全文获取类型
收费全文 | 5049篇 |
免费 | 97篇 |
国内免费 | 15篇 |
专业分类
化学 | 3573篇 |
晶体学 | 19篇 |
力学 | 145篇 |
数学 | 646篇 |
物理学 | 778篇 |
出版年
2023年 | 30篇 |
2022年 | 41篇 |
2021年 | 42篇 |
2020年 | 68篇 |
2019年 | 52篇 |
2018年 | 33篇 |
2017年 | 32篇 |
2016年 | 127篇 |
2015年 | 90篇 |
2014年 | 86篇 |
2013年 | 182篇 |
2012年 | 219篇 |
2011年 | 321篇 |
2010年 | 157篇 |
2009年 | 166篇 |
2008年 | 284篇 |
2007年 | 246篇 |
2006年 | 240篇 |
2005年 | 215篇 |
2004年 | 216篇 |
2003年 | 156篇 |
2002年 | 137篇 |
2001年 | 88篇 |
2000年 | 93篇 |
1999年 | 63篇 |
1998年 | 56篇 |
1997年 | 75篇 |
1996年 | 78篇 |
1995年 | 72篇 |
1994年 | 86篇 |
1993年 | 87篇 |
1992年 | 56篇 |
1991年 | 92篇 |
1990年 | 62篇 |
1989年 | 68篇 |
1988年 | 74篇 |
1987年 | 54篇 |
1986年 | 51篇 |
1985年 | 75篇 |
1984年 | 62篇 |
1983年 | 65篇 |
1982年 | 62篇 |
1981年 | 62篇 |
1980年 | 52篇 |
1979年 | 38篇 |
1978年 | 45篇 |
1977年 | 36篇 |
1976年 | 48篇 |
1975年 | 41篇 |
1973年 | 28篇 |
排序方式: 共有5161条查询结果,搜索用时 0 毫秒
51.
The synthesis and the photochemical behaviour of 4 new 2H-1.4-oxazines and 3 new spiro-2H-1.4-oxazines is described. 相似文献
52.
Synthesis of 4-Substituted Bicyclo[2.2.2]oct-1-yl-p-Nitrobenzenesulfonates The syntheses of the 4-substituted bicyclo[2.2.2]oct-1-yl-p-nitrobenzenesulfonates 1a–2s are described. 相似文献
53.
(all-Z)-(9,10,12,13,15,16-2H6)Octadeca-9,12,15-trienoic acid ( = α-linolenic acid; D6- 4 ) was synthesized to investigate the biochemical formation of linolenic-acid-derived aroma compounds in cultures of the yeast Sporobolomyces odorus, using an established gas chromatographic/mass spectrometric (GC/MS) method. Three compounds were identified as labeled: (Z)-dec-7-eno-5-lactone (δ-jasmin lactone), (Z,Z)-dodeca-6,9-dieno-4-lactone, and (2E,4Z)-hepta-2,4-dienoic acid. Both lactones were biosynthesized mostly under conservation of the initial configuration from their corresponding oxygenated linolenic-acid intermediates. The application of (13S,9Z,11E,15Z)-13-hydroxy(9,10,12,13,15, 16-2H6)octadeca-9,11,15-trienoic acid (D6- 7 ) as a OH-functionalized precursor of δ-jasmin lactone allowed to gain insight into the stereochemical course of the biosynthesis to both enantiomers of this lactone. In this experiment, 88.3% of the metabolized labeled precursor was transformed under retention of the original configuration of the (R)-enantiomer. This investigation is also a contribution to a better understanding of the C?C bond isomerization steps which took place during the β-oxidative degradation of the substrate. 相似文献
54.
Richard L. Tolman Roland K. Robins Leroy B. Townsend 《Journal of heterocyclic chemistry》1971,8(5):703-706
A study involving the reactivity of the pyrrolo[2,3-d] pyrimidine ring system at position 6 with another exocyclic group (CN or -NH2) already residing at C5 has established that hydrogen and bromine are susceptible to electrophilic and acid-catalyzed nucleophilic substitution, respect-tively. In one instance a strong nucleophile (hydrazine) gave nucleophilic substitution at position 6 which was followed by a reaction with the o-nitrile group to afford the tricyclic nucleoside 4,5-diamino-8-(β-D-ribofuranosyl)pyrazolo[3′, 4′ :5,4] pyrrolo[2,3-d] pyrimidine (4). 相似文献
55.
Cornelissen C Erker G Kehr G Fröhlich R 《Dalton transactions (Cambridge, England : 2003)》2004,(23):4059-4063
Treatment of the complexes [(C(5)H(4)PR(2))(2)Zr(CH(3))(2)](b: R = isopropyl; c: R = cyclohexyl) with the reagent HIr(CO)(PPh(3))(3) (2b) yield the heterobimetallic complexes [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Ir(CO)(PPh(3)))] (4b, 4c) with evolution of methane. The reaction of the -PPh(2) substituted analogue with initially yields an intermediate [(H(3)C)(2)Zr(mu-C(5)H(4)PPh(2))(2)Ir(H)(CO)(PPh(3))] 5a, that still contains both methyl groups at zirconium and does not contain a metal-metal bond. At room temperature, the intermediate reacts further with methane formation to eventually yield the (Zr-Ir) complex 4a. The corresponding [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Rh(CO)(PPh(3)))] complexes 3a (R = Ph) and 3b (R = isopropyl) react cleanly with isopropyl alcohol to liberate methane and yield the corresponding [mu-C(5)H(4)PR(2))(2)(Me(2)CHO-Zr-Rh(CO)(PPh(3)))] products (7a, 7b). Carefully monitoring the reaction of with Me(2)CHOH by NMR revealed that the Zr-Rh functionality is attacked first to give the intermediate [Me(Me(2)CHO)Zr([micro sign]-C(5)H(4)PR(2))(2)Rh(H)(CO)(PPh(3))] (6b). This intermediate then reacts further to cleave off methane and re-form the (Zr-Rh) metal-metal bond to yield the product 7b. The tetrametallic mu-oxo-(Zr-Rh) metallocene derivate 11a was obtained starting from the (Zr-Rh) complex 3a and it was characterized by X-ray diffraction. It may be that this reaction is also initiated by H-OH addition to the [Zr-Rh] metal-metal bond. 相似文献
56.
57.
Max Winter Karl H. Schulte-Elte Alain Velluz Josef Limacher Wilhelm Pickenhagen Günther Ohloff 《Helvetica chimica acta》1979,62(1):131-134
Aroma Constituents of the Purple Passion Fruit. Two New Edulan Derivatives The isolation of (2R*, 4S*, 4aS*, 8aS*)-4,4a-epoxy-4,4a-dihydroedulan ( 1 ), and (2R*, 3S*, 8aS*)-3-hydroxyedulan ( 2 ), two new constituents of the purple passion-fruit (Passiflora edulis SIMS ), is reported. Racemic epoxide 1 was synthesized by oxidation of edulan 6 with peracidic acid, and racemate of alcohol 2 was obtained by reduction of ketone 7 , one of the chromic acid oxidation-products of edulan 6 . 相似文献
58.
Roland A. Bougon Karl O. Christe William W. Wilson 《Journal of fluorine chemistry》1985,30(2):237-239
Evidence is presented for the existence of (O2+)2MF62?(M=Ni,Mn) salts. These salts are marginally stable up to about 10°C and are characterized by an OO stretching frequency of about 1805 cm?1. 相似文献
59.
60.