全文获取类型
收费全文 | 1526篇 |
免费 | 37篇 |
国内免费 | 5篇 |
专业分类
化学 | 1140篇 |
晶体学 | 13篇 |
力学 | 19篇 |
数学 | 215篇 |
物理学 | 181篇 |
出版年
2022年 | 11篇 |
2021年 | 16篇 |
2020年 | 25篇 |
2019年 | 14篇 |
2018年 | 13篇 |
2017年 | 8篇 |
2016年 | 20篇 |
2015年 | 25篇 |
2014年 | 35篇 |
2013年 | 60篇 |
2012年 | 84篇 |
2011年 | 88篇 |
2010年 | 57篇 |
2009年 | 38篇 |
2008年 | 70篇 |
2007年 | 63篇 |
2006年 | 72篇 |
2005年 | 84篇 |
2004年 | 70篇 |
2003年 | 57篇 |
2002年 | 55篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 16篇 |
1998年 | 14篇 |
1997年 | 13篇 |
1996年 | 11篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 31篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 19篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1984年 | 13篇 |
1983年 | 18篇 |
1982年 | 24篇 |
1981年 | 15篇 |
1980年 | 19篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 15篇 |
1975年 | 10篇 |
1974年 | 15篇 |
1973年 | 16篇 |
1970年 | 9篇 |
1882年 | 7篇 |
排序方式: 共有1568条查询结果,搜索用时 31 毫秒
941.
A new design of pulse-contrast-measuring correlator, based on optical parametric amplification, is presented. It amplifies the wings of the pulse and allows high-contrast measurement with a low-sensitivity detector. The test system demonstrates an ability to measure a dynamic range of >10(11):1. 相似文献
942.
Zhou Tang Dr. Edwin Otten Prof. Dr. Joost N. H. Reek Dr. Ir. Jarl Ivar van der Vlugt Prof. Dr. Bas de Bruin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(36):12683-12693
Ligand cooperativity provides (transition) metal complexes with new reactivities in substrate activation and catalytic reactions, but usually the ligand acts as an internal (Brønsted) base, while the metal acts as a (Lewis) acid. We describe the synthesis and stepwise activation of a new phosphane‐pyridine‐amide ligand PNNH2 in combination with RhI. The ligand is susceptible to stepwise proton and hydride loss from the nitrogen arm (imine formation) and deprotonation at the pyridylphosphine arm (dearomatization), giving rise to amine complex 1 , amido species 2 , imine complex 3 and dearomatized compound 4 . Complex 4 bears a dual‐mode cooperative PNN′ ligand containing both a (nucleophilic) basic methine fragment and a reactive (electrophilic) imine moiety. The basic ligand arm enables substrate deprotonation while the imine ligand arm enables reversible “storage” of the activated (nucleophilic) form of a sulfonamide substrate at the ligand. In combination with metal‐based reactivity, this allows for the mono‐alkylation of o‐toluenesulfonamide with iodomethane. Compounds 1 , 3 and 4 are structurally characterized. We also report the first structurally characterized example of an aminal in the coordination sphere of rhodium, complex 5 , [Rh(CO)( PNN′′ )], formed by sequential N?H activation of sulfonamide by the dearomatized ligand PNN′ and follow‐up nucleophilic attack of anionic sulfonamide onto the imine fragment. 相似文献
943.
Cyclic anhydrides such as naphtho[2,3-c]furan-1,3-dione derivatives were synthesized from the reaction of arylpropiolic acids and 2-chloro-4,6-dimethoxy-1,3,5-triazine in the presence of N-methylmorpholine at room temperature. This mild condition provided the naphtho[2,3-c]furan-1,3-dione derivatives in good yields. Spectroscopic analysis suggested that the formation of arylpropiolate is the rate-determining step. 相似文献
944.
