全文获取类型
收费全文 | 2050篇 |
免费 | 45篇 |
国内免费 | 8篇 |
专业分类
化学 | 1828篇 |
晶体学 | 3篇 |
力学 | 7篇 |
数学 | 84篇 |
物理学 | 181篇 |
出版年
2020年 | 13篇 |
2018年 | 10篇 |
2017年 | 9篇 |
2016年 | 22篇 |
2015年 | 32篇 |
2014年 | 26篇 |
2013年 | 59篇 |
2012年 | 51篇 |
2011年 | 64篇 |
2010年 | 41篇 |
2009年 | 36篇 |
2008年 | 47篇 |
2007年 | 58篇 |
2006年 | 41篇 |
2005年 | 56篇 |
2004年 | 32篇 |
2003年 | 27篇 |
2002年 | 30篇 |
2001年 | 27篇 |
2000年 | 29篇 |
1999年 | 20篇 |
1998年 | 16篇 |
1997年 | 22篇 |
1996年 | 25篇 |
1995年 | 12篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 19篇 |
1991年 | 17篇 |
1990年 | 27篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 13篇 |
1973年 | 9篇 |
1966年 | 76篇 |
1965年 | 129篇 |
1964年 | 144篇 |
1963年 | 123篇 |
1962年 | 128篇 |
1961年 | 146篇 |
1960年 | 125篇 |
1959年 | 63篇 |
排序方式: 共有2103条查询结果,搜索用时 546 毫秒
31.
Summary Multiflorine (1) — a minor lupine alkaloid — treated by methyl lithium or methyl magnesium iodide affords 4S-4-hydroxy-4-methyl-2,3-didehydrosparteine (2) and 2S-2-methyl-4-oxosparteine (3), respectively, as the dominating products. Their steric structure, determined by1H and13C NMR techniques, points to stereospecific preferences of these reactions. The observed nucleophilic 1,2- and 1,4-additions indicate that regiospecificity of the action of MeLi or MeMgI on multiflorine is different from that of the so far known similar alkylation of other enamino ketones.
Die räumliche Struktur von Methylierungsprodukten des Multiflorins
Zusammenfassung Multiflorin (1), ein Lupin-Nebenalkaloid, ergibt bei Umsetzung mit Methyllithium oder Methylmagnesiumiodid 4S-4-Hydroxy-4-methyl-2,3-didehydrospartein (2) und 2S-2-Methyl-4-oxospartein (3) als Hauptprodukte. Ihre NMR-spektroskopisch (1H und13C) aufgeklärte räumliche Struktur weist auf eine Stereoselektivität der erwähnten Reaktionen hin. Die beobachteten nucleophilen 1,2- und 1,4-Additionen zeigen, daß sich die Regiospezifität der Einwirkung von MeLi oder MeMgl auf Multiflorin von jener bis jetzt bekannter Alkylierungen von Enaminoketonen unterscheidet.相似文献
32.
33.
Different methods for representing the upper orbitals in Rydberg transitions are tested by means of a series of ab initio SCF and CI calculations for ethylene and various properties for such diffuse united-atom species are reported. Calculated transition energies indicates the maximum separation between individual components of the (π,3d) Rydberg species to be in the 0.1-0.2 range, with similarly small energy separations being obtained for the corresponding (π, 3p) states 相似文献
34.
35.
A hybrid heterogeneous catalyst system, which has been synthesized by covalently anchoring oxodiperoxo molybdenum chelate complexes onto the surface of mesoporous MCM-41, is highly active and truly heterogeneous for the liquid-phase epoxidation of cyclooctene with tBuOOH as the oxygen source. 相似文献
36.
Ursula Baumann 《Fresenius' Journal of Analytical Chemistry》1963,193(6):456
Ohne Zusammenfassung
Folin, O.: Hoppe-Seylers Z. physiol. Chem. 41, 223 (1904); vgl. diese Z. 44, 459 (1905). 相似文献
37.
A self-consistent formalism is proposed for the two-center electron repulsion integrals in the NDDO approximation, based on their expansion in terms of multipole-multipole interactions and free from adjustable parameters. 相似文献
38.
