全文获取类型
收费全文 | 3638篇 |
免费 | 14篇 |
国内免费 | 4篇 |
专业分类
化学 | 2547篇 |
晶体学 | 19篇 |
力学 | 54篇 |
数学 | 506篇 |
物理学 | 530篇 |
出版年
2018年 | 28篇 |
2017年 | 32篇 |
2016年 | 50篇 |
2015年 | 40篇 |
2014年 | 37篇 |
2013年 | 98篇 |
2012年 | 85篇 |
2011年 | 112篇 |
2010年 | 88篇 |
2009年 | 84篇 |
2008年 | 113篇 |
2007年 | 133篇 |
2006年 | 105篇 |
2005年 | 121篇 |
2004年 | 113篇 |
2003年 | 96篇 |
2002年 | 105篇 |
2001年 | 68篇 |
2000年 | 91篇 |
1999年 | 78篇 |
1998年 | 61篇 |
1997年 | 69篇 |
1996年 | 51篇 |
1995年 | 52篇 |
1994年 | 45篇 |
1993年 | 74篇 |
1992年 | 60篇 |
1991年 | 46篇 |
1990年 | 33篇 |
1989年 | 35篇 |
1988年 | 43篇 |
1987年 | 42篇 |
1986年 | 57篇 |
1985年 | 73篇 |
1984年 | 46篇 |
1983年 | 37篇 |
1982年 | 45篇 |
1981年 | 57篇 |
1980年 | 46篇 |
1979年 | 54篇 |
1978年 | 49篇 |
1977年 | 43篇 |
1976年 | 39篇 |
1975年 | 33篇 |
1974年 | 40篇 |
1973年 | 24篇 |
1972年 | 32篇 |
1971年 | 30篇 |
1970年 | 35篇 |
1969年 | 29篇 |
排序方式: 共有3656条查询结果,搜索用时 0 毫秒
21.
The synthesis of endo- and exo-13-oxo-4b, 5, 12, 12a-tetrahydro-5, 12-ethanoindeno[2,3-b]anthracene ( 23 ; Schemes 1 and 2), exo- and endo-11-oxo-4b, 5, 10, 10a-tetrahydro-5, 10-ethano-indeno[2,3-b]naphthalene ( 31 ; Scheme 3), 1,2,3,4,4a,9a-hexahydro?1,4-(peri-naphthaleno)-fluoren-9-one (36; Scheme 4), and the corresponding hydrocarbons of the stereoisomeric ketone pairs 23 and 36 , is described. 相似文献
22.
Kurt Rüfenacht 《Helvetica chimica acta》1975,58(6):1521-1527
Aza-analogues of the known pesticidal dithiophosphates 2, 4 and 5 have been prepared by replacing the phthalimide, benzotriazole and 1,2,3-benzotriazin-4(3H)-one moieties resp. in 2 by those of quinolinic acid imide (6), cinchomeronic acid imide (7) and pyrazine-2,3-dicarboxylic acid imide (12), in 4 by 1H-v-triazolo[4,5-b]pyridine (15) and in 5 by pyrido[2,3-e]1,2,3-triazin-4(3H)-one (21). Compared to the known compounds the new esters are less or at best equally pesticidal with an equal or even higher mammalian toxicity. 相似文献
23.
24.
The preparation, some properties and the vibrational spectra of NMe4 - [Me3TlCN] (I) and NMe4[(Me3Tl)2F] (II) are reported. A C3v point-group can be assigned to the complex anion in (I), whereas in (II) a linear TlFTl-bridged system (symmetry D3h) is present. 相似文献
25.
26.
