 |
2009年 | 15篇 |
2008年 | 51篇 |
2007年 | 37篇 |
2006年 | 52篇 |
2005年 | 35篇 |
2004年 | 38篇 |
2003年 | 28篇 |
2002年 | 27篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 10篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 17篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有813条查询结果,搜索用时 62 毫秒
221.
Pascal Vermeeren Trevor A. Hamlin Israel Fernndez F. Matthias Bickelhaupt 《Chemical science》2020,11(31):8105
The Diels–Alder reactions between cyclopentadiene and various α,β-unsaturated aldehyde, imine, and iminium dienophiles were quantum chemically studied using a combined density functional theory and coupled-cluster theory approach. Simple iminium catalysts accelerate the Diels–Alder reactions by lowering the reaction barrier up to 20 kcal mol−1 compared to the parent aldehyde and imine reactions. Our detailed activation strain and Kohn–Sham molecular orbital analyses reveal that the iminium catalysts enhance the reactivity by reducing the steric (Pauli) repulsion between the diene and dienophile, which originates from both a more asynchronous reaction mode and a more significant polarization of the π-system away from the incoming diene compared to aldehyde and imine analogs. Notably, we establish that the driving force behind the asynchronicity of the herein studied Diels–Alder reactions is the relief of destabilizing steric (Pauli) repulsion and not the orbital interaction between the terminal carbon of the dienophile and the diene, which is the widely accepted rationale.Quantum chemical activation strain analyses revealed that iminium catalysts accelerate Diels–Alder reactions by reducing the Pauli repulsion between reactants. 相似文献
222.
Christos Papageorgiou Trevor J. Petcher Erwin Waldvogel 《Helvetica chimica acta》1989,72(7):1463-1470
In 7 steps, 6- or 9-hydroxylated or -methoxylated trans-octahydrobenzo[g]isoquinolines were efficiently synthesized starling from dimethoxynaphtbalenes (Scheme), as potential new selective ligands for serotonin receptors. The 6-substituted compounds had very little affinity to common neurotransmitter receptors, with the exception of adrenergic α2. The 9-substituted compounds, while showing interesting affinity for 5HT1a receptors, had comparable affinities for adrenergic α1 and β2, and in one case for dopamine D2 receptors. 相似文献
223.
Levina A Codd R Foran GJ Hambley TW Maschmeyer T Masters AF Lay PA 《Inorganic chemistry》2004,43(3):1046-1055
Structures of the complexes [Cr(V)O(ehba)(2)](-), [Cr(IV)O(ehbaH)(2)](0), and [Cr(III)(ehbaH)(2)(OH(2))(2)](+) (ehbaH(2) = 2-ethyl-2-hydroxybutanoic acid) in frozen aqueous solutions (10 K, [Cr] = 10 mM, 1.0 M ehbaH(2)/ehbaH, pH 3.5) have been determined by single- and multiple-scattering fitting of X-ray absorption fine structure (XAFS) data. An optimal set of fitting parameters has been determined from the XAFS calculations for a compound with known crystal structure, Na[Cr(V)O(ehba)(2)] (solid, 10 K). The structure of the Cr(V) complex [Cr(V)O(ehba)(2)](-) does not change in solution in the presence of excess ligand. Contrary to the earlier suggestions made from the kinetic data (Ghosh, M. C.; Gould, E. S. J. Chem. Soc., Chem. Commun. 1992, 195-196), the structure of the Cr(IV) complex (generated by the Cr(VI) + As(III) + ehbaH(2) reaction) is close to that of the Cr(V) complex (five-coordinate, distorted trigonal bipyramidal) and different from that of the Cr(III) complex (six-coordinate, octahedral). For both Cr(V) and Cr(IV) complexes, some disorder in the position of the oxo group is observed, which is consistent with but not definitive for the presence of geometric isomers. The structure of the Cr(IV) complex differs from that of Cr(V) by protonation of alcoholato groups of the ligands, which leads to significant elongation of the corresponding Cr-O bonds (2.0 vs 1.8 A). This is reflected in the different chemical properties reported previously for the Cr(IV) and Cr(V) complexes, including their reactivities toward DNA and other biomolecules in relation to Cr-induced carcinogenicity. 相似文献
224.
Texter J Ge L Mourey TH Bryan TG 《Langmuir : the ACS journal of surfaces and colloids》2004,20(26):11288-11292
A hygroscopic and polymerizable salt ([2-methacryloyloxy]ethyl trimethylammonium chloride) is used to ion exchange the sodium ion in AOT (bis[2-ethylhexyl]sulfosuccinate, sodium salt) to produce a polymerizable form of AOT, MDOS ([2-methacryloyloxy]ethyl trimethylammonium bis[2-ethylhexyl]sulfosuccinate). A partial ternary phase diagram of water, MDOS, and methyl methacrylate (MMA) was determined at room temperature (22 +/- 1 degrees C). A relatively large L2 domain is obtained, but this domain is smaller than that obtained with AOT. Microemulsion polymerization in this domain at 70 degrees C, using AIBN (azoisobutyronitrile) as an initiator, produces an optically clear copolymer solid domain nearly as large as the L2 domain. This interesting behavior contrasts with similar studies of Pavel and Mackay [Langmuir 2000, 16, 8528] using a polymerizable surfactant DDAMA (didecyldimethylammonium methacrylate) that produced a much larger L2 domain than MDOS but yielded a much smaller optically clear domain after thermally initiated polymerization. Thermogravimetric analysis indicates that optically clear composites obtained at an MDOS/MMA weight ratio of 1:4 and containing 5% water (w/w; weight % water in microemulsion) released the water in a transition commencing around 160 degrees C and continuing to 250 degrees C. Thereafter, the thermal decomposition was substantially impeded relative to poly(methyl methacrylate) as a control, which was due to the fire-resistant nature of the MDOS monomer. Molecular weight measurements indicate MDOS/MMA copolymers form substantially higher molecular weights as the proportion of MDOS increases. At a given radius of gyration, higher MDOS-containing copolymers exhibit higher molecular weights, suggesting a more compact structure with increasing MDOS. 相似文献
225.
