全文获取类型
收费全文 | 6397篇 |
免费 | 102篇 |
国内免费 | 29篇 |
专业分类
化学 | 4254篇 |
晶体学 | 28篇 |
力学 | 102篇 |
数学 | 1251篇 |
物理学 | 893篇 |
出版年
2020年 | 45篇 |
2019年 | 53篇 |
2016年 | 99篇 |
2015年 | 67篇 |
2014年 | 53篇 |
2013年 | 157篇 |
2012年 | 162篇 |
2011年 | 204篇 |
2010年 | 142篇 |
2009年 | 142篇 |
2008年 | 200篇 |
2007年 | 194篇 |
2006年 | 208篇 |
2005年 | 185篇 |
2004年 | 158篇 |
2003年 | 145篇 |
2002年 | 148篇 |
2001年 | 103篇 |
2000年 | 118篇 |
1999年 | 90篇 |
1998年 | 79篇 |
1997年 | 84篇 |
1996年 | 105篇 |
1995年 | 106篇 |
1994年 | 95篇 |
1993年 | 109篇 |
1992年 | 142篇 |
1991年 | 134篇 |
1990年 | 122篇 |
1989年 | 118篇 |
1988年 | 96篇 |
1987年 | 94篇 |
1986年 | 115篇 |
1985年 | 129篇 |
1984年 | 125篇 |
1983年 | 115篇 |
1982年 | 115篇 |
1981年 | 137篇 |
1980年 | 90篇 |
1979年 | 114篇 |
1978年 | 112篇 |
1977年 | 131篇 |
1976年 | 96篇 |
1975年 | 110篇 |
1974年 | 118篇 |
1973年 | 102篇 |
1972年 | 83篇 |
1971年 | 71篇 |
1970年 | 62篇 |
1967年 | 53篇 |
排序方式: 共有6528条查询结果,搜索用时 31 毫秒
71.
72.
73.
74.
Abstract Photoreactions, such as base release and decomposition of the base moiety, induced by either 20 ns laser pulses at 193 nm or continuous 254 nm irradiation, were studied for a series of uracil and adenine derivatives in neutral aqueous solution. The quantum yield of chromophore loss (φ) depends significantly on the nature of the nucleic acid constituent and the saturating gas (Ar, N2O or O2). In the case of polynucleotides the destruction of nucleotides was measured by high-performance liquid chromatography after hydrolysis; the quantum yields (φ) are comparable to those of chromophore loss or larger. The φcl and aφdn of 0.04–0.1 for poly(U) and poly(dU), obtained for both wavelengths of irradiation, are due to processes originating from the lowest excited singlet state, i.e. formation of photohydrates and photodimers, and a second part from photoionization using λirr= 193 nm. Irradiation at 193 nm effectively splits pyrimidine dimers and thus reverts them into monomers. The quantum yield for release of undamaged bases (φbr) from nucleosides, nucleotides and polynucleotides upon irradiation at 254 nm is typically φbr= (0.1–1) × 10?4 Breakage of the N-glycosidic bond is significantly more efficient for λirr=193 nm, e.g. φbr= 1.1 × 10?3, 0.8 × 10?3, 4.3 × 10?3 and 0.5 × 10?3 for poly(A), poly(dA), poly(U) and poly(dU) in Ar-saturated solution, respectively. Enhanced φ values for λirr= 193 nm, essentially for adenine and its derivatives, are caused by photo-processes that are initiated by photoionization. 相似文献
75.
Helmut Schmidhammer Felizia Kaspar Arpad Marki Anna Borsodi 《Helvetica chimica acta》1994,77(4):999-1002
The mixed azines 4 and 5 were prepared by reaction of naloxazone ( 2 ) with either oxymorphone ( 6 ) or 14-O-methyloxymorphone ( 7 ) and tested in vitro using opioid receptor binding assays and in vivo using the AcOH-writhing test in mice. Compound 4 was found to be a partial agonist, while compound 5 was a potent opioid agonist with higher potency than morphine. 相似文献
76.
The photoprocesses of a series of N-substituted dibenz[b,f]azepines (iminostilbenes) were studied by absorption and emission spectroscopy, by laser flash photolysis, and by preparative irradiation with NMR analysis. In solutions, 2pi+2pi photodimers of N-cyano and N-acyl dibenzazepines are formed via the triplet state upon acetone- or benzophenone-sensitized energy transfer. T-T absorption spectra were measured and absorption coefficients were determined. The triplet energy transfer is equally efficient for N-alkyl dibenzazepines, which do not dimerize. Excited states of npi* character in the latter cases are discussed to rationalize the different reactivities. In spite of negligible intersystem crossing of 21 dibenzazepine derivatives, photodimers of N-acyl and N-cyano dibenzazepines are formed upon direct excitation in concentrated solutions (0.01-0.1 mol dm(-3)) as well as in the solid state. A selective anti-configuration of the photodimers was found throughout. 相似文献
77.