Hendrik Klien Wilhelm Seichter Konstantinos Skobridis Edwin Weber 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(9):768-775
Having reference to an elongated structural modification of 2,2′‐bis(hydroxydiphenylmethyl)biphenyl, (I), the two 1,1′:4′,1′′‐terphenyl‐based diol hosts 2,2′′‐bis(hydroxydiphenylmethyl)‐1,1′:4′,1′′‐terphenyl, C44H34O2, (II), and 2,2′′‐bis[hydroxybis(4‐methylphenyl)methyl]‐1,1′:4′,1′′‐terphenyl, C48H42O2, (III), have been synthesized and studied with regard to their crystal structures involving different inclusions, i.e. (II) with dimethylformamide (DMF), C44H34O2·C2H6NO, denoted (IIa), (III) with DMF, C48H42O2·C2H6NO, denoted (IIIa), and (III) with acetonitrile, C48H42O2·CH3CN, denoted (IIIb). In the solvent‐free crystals of (II) and (III), the hydroxy H atoms are involved in intramolecular O—H...π hydrogen bonding, with the central arene ring of the terphenyl unit acting as an acceptor. The corresponding crystal structures are stabilized by intermolecular C—H...π contacts. Due to the distinctive acceptor character of the included DMF solvent species in the crystal structures of (IIa) and (IIIa), the guest molecule is coordinated to the host via O—H...O=C hydrogen bonding. In both crystal structures, infinite strands composed of alternating host and guest molecules represent the basic supramolecular aggregates. Within a given strand, the O atom of the solvent molecule acts as a bifurcated acceptor. Similar to the solvent‐free cases, the hydroxy H atoms in inclusion structure (IIIb) are involved in intramolecular hydrogen bonding, and there is thus a lack of host–guest interaction. As a result, the solvent molecules are accommodated as C—H...N hydrogen‐bonded inversion‐symmetric dimers in the channel‐like voids of the host lattice. 相似文献
945.
946.
Highly substituted, tethered alkyne dipolarophiles participate in the internal 2 + 3 cycloaddition with azomethine ylides generated by treatment of oxazolium salts with cyanide ion. Starting from oxazole 26, a sequence of N-methylation, cyanide addition, and electrocyclic ring opening of a 4-oxazoline intermediate affords the indoloquinone 31 in a one-pot process. A similar reaction from the protected alkynol derivative 25 affords the sensitive, but isolable, enone 32, and subsequent oxidation affords 31 and the deprotected quinone alcohol 34. Related azomethine cycloaddition methodology via intramolecular oxazolium salt formation from 43 or 46 is also demonstrated and allows the synthesis of quinone 45 and derived structures having the substitution pattern of aziridinomitosene A. Removal of the N-trityl protecting group could not be achieved without aziridine cleavage. 相似文献
947.
Wang J Kubicki J Hilinski EF Mecklenburg SL Gustafson TL Platz MS 《Journal of the American Chemical Society》2007,129(44):13683-13690
Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.9 ps in acetonitrile and decays to the lower energy singlet state (1Fl), which relaxes to the triplet ground state (3Fl) in acetonitrile, cyclohexane, benzene, and hexafluorobenzene. The equilibrium mixture of singlet and triplet fluorenylidene reacts with these solvents. Singlet fluorenylidene reacts with methanol and cyclohexene in competition with relaxation to 3Fl. One of the reaction products in methanol is the 9-fluorenyl cation. The rate of intersystem crossing (ISC) in hexafluorobenzene and other halogenated solvents is remarkably slow given that carbene ISC rates are generally fastest in nonpolar solvents. An explanation of this effect is advanced. 相似文献
948.
949.
Clifford JN Bell TD Tinnefeld P Heilemann M Melnikov SM Hotta J Sliwa M Dedecker P Sauer M Hofkens J Yeow EK 《The journal of physical chemistry. B》2007,111(25):6987-6991
The single-molecule fluorescence blinking behavior of the organic dye Atto647N in various polymer matrixes such as Zeonex, PVK, and PVA as well as aqueous media was investigated. Fluorescence blinking with off-times in the millisecond to second time range is assigned to dye radical ions formed by photoinduced electron transfer reactions from or to the environment. In Zeonex and PVK, the measured off-time distributions show power law dependence, whereas, in PVA, no such dependence is observed. Rather, in this polymer, off-time distributions can be best fitted to monoexponential or stretched exponential functions. Furthermore, treatment of PVA samples to mild heating and low pressure greatly reduces the frequency of blinking events. We tentatively ascribe this to the removal of water pockets within the polymer film itself. Measurements of the dye immobilized in water in the presence of methylviologen, a strongly oxidizing agent, reveal simple exponential on- and off-time distributions. Thus, our data suggest that the blinking behavior of single organic molecules is sensitive to their immediate environment and, moreover, that fluorescence blinking on- and off-time distributions do not inherently and uniquely obey a power law. 相似文献
950.
Moya SE Azzaroni O Kelby T Donath E Huck WT 《The journal of physical chemistry. B》2007,111(25):7034-7040
The conformational behavior of poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes with different chain density in the presence of large benzyltributylammonium chloride (BTBAC) ions has been studied by a Quartz Crystal Microbalance with Dissipation (QCM-D) and Scanning Force Microscopy. Dense brushes do not collapse in the presence of BTBAC solutions of increasing ionic strength, contrary to what is observed in the presence of NaCl. Brush collapse can be observed for low-ionic-strength solutions of BTBAC when the brush density has been reduced. These phenomena can be explained by considering the Hofmeister series as well as ion size and free space in the brush. 相似文献