A new octapeptide has been isolated and characterized as its potassium complex from acidic aqueous methanol solutions of ascidiacyclamide, a cyclic peptide isolated from the ascidian Lissoclinum patella. Crystals suitable for X-ray structure determination were orthorhombic, space group P2(1)2(1)2(1), with a = 15.442(3) ?, b = 36.167(1) ?, c = 9.567(3) ?, V = 5343(2) ?(3), Z = 4, and R = 0.069. The structure consists of the macrocyclic cation, three perchlorate anions, and two water molecules. The macrocyclic ring of ascidiacyclamide remains intact, but two oxazoline rings have been opened to form two amino lactones, with the amine protonated. A potassium ion is bound to the N atoms of the thiazole rings (K(1).N(1), 2.47(2) ?; K(1).N(5), 2.50(2) ?) and to the adjacent O (amide) atoms (K(1).O(1), 2.34(1) ?; K(1).O(5), 2.34(1) ?). Two perchlorate anions, located on either side of the plane of the macrocyclic ring, are weakly associated with the potassium cation (K(1).O(10), 2.72(3) ?; K(1).O(16), 2.48(3) ?). This cyclic octapeptide reacts with copper(II) to give a purple solution (lambda(max) 590 nm). 相似文献
226.
The protonation of 4-(2-pyridylazo)-N,N-dimethylaniline (PYAD) in aqueous solution and its adsorption on oxide surfaces has been studied by resonance Raman (RR) spectroscopy. The gas phase structures of neutral, protonated and diprotonated forms of PYAD were modelled by SCF-DFT calculations at the B3-LYP/DZ level, enabling determination of the simulated vibrational spectra of these species, together with vibrational assignments, and providing confirmation that protonation occurs initially at the pyridyl nitrogen atom. Electronic absorption spectra were interpreted using time-dependent DFT calculations. Adsorption of PYAD on SiO2 or Al2O3 surfaces is mainly via the neutral species, hydrogen bonded to surface OH groups, although a small proportion of adsorbed molecules are protonated. By contrast, adsorption on SiO2–Al2O3 results in complete protonation, indicating the presence of Brønsted acidic sites with pKa values ? 4.5, whereas adsorption on H-mordenite results in diprotonation, indicating the presence of Brønsted acidic sites with pKa values ? 2. 相似文献
227.
Gahan LR Grillo VA Hambley TW Hanson GR Hawkins CJ Proudfoot EM Moubaraki B Murray KS Wang D 《Inorganic chemistry》1996,35(4):1039-1044
Reaction of manganese(II) perchlorate hexahydrate with a methanol solution of 1-thia-4,7-diazacyclononane ([9]aneN(2)S) resulted in the isolation of the manganese(II) complex [Mn([9]aneN(2)S)(2)](ClO(4))(2). The X-ray structure of this complex is reported: crystal system orthorhombic, space group Pbam, No. 55, a = 7.937(2) ?,b = 8.811(2) ?, c = 15.531(3) ?, Z = 2, R = 0.0579. The complex is high spin (S = (5)/(2)) with an effective magnetic moment (&mgr;(eff)) 5.82 &mgr;(B) at 298 K and 5.65 &mgr;(B) at 4.2 K. Computer simulation of the Q-band EPR spectrum of [Mn([9]aneN(2)S)(2)](ClO(4))(2) yields g = 1.99 +/- 0.01, |D| = 0.19 +/- 0.005 cm(-)(1), and E/D = 0.04 +/- 0.02. For the analogous hexaamine complex [Mn([9]aneN(3))(2)](ClO(4))(2) ([9]aneN(3) = 1,4,7-triazacyclononane) analysis of the EPR spectra produced the following values: g = 1.98 +/- 0.01, |D| = 0.09 +/- 0.003 cm(-)(1), and E/D = 0.1 +/- 0.01. The spin Hamiltonian parameters for [Mn([9]aneN(2)S)(2)](ClO(4))(2) derived from the EPR spectra produced a good fit to the magnetic susceptibility data. 相似文献
228.
Anionic polymerization of propylene oxide: Isomerization of allyl ether to propenyl ether end groups
Ga-Er Yu Frank Heatley Colin Booth Trevor G. Blease 《Journal of polymer science. Part A, Polymer chemistry》1994,32(6):1131-1135
The isomerization of allyl ether to propenyl ether end group in anionically-polymerized poly (propylene oxide) was monitored by 1H NMR spectroscopy. It was confirmed that the reaction followed a simple second-order rate law: ?d[allyl]dt = k2[allyl] [O?]. Values of k2 determined over the 90–130°C temperature range, indicated an activation energy of 116 kJ mol?1. © 1994 John Wiley & Sons, Inc. 相似文献
229.
Time-resolved frequency-modulation spectroscopy is shown to be an effective method for measuring the rates of gas-phase reactions. As an example, the rate constant for the reaction of CN with C2H4 at 298 K is measured to be 2.5 ± 0.2 × 10−10 cm3s−1, in good agreement with other literature values. The efficiency and sensitivity of this technique will be of interest to the chemical kineticist. © 1997 John Wiley & Sons, Inc. 相似文献
230.