全文获取类型
收费全文 | 1547篇 |
免费 | 204篇 |
国内免费 | 152篇 |
专业分类
化学 | 482篇 |
晶体学 | 5篇 |
力学 | 204篇 |
综合类 | 29篇 |
数学 | 599篇 |
物理学 | 584篇 |
出版年
2024年 | 3篇 |
2023年 | 21篇 |
2022年 | 34篇 |
2021年 | 36篇 |
2020年 | 32篇 |
2019年 | 42篇 |
2018年 | 41篇 |
2017年 | 48篇 |
2016年 | 36篇 |
2015年 | 41篇 |
2014年 | 82篇 |
2013年 | 92篇 |
2012年 | 93篇 |
2011年 | 94篇 |
2010年 | 79篇 |
2009年 | 78篇 |
2008年 | 74篇 |
2007年 | 96篇 |
2006年 | 89篇 |
2005年 | 83篇 |
2004年 | 65篇 |
2003年 | 84篇 |
2002年 | 81篇 |
2001年 | 53篇 |
2000年 | 60篇 |
1999年 | 58篇 |
1998年 | 46篇 |
1997年 | 43篇 |
1996年 | 35篇 |
1995年 | 26篇 |
1994年 | 25篇 |
1993年 | 21篇 |
1992年 | 17篇 |
1991年 | 18篇 |
1990年 | 7篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有1903条查询结果,搜索用时 15 毫秒
101.
Eberhard Reimann Wolfgang Erdle Eugen Hargasser Hermann Lotter 《Monatshefte für Chemie / Chemical Monthly》2002,133(7):1017-1030
Summary. The stereoselective synthesis of cis-ergoline is presented. Starting from rac-N-benzoyl tryptophan methyl ester, the key compound indolinylmethylpyridin-3-one was prepared via a seven-step reaction in good yield. Since its cyclization to the desired ergolinone failed, the key compound was reduced
to yield the two diastereomeric pyridin-3-ols; only one of them cyclized in trifluoromethanesulfonic acid, affording cis-ergoline. Catalytic hydrogenation of the latter gave N,N′-dimethyldihydroergoline, the X-ray crystallography of which revealed both the correct structure and identical relative configurations
at C-5a and C-6a (SS or RR). Hydroboration and subsequent perruthenate oxidation of the Δ9-ergoline provided access to the regioisomeric ergolinols and ergolinones.
Received December 27, 2001. Accepted January 15, 2002 相似文献
102.
Mohamed El Guendouzi Abderrahim Dinane Abdelfetah Mounir 《Journal of solution chemistry》2002,31(2):119-129
The mixed aqueous electrolyte system of ammonium and lithium chlorides has been studied by the hygrometric method at 25°C. The relative humidities of this system are measured at total molalities from 0.3 to 6 mol-kg– 1 for different ionic-strength fractions y of NH4Cl with y = 0.33, 0.50, and 0.67. The data obtained allow the deduction of new water activities and osmotic coefficients. The experimental results are compared with the predictions of the ECA (extended composed additivity) law proposed in our previous work. The Zdanovskii–Stokes–Robinson (ZSR), the Robinson–Stokes (RS), Reilly–Wood–Robinson (RWR), the Pitzer, and the Lietzke–Stoughton (LS II) models are also compared with our results. Predictions made using these models are, in general, consistent with our results. From these measurements, new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture for different ionic-strength fractions. 相似文献
103.
~(19)F-NMR法测定聚醚多元醇中伯羟基的相对含量 总被引:2,自引:0,他引:2
<正> 聚醚多元醇是聚氨酯材料的主要原料之一.多元醇中伯羟基含量的多少对反应活性有着直接的影响,因此,准确地测定伯羟基含量已成为许多研究者关心的课题,并已报道了不少测定方法,其中主要包括化学动力学方法、~1H-核磁共振(NMR)法和~(19)F-NMR法,~1H-NMR法系利用不同的酰化剂,将聚醚多元醇酰化后测其酰化物的~1H-NMR谱,从伯、仲羟基相应的酯基氢的积分比例来计算伯羟基的相对含量,但由于体系中存在其它含氢基因,且是大量的,使测量困难或应用受到限制.~(19)F-NMR法测伯羟 相似文献
104.
NMR chemical shifts (CSs: δN(NH), δC(α), δC(β), δC', δH(NH), and δH(α)) were computed for the amino acid backbone conformers (α(L), β(L), γ(L), δ(L), ε(L), α(D), γ(D), δ(D), and ε(D) [Perczel et al., J Am Chem Soc 1991, 113, 6256]) modeled by oligoalanine structures. Topological differences of the extended fold were investigated on single β-strands, hairpins with type I and II β-turns, as well as double- and triple-stranded β-sheet models. The so-called "capping effect" was analyzed: residues at the termini of a homoconformer sequence unit usually have different CSs than the central residues of an adequately long homoconformer model. In heteroconformer sequences capping effect ruins the direct applicability of several chemical shift types (δH(NH), δC', and δN(NH)) for backbone structure determination of the parent residue. Experimental δH(α), δC(α), and δC(β) values retrieved from protein database are in good agreement with the relevant computed data in the case of the common backbone conformers (α(L), β(L), γ(L), and ε(L)), even though neighboring residue effects were not accounted for. Experimental and computed ΔδH(α)-ΔδC(α), ΔδH(α)-ΔδC(β), and ΔδC(α)-ΔδC(β) maps give qualitatively the same picture, that is, the positions of the backbone conformers relative to each other are very similar. This indicates that the H(α), C(α), and C(β) chemical shifts of alanine depend considerably on the backbone fold of the parent residue also in proteins. We provide tabulated CSs of the chiral amino acids that may predict the various structures of the residues. 相似文献
105.