Gertraud Chiste Helmut Egg Klemens Kraus Elisabeth Pipitz Elisabeth Spanyar 《Monatshefte für Chemie / Chemical Monthly》1991,122(6-7):537-544
Summary A new method for the synthesis of 1,2-diaryl-1,2-dihydro-5-methyl-3H-pyrazol-3-ones3 and 4-acetyl-1,2-diaryl-1,2-dihydro-5-methyl-3H-pyrazol-3-ones5 is presented. The reaction of 4,4-disubstituted 1,2-diarylhydrazines1 with acetic anhydride in the presence of an equimolar amount of 4-(dimethylamino)pyridine leads to mixtures of the corresponding acetyl derivatives2 and3. Under the same conditions, 2,2-disubstituted 1,2-diarylhydrazines yield mixtures of3 and5.
4-(Dimethylamino)pyridin-katalysierte Reaktion von 1,2-Diarylhydrazinen mit Essigsäureanhydrid
Zusammenfassung Eine neue Methode zur Synthese von 1,2-Diaryl-1,2-dihydro-5-methyl-3H-pyrazol-3-onen3 und 4-Acetyl-1,2-diaryl-1,2-dihydro-5-methyl-3H-pyrazol-3-onen5 wird beschrieben. Die Reaktion von 4,4-disubstituierten 1,2-Diaryl-hydrazinen1 mit Essigsäureanhydrid führt in Gegenwart eines Äquivalentes 4-(Dimethylamino)pyridin zu Gemischen der entsprechenden Acetylderivate2 und3. Unter den gleichen Bedingungen werden aus 2,2-disubstituierten 1,2-Diarylhydrazinen Gemische aus3 und5 erhalten.相似文献
78.
The preparation of η-C3H5V(CO)5 from allyl chloride and sodium hexacarbonylvanadate (–I) is reported. Composition, IR and NMR spectra are in accordance with a π-bonded C3H5-moiety; the preparation has been extended to give complexes with substituted allyl groups. Furthermore (1-3-η-2-butenyl)pentacarbonylvanadium can be obtained by the addition of butadiene to HV (CO)6. 相似文献
79.
ValeryN. Kalinin IgorV. Shishkov SergeyK. Moiseev ElviraE. Shults GenrikhA. Tolstikov NataliaI. Sosnina PavelV. Petrovskii KonstantinA. Lyssenko Helmut Schmidhammer 《Helvetica chimica acta》2006,89(5):861-869
In a search for starting materials for the preparation of 7,8‐fused morphine alkaloid derivatives, 8‐[(1E‐2‐phenylethenyl]codeinone dimethyl ketal ( 4 ) and 8‐[(1E‐2‐phenylethenyl]codeine ( 5 ) were prepared. These dienes were used as substrates in the Diels–Alder reactions. Compound 5 formed the ‘normal’ adduct 12 with N‐phenylmaleimide, while compound 4 behaved in reactions with dienophiles as the ‘masked’ diene 11 , a 8‐[(1E)‐2‐phenylethenyl]‐substituted thebaine, yielding the corresponding 19‐substituted 6,14‐endo‐etheno‐6,7,8,14‐tetrahydrothebaines. Specifically, reaction of 4 with methyl vinyl ketone gave rise to 19‐[(1E)‐phenylethenyl]thevinone ( 14 ) whose structure was elucidated by an X‐ray diffraction analysis. The thebaine derivative 11 was also prepared from 4 . 相似文献
80.
In a weakly acidic solution, the addition of HBr to 1-phenylprop-1-yne produces predominantly the anti-Markovnikov product. In this paper, we consider five possible explanations for this behavior and conclude that the concerted addition is occurring on the acetylenic pi bond orthogonal to the extended aromatic pi system. The electronic effect of the distal methyl group and the steric hindrance of the coplanar phenyl ring combine to promote bromide attack at the beta carbon. Attack on this pi bond is insensitive to the electronic effect of meta and para substituents on the ring but is very (sterically) sensitive toward all ortho substituents. 相似文献