全文获取类型
收费全文 | 907篇 |
免费 | 14篇 |
国内免费 | 1篇 |
专业分类
化学 | 458篇 |
晶体学 | 7篇 |
力学 | 31篇 |
数学 | 89篇 |
物理学 | 337篇 |
出版年
2021年 | 7篇 |
2019年 | 6篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 7篇 |
2015年 | 9篇 |
2014年 | 13篇 |
2013年 | 41篇 |
2012年 | 39篇 |
2011年 | 45篇 |
2010年 | 20篇 |
2009年 | 13篇 |
2008年 | 42篇 |
2007年 | 34篇 |
2006年 | 37篇 |
2005年 | 43篇 |
2004年 | 33篇 |
2003年 | 32篇 |
2002年 | 32篇 |
2001年 | 24篇 |
2000年 | 17篇 |
1999年 | 10篇 |
1998年 | 8篇 |
1997年 | 12篇 |
1996年 | 17篇 |
1995年 | 12篇 |
1994年 | 28篇 |
1993年 | 21篇 |
1992年 | 32篇 |
1991年 | 17篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1974年 | 10篇 |
1973年 | 13篇 |
1972年 | 6篇 |
1970年 | 8篇 |
1969年 | 6篇 |
排序方式: 共有922条查询结果,搜索用时 15 毫秒
171.
Salvi L Jeon SJ Fisher EL Carroll PJ Walsh PJ 《Journal of the American Chemical Society》2007,129(51):16119-16125
A one-pot method for the direct preparation of enantioenriched (Z)-disubstituted allylic alcohols is introduced. Hydroboration of 1-halo-1-alkynes with dicyclohexylborane, reaction with t-BuLi, and transmetalation with dialkylzinc reagents generate (Z)-disubstituted vinylzinc intermediates. In situ reaction of these reagents with aldehydes in the presence of a catalyst derived from (-)-MIB generates (Z)-disubstituted allylic alcohols. It was found that the resulting allylic alcohols were racemic, most likely due to a rapid addition reaction promoted by LiX (X = Br and Cl). To suppress the LiX-promoted reaction, a series of inhibitors were screened. It was found that 20-30 mol % tetraethylethylenediamine inhibited LiCl without inhibiting the chiral zinc-based Lewis acid. In this fashion, (Z)-disubstituted allylic alcohols were obtained with up to 98% ee. The asymmetric (Z)-vinylation could be coupled with tandem diastereoselective epoxidation reactions to provide epoxy alcohols and allylic epoxy alcohols with up to three contiguous stereogenic centers, enabling the rapid construction of complex building blocks with high levels of enantio- and diastereoselectivity. 相似文献
172.
Krow GR Huang Q Szczepanski SW Hausheer FH Carroll PJ 《The Journal of organic chemistry》2007,72(9):3458-3466
Diene substituent effects on the regiochemical and stereochemical outcomes of uncatalyzed Diels-Alder reactions of N-alkoxycarbonyl-1,2-dihydropyridines with both styrene and methyl vinyl ketone (MVK) were studied. Alkyl substitution on the diene in all cases examined resulted in a kinetic preference for 7-endo isomers (7-phenyl 51-96% exo and 7-acetyl 54-96% exo). For both dienophiles, the highest stereoselectivities (>or=89% endo) were observed with 5-methyl or 6-methyl substituents in the dihydropyridine. Theoretical calculations of the energies of gas phase endo and exo transition states at the RHF/3-21G(*) predict that total entropy, DeltaStotal, considerations favor endo cycloadducts for both dienophiles with DHP, while total energy considerations, DeltaEo, favor endo cycloadducts for styrene and exo cycloadducts for MVK. At this level, favored endo-phenyl isomers are correctly predicted for styrene reactions, but the calculation of 7-acetyl exo or endo isomer dominance is diene-substituent-dependent for MVK reactions. The preference for endo addition of MVK to the parent, 5-methyl, and 6-methyl-DHPs was successfully predicted by calculations at the B3LYP/6-31G* theory level. 相似文献
173.
Shibasaki's heterobimetallic complexes M3(THF)n(BINOLate)3Ln [M = Li, Na, K; Ln = lanthanide(III)] are among the most successful asymmetric Lewis acid catalysts. Why does M3(THF)n(BINOLate)3Ln readily bind substrates when M = Li but not when M = Na or K? Structural studies herein indicate Na- and K-C cation-pi interactions and alkali metal radius may be more important than even lanthanide radius. Also reported is a novel polymeric [K3(THF)2(BINOLate)3Yb]n structure that provides the first evidence of interactions between M3(THF)n(BINOLate)3Ln complexes. 