A molecular thermodynamic model for polyelectrolyte systems—called pePC-SAFT—is proposed. The effect of charged monomers within the polyelectrolyte chain is explicitly taken into account in the reference term by replacing the hard-chain contribution of the PC-SAFT model by a charged-hard-chain contribution. Moreover, counterion condensation is accounted for to determine the effective number of charges along the polyion as well as of free counterions. The electrostatic contribution of the free counterions is described by a Debye–Hückel term. 相似文献
106.
Guerrero-Alvarez JA Moncayo-Bautista A Ariza-Castolo A 《Magnetic resonance in chemistry : MRC》2004,42(6):524-533
The relative configuration of 11 1,4-diazaspiro[4.5]decanes (1a-1j and 1m), 15 1,4-oxazaspiro[4.5]decanes (2a-2o) and 10 1,4-dioxaspiro[4.5]decanes (3a-3n) substituted at the 2-, 6-, 7- or 8-position by a methyl group or using the tert-butyl group as a model for the ananchomeric structure is reported. The relative stereochemistry was analyzed by 1H, 13C, 15N and 17O NMR and all isomers present were characterized spectroscopically. Compounds with a methyl group in the six-membered ring show a chair conformation preference with the methyl group in the equatorial position. Compounds with one or two nitrogens exhibit a tautomeric equilibrium between the imine-diazolidine forms, as demonstrated by IR and 13C NMR. 相似文献
107.
The reaction of fluorobenzene with urotropine in trifluoroacetic acid (TFAA) at high pressures and temperatures affords predominantly fluorobenzaldehydes andN-(fluorophenylmethyl)trifluoroacetamides. The yields of these products depend considerably on the reaction conditions. The rates of their formation have the maximum values at the momemt of the phase transition (PT) of TFAA. A new efficient cyclic (dynamic) regime is proposed for the synthesis at high pressures. The regime involves periodically occurring PT of the solvent. The change in the relative rate of product formation with the degree of fluorobenzene conversion is wave-like.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 319–323, February, 1995. 相似文献
108.
The relative permittivity of 1,2-dimethoxyethane and N,N-dimethylformamide mixtures from −10 to 40°C
Gianni Goldoni Luigi Marcheselli Andrea Marchetti Lorenzo Tassi Giuseppe Tosi 《Journal of solution chemistry》1992,21(9):953-962
Binary solutions of N,N-dimethylformamide and 1,2-dimethoxyethane have been investigated by means of dielectric measurements at temperatures ranging from –10 to +40°C, and for nine mixtures covering the whole miscibility field expressed by the mole fraction of one component (0X11). The experimental data were used to study the dependence of on T and X1, of the type = (T), = (X1), and = (T,X1). Further, the excess mixing function E has been evaluated in order to identify particular patterns of interaction between unlike molecules and any other factor that could modify such patterns. The minimum in the E vs. composition plots suggests the formation of an adduct of stoichiometric ratio DMFDME=11 at all the investigated temperatures. 相似文献
109.
L. S. Zakharov G. N. Molchanova T. M. Shcherbina P. V. Petrovskii M. I. Kabachnik 《Russian Chemical Bulletin》1998,47(9):1718-1724
A number of trialkylsilylmethyl diphenyl phosphates MeRR′SiCH2OP(O)(OPh)2 (1a-e: R=Et (a), Pr (b), CF3CH2CH2 (c, e), Me3SiCH2 (d); R′=Me (a-d), Et (e)) were synthesized and their thermal rearrangement, of the 1,2-shift type, was studied. The rearrangement consists of the
migration of an alkyl group from Si atom to the methylene carbon atom and gives the corresponding silyl esters. The rate of
the rearrangement was found to increase in the order1d<1b<1a<1 (R=R′=Me)<1c corresponding to the enhancement of the total inductive effect (−I) of the substituents at the Si atom. The relative migration ability of the substituents at the Si atom, determined by GC/MS
analysis of the disiloxane fraction resulting from hydrolysis of pyrolyzed phosphates1a-e, increases in the order R=Pr<Et<CF3CH2CH2<Me≪Me3SiCH2, which differs substantially from the order in which the rate of the rearrangement of phosphates1a-d changes. The electronegativity of the migrating group affects noticeably the relative ability to migrate.
For Part 4, see Ref. 1.
Deceased.
Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 1767–1772, September, 1998. 相似文献
110.
Abdelfetah Mounir Mohamed El Guendouzi Abderrahim Dinane 《Journal of solution chemistry》2002,31(10):793-799
Ternary aqueous solutions of MgSO4 and K2SO4 have been studied by the hygrometric method at 25°C. The relative humidity of this system is measured at total molalities from 0.35 mol-kg–1 to about saturation for three ionic-strength fractions (y = 0.25, 0.50, and 0.80 of MgSO4. The data allow calculation of water activities and osmotic coefficients. From these measurements, the Pitzer ionic mixing parameters are determined and used to predict the solute activity coefficients in the mixture. The results are used to calculate the excess Gibbs energy at total molalities for ionic-strength fraction y. 相似文献