Ten tetraoxygenated xanthones (1-hydroxy-3, 7, 8-trimethoxyxanthone I; 1, 7-dihydroxy-3, 8-dimethoxyxanthone II; 1, 7, 8-trihydroxy-3-methoxyxanthone III; 1, 3, 7, 8-tetrahydroxyxanthone IV; 3, 7, 8-trimethoxyxanthone-1-O-primeveroside V; 7-hydroxy-3, 8-dimethoxyxanthone-1-O-primeveroside VI; 1, 8-dihydroxy-3-methoxyxanthone-7-O-acetylrutinoside VII; 7, 8-dihydroxy-3-methoxyxanthone-1-O-primeveroside VIII; 3, 7, 8-trihydroxyxanthone-1-O-primeveroside IX; 3, 7, 8-trihydroxyxanthone-1-O-glucoside X) have been isolated from leaves of Gentiana bavarica L . by means of column chromatography on polyamid. Among these xanthones, VI, VII, VIII and IX were not encountered before in nature. 相似文献
27.
A new xanthone-O-glycoside, the 1,7-dihydroxy-3-methoxyxanthone-8-O-β-D-glucopyranoside ( 7 ), has been isolated from the leaves of Gentiana verna L. by means of column chromatography on polyamid. Six known xanthones: 1-hydroxy-3,7,8-trimethoxyxanthone-1-O-primerveroside ( 2 ); 1,7,8-trihydroxy-3-methoxyxanthone ( 3 ); 7-hydroxy-3,8-dimethoxyxanthone-1-O-primeveroside ( 4 ); 7,8-dihydroxy-3-methoxyxanthone-1-O-primeveroside ( 5 ); mangiferin 6 and the flavone C-glycoside isoorientin 8 have also been isolated and identified. 相似文献
28.
When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S2(π,π*) state, certain unimolecular reactions can be observed to compete with S2 → S1 internal conversion. These reactions do not occur from the S1(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S1(n,π*) state, and it is either identical with the thermally equilibrated S2(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones. 相似文献
29.
Helga Wittmann Johanna Kuhn-Kuhnenfeld H. Binder H. Sterk E. Ziegler 《Monatshefte für Chemie / Chemical Monthly》1971,102(2):404-411
Zusammenfassung IR-spektroskopische Untersuchungen an den Malonesterbetainen1–7 geben Hinweise auf die Schwerpunkte der Ladungsverteilung. Während die Lage der CO-Banden der Betaine1 und2 mit rein aliphatischer Esterkomponente auf vollständige Resonanz zwischen den Estercarbonylen hinweist, ist bei jenen mit aromatischen Estergruppen (4, 5 und7) eine CO-Funktion zu beobachten, die schon auf einen gewissen Ylidanteil schließen läßt. Damit im Einklang stehen auch ihre chemischen Eigenschaften. Z. B. zersetzt sich das Bisphenoxycarbonylbetain4 beim Schmelzpunkt vollständig. Die Thermolyse von1 und2 führt dagegen sowohl zu Picolin- als auch zu Kohlensäureestern. Der Reaktionsmechanismus wird diskutiert.
Herrn Direktor Dr. techn., Dr. rer. nat. h. c., Dr. phil. h. c.W. G. Stoll, Basel, mit besten Wünschen zum 60. Geburtstag gewidmet. 相似文献
Reactions with betain, VI: The chemistry of several malonate betaines
An IR-spectroscopic investigation concerning the malonate betaines1–7 gives an insight in the charge density distribution. The position of the C=O frequencies of the betaines1 and2 with alkyl ester groups shows complete resonance between the ester carbonyls, while with the phenolic esters4,5 and7 and C=O absorption is observed which shows a considerable ylid participation. the chemical properties of the betaines agree with this; e.g. the bis-phenoxycarbonylbetaine4 is decomposed totally at its melting point. However, the thermolysis of1 and2 gives picolinic acid esters and carbonic acid esters. The mechanism of these reactions is discussed.
Herrn Direktor Dr. techn., Dr. rer. nat. h. c., Dr. phil. h. c.W. G. Stoll, Basel, mit besten Wünschen zum 60. Geburtstag gewidmet. 相似文献
30.