全文获取类型
收费全文 | 406篇 |
免费 | 20篇 |
国内免费 | 1篇 |
专业分类
化学 | 334篇 |
晶体学 | 1篇 |
力学 | 11篇 |
数学 | 31篇 |
物理学 | 50篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 7篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 12篇 |
2015年 | 15篇 |
2014年 | 7篇 |
2013年 | 10篇 |
2012年 | 27篇 |
2011年 | 38篇 |
2010年 | 15篇 |
2009年 | 16篇 |
2008年 | 30篇 |
2007年 | 30篇 |
2006年 | 33篇 |
2005年 | 24篇 |
2004年 | 31篇 |
2003年 | 14篇 |
2002年 | 15篇 |
2001年 | 9篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1967年 | 1篇 |
1966年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有427条查询结果,搜索用时 0 毫秒
61.
62.
The electrophilic addition of RPX2·AlX3 (1) (R=Ph or Me; X=Cl or Br) to [1,n]dienes was found to be an efficient and useful reaction for the synthesis of a variety of new phospha-heterocycles such as substituted phospholanes, dihalophosphorinanes, phosphorinenes, 3-phosphabicyclo[3.1.0]hexanes, 2-phosphabicyclo[2.2.1] heptanes, 2-phosphabicyclo[3.2.1]octanes and substituted phosphetanes. An ionic multistep mechanism, governed by the stability of the intermediate carbonium ions, is suggested. The phosphorylation reaction was found to be most sensitive to water and in its presence (even 0.1 eq.) it takes a different course. The products, results from hydrogen and RP(O)X addition to the double bond. The phosphorylation is initiated by an unusual protonation process which employs the specific catalytic properties of the AlCl3·H2O complex (HX + RPX2 in the presence or absence of AlCl3 do not give any defined phosphorylation products but polymers). Some of the examined dienes give with water-doped RPX2·AlX3 phosphorylations and/or HX additions, involving in several cases participation of both double bonds to give intramolecular cyclisation products. These products may explain the substances obtained following phosphorylation of simple n-alkenes. Compound 16, one of the new addition compounds, was found to cyclise to the 2-oxaphospholane system (19). 相似文献
63.
The 13C-NMR spectrum of heteronemin (1), a new sesterterpene from marine origin is reported. Assignment of most of the signals was accomplished by a combination of off-resonance decoupling, PRFT measurement, comparison with suitable known model compounds and LIS measurements. An all trand-anti-trans configuration is suggested from 1 according to the 13C-NMR data. 相似文献
64.
65.
We investigate polyelectrolyte bridging interactions mediated by charged, flexible, polyelectrolyte chains between fixed cylindrical macroions of opposite charge in a two-dimensional hexagonal crystalline array. We show that in the asymptotic regime of small macroion density, the polyelectrolyte-mediated attraction is long range, falling off approximately linearly with the macroion array density. We investigate the polyelectrolyte free energy as a function of the macroion density and derive several analytic limiting laws valid in different regimes of the parameter space. 相似文献
66.
67.
An amorphous ultra-thin film of Fe2O3 was prepared on partially oriented graphite (Papyex) by surface oxidation of an adsorbed monolayer of Fe(CO)5. Mössbauer spectra exhibit super-paramagnetism below 79 K. The Mössbauer fraction parallel to the film exceeds that perpendicular by 16% at room temperature. 相似文献
68.
The segregation of solute particles on a moving interface leads to the appearance of two types of instabilities near competing velocity thresholds. This behavior is shown to occur in a variety of exactly solvable models where the interface motion is coupled to a diffusion process of the solute particles. These models directly apply to the propagation of internal domain walls, but can also be generalized to surfaces of growing crystals in the kinetics-limited regime. 相似文献
69.
Puchta R van Eikema Hommes N Meier R van Eldik R 《Dalton transactions (Cambridge, England : 2003)》2006,(28):3392-3395
Based on DFT calculations (RB3LYP/LANL2DZp), the unexpected single-line 1H NMR spectrum of Zn(II)(nta), nta = 2,2',2'-nitrilotriacetate, can be ascribed to a non-dissociative enantiomerization process (deltadeltadelta<=>lambdalambdalambda) from C3viaC3v to C3 symmetry. The energy barrier is rather low and depends to a lesser extent on the nature of the co-ligand in [Zn(nta)X]2- (X: H-, CH3- NH2-, OCH3-, F-, Cl-, Br-, I-) and [Zn(nta)Y]- (Y: NCH, CO, N2, O(CH3)2), but more so on the overall charge of the complex. The energy barrier for enantiomerization of [Zn(nta)X]2- is between 5.7 and 6.7 kcal mol-1, and for [Zn(nta)Y]- between 2.2 and 3.1 kcal mol-1. 相似文献
70.
Bugarcić ZD Nandibewoor ST Hamza MS Heinemann F van Eldik R 《Dalton transactions (Cambridge, England : 2003)》2006,(24):2984-2990
Substitution reactions of the complexes [Pd(bpma)(H2O)]2+, [Pd(bpma)Cl]+, [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+, where bpma = bis(2-pyridylmethyl)amine and dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as triazole, pyrazole, pyrimidine, pyrazine and pyridazine, were studied in an aqueous 0.10 M NaClO4 at pH 2.8 using variable-temperature and -pressure stopped-flow spectrophotometry. The second-order rate constants indicate that the Pd(II) complexes of bpma, viz. [Pd(bpma)(H2O)]2+ and [Pd(bpma)Cl]+, are more reactive than the complexes of dien, viz. [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+. Also, the aqua complexes, [Pd(bpma)(H2O)]2+ and [Pd(dien)(H2O)]2+, are much more reactive than the corresponding chloro complexes. The most reactive nucleophile of the five-membered rings is triazole and for the six-membered rings the most reactive one is pyridazine. Activation parameters were determined for all reactions and the negative entropies and volumes of activation (Delta S++, Delta V++) support an associative ligand substitution mechanism. The crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O was determined by X-ray diffraction. Crystals are monoclinic with the space group P2(1)/c. The coordination geometry of [Pd(bpma)(H2O)]2+ is distorted square-planar. The Pd-N (central) bond distance, 1.958(5) A, is shorter than the other two Pd-N distances, 2.007(5) and 2.009(5) A. The Pd-O distance is 2.043(5) A. 相似文献