全文获取类型
收费全文 | 3241篇 |
免费 | 188篇 |
国内免费 | 14篇 |
专业分类
化学 | 2600篇 |
晶体学 | 14篇 |
力学 | 58篇 |
数学 | 339篇 |
物理学 | 432篇 |
出版年
2023年 | 56篇 |
2022年 | 42篇 |
2021年 | 90篇 |
2020年 | 143篇 |
2019年 | 114篇 |
2018年 | 82篇 |
2017年 | 55篇 |
2016年 | 122篇 |
2015年 | 102篇 |
2014年 | 101篇 |
2013年 | 159篇 |
2012年 | 275篇 |
2011年 | 354篇 |
2010年 | 129篇 |
2009年 | 97篇 |
2008年 | 216篇 |
2007年 | 204篇 |
2006年 | 214篇 |
2005年 | 141篇 |
2004年 | 134篇 |
2003年 | 103篇 |
2002年 | 80篇 |
2001年 | 22篇 |
2000年 | 32篇 |
1999年 | 16篇 |
1998年 | 21篇 |
1997年 | 11篇 |
1996年 | 23篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 23篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 15篇 |
1989年 | 13篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1976年 | 7篇 |
1975年 | 9篇 |
1973年 | 10篇 |
1968年 | 7篇 |
1966年 | 6篇 |
排序方式: 共有3443条查询结果,搜索用时 15 毫秒
131.
Ryan M. Rich Dorota L. Stankowska Badri P. Maliwal Thomas Just Sørensen Bo W. Laursen Raghu R. Krishnamoorthy Zygmunt Gryczynski Julian Borejdo Ignacy Gryczynski Rafal Fudala 《Analytical and bioanalytical chemistry》2013,405(6):2065-2075
Sample autofluorescence (fluorescence of inherent components of tissue and fixative-induced fluorescence) is a significant problem in direct imaging of molecular processes in biological samples. A large variety of naturally occurring fluorescent components in tissue results in broad emission that overlaps the emission of typical fluorescent dyes used for tissue labeling. In addition, autofluorescence is characterized by complex fluorescence intensity decay composed of multiple components whose lifetimes range from sub-nanoseconds to a few nanoseconds. For these reasons, the real fluorescence signal of the probe is difficult to separate from the unwanted autofluorescence. Here we present a method for reducing the autofluorescence problem by utilizing an azadioxatriangulenium (ADOTA) dye with a fluorescence lifetime of approximately 15 ns, much longer than those of most of the components of autofluorescence. A probe with such a long lifetime enables us to use time-gated intensity imaging to separate the signal of the targeting dye from the autofluorescence. We have shown experimentally that by discarding photons detected within the first 20 ns of the excitation pulse, the signal-to-background ratio is improved fivefold. This time-gating eliminates over 96 % of autofluorescence. Analysis using a variable time-gate may enable quantitative determination of the bound probe without the contributions from the background. 相似文献
132.
Friedel–Crafts acetylation at the 4-position of 3-phenylsydnone (1) was achieved via thermal heating overnight in moderate to good yields by employing various metal triflate catalysts (5–20 mol%), lithium perchlorate (0–20 mol%), and acetic anhydride (4 equivalents) using either acetonitrile or acetic anhydride, in excess, as solvent. Six commercially available, homogeneous metal triflate catalysts were investigated, and optimal conditions were determined. The best yields overall were achieved with indium triflate. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications ® to view the free supplemental file. 相似文献
133.
Richard F. Smith Laurie A. Olson William J. Ryan Karen J. Coffman Julienne M. Galante Timothy S. Wojdan 《合成通讯》2013,43(5):585-596
A simple, efficient procedure for the conversion of phenylhydrazine to 1-alkyl-1-phenylhydrazines via base-promoted hydrolysis of 1-alkyl-3-amino-4,5-dihydro-1-phenyl-1H-pyrazolium iodides (2) is described. N1, N1-disubstituted hydrazones of trans-cinnamamide and acrylamide are obtained by Hofmann-type ring openings of 1,1 -disubstituted-3-amino-4,5-dihydro-5-phenyl (or 5-unsubstituted) -1H-pyrazolium halides (5, 7). 相似文献
134.
Susann Paul W. Bernd Schweizer Graham Rugg Hans Martin Senn Ryan Gilmour 《Tetrahedron》2013,69(27-28):5647-5659
Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group, which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (?[NCCF]~60°) with respect to the triazolium nitrogen at the vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects and evaluate the influence of electronic modulation of the carbenic carbon [(CN+); neutral carbene (C:); metal-bound carbene (CM)], on the relative gauche/anti preference, thus highlighting the potential of this conformational phenomenon as a useful molecular design strategy for controlling the topology of organometallic complexes. 相似文献
135.
136.
Paul D. Sylvester Helen E. Ryan Craig D. Smith Aaron S. Micallef Carl H. Schiesser Uta Wille 《Polymer Degradation and Stability》2013
A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering. 相似文献
137.
138.
Dr. Lingui Zhu Dr. Shasha Liu Dr. Justin T. Douglas Prof. Dr. Ryan A. Altman 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(38):12800-12805
The conversion of an alcohol‐based functional group, into a trifluoromethyl analogue is a desirable transformation. However, few methods are capable of converting O‐based electrophiles into trifluoromethanes. The copper‐mediated trifluoromethylation of benzylic xanthates using Umemoto’s reagent as the source of CF3 to form C? CF3 bonds is described. The method is compatible with an array of benzylic xanthates bearing useful functional groups. A preliminary mechanistic investigation suggests that the C? CF3 bond forms by reaction of the substrate with in situ generated CuCF3 and CuOTf. Further evidence suggests that the reaction could proceed via a radical cation intermediate. 相似文献
139.
Ryan Gianatassio Shuhei Kawamura Cecil L Eprile Klement Foo Jason Ge Aaron C. Burns Michael R. Collins Phil S. Baran 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(37):10009-10013
A simple method to convert readily available carboxylic acids into sulfinate salts by employing an interrupted Barton decarboxylation reaction is reported. A medicinally oriented panel of ten new sulfinate reagents was created using this method, including a key trifluoromethylcyclopropanation reagent, TFCS‐Na. The reactivity of six of these salts towards C H functionalization was field‐tested using several different classes of heterocycles. 相似文献
140.
Ryan M. Tipker Jake A. Muldoon Daniel H. Pham Balazs R. Varga Russell P. Hughes David S. Glueck Gary J. Balaich Arnold L. Rheingold 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2022,134(1):e202110753
Tetrahedral main-group compounds are normally configurationally stable, but P-epimerization of the chiral phosphiranium cations syn- or anti-[Mes*P(Me)CH2CHPh][OTf] (Mes*=2,4,6-(t-Bu)3C6H2) occurred under mild conditions at 60 °C in CD2Cl2, resulting in isomerization to give a syn-enriched equilibrium mixture. Ion exchange with excess [NBu4][Δ-TRISPHAT] (Δ-TRISPHAT=Δ-P(o-C6Cl4O2)3) followed by chromatography on silica removed [NBu4][OTf] and gave mixtures of syn- and anti-[Mes*P(Me)CH2CHPh][Δ-TRISPHAT]?x[NBu4][Δ-TRISPHAT]. NMR spectroscopy showed that isomerization proceeded with epimerization at P and retention at C. DFT calculations are consistent with a mechanism involving P-C cleavage to yield a hyperconjugation-stabilized carbocation, pyramidal inversion promoted by σ-interaction of the P lone pair with the neighboring β-carbocation, and ring closure with inversion of configuration at P. 相似文献