Ring-closing metathesis (RCM) is the key step in a recently reported synthesis of salicylihalamide and related model compounds. Experimentally, the stereochemistry of the resulting cycloolefin (cis/trans) depends strongly on the substituents that are present in the diene substrate. To gain insight into the factors that govern the observed stereochemistry, density functional theory (DFT) calculations have been carried out for a simplified dichloro(2-propylidene)(imidazole-2-ylidene)ruthenium catalyst I, as well as for the real catalyst II with two mesityl substituents on the imidazole ring. Four model substrates are considered, which are closely related to the systems studied experimentally, and in each case, two pathways A and B are possible since the RCM reaction can be initiated by coordination of either of the two diene double bonds to the metal center. The first metathesis yields a carbene intermediate, which can then undergo a second metathesis by ring closure, metallacycle formation, and metallacycle cleavage to give the final cycloolefin complex. According to the DFT calculations, the stereochemistry is always determined in the second metathesis reaction, but the rate-determining step may be different for different catalysts, substrates, and pathways. The ancillary N-heterocyclic carbene ligand lies in the Ru-Cl-Cl plane in the simplified catalyst I, but is perpendicular to it in the real catalyst II, and this affects the relative energies of the relevant intermediates and transition states. Likewise, the introduction of methyl substituents in the diene substrates influences these relative energies appreciably. Good agreement with the experimentally observed stereochemistry is only found when using the real catalyst II and the largest model substrates in the DFT calculations. 相似文献
39.
Ursula Schmid Paul Gilgen Heinz Heimgartner Hans-Jürgen Hansen Hans Schmid 《Helvetica chimica acta》1974,57(5):1393-1403
Irradiation of 2, 3-diphenyl-2H-azirine ( 1a ) and 1-azido-1-phenyl-propene, the precursor of 2-methyl-3-phenyl-2H-azirine ( 1b ), in benzene, with a high pressure mercury lamp (pyrex filter) in the presence of acid chlorides yields the oxazoles 5a–d (Scheme 2). Photolysis of 2, 2-dimethyl-3-phenyl-2H-azirine ( 1c ) under the same conditions gives after methanolysis the 5-methoxy-2, 2-dimethyl-4-phenyl-3-oxazolines 7a, b, d , while hydrolysis of the reaction mixture leads to the formation of the 1, 2-diketones 8a, c, d (Scheme 4). The suggested reaction path for all these reactions is a 1, 3-dipolar cycloaddition of the photochemically generated benzonitrilemethylides 2 to the carbonyl double bond of the acid chlorides to give the intermediates 4 , followed by either elimination of hydrogen chloride or solvolysis (Schemes 2 and 4). Irradiation of 1c in the presence of acetic acid anhydride leads via the intermediate 9 to the 5-hydroxy-3-oxazoline 10 and the 5-methylidene-3-oxazoline 11 (Scheme 5). 相似文献
40.
Hartmut Schmidt‡ Ahmed Al-Ibrahim Ursula Dietzel Ludwig Bieker 《Photochemistry and photobiology》1981,33(1):127-130
Abstract— The quantum yield of the photodynamic inactivation of lysozyme increases in the sequence acridine orange, methylene blue, proflavine and acriflavine (1:5:6:12). At least up to protein concentrations of 0.1 m M , singlet oxygen is exclusively responsible for the inactivation of the enzyme. For methylene blue, acriflavine and proflavine the quantum yields decrease considerably with increasing dye concentrations. From measurements in H2 O and D2 O buffer solutions it was concluded that in the case of methylene blue the effect is mainly caused by the quenching of singlet oxygen [rate constant (3–4) × 108 M −1 s−1 ]. For the acridine sensitizers both singlet oxygen and dye triplet quenching processes have to be taken into consideration. It has been found that all sensitizers act as competitive inhibitors of the enzymatic reaction of lysozyme. However, the dye-protein interaction near the active center cannot be responsible for the observed dye self-quenching effect. 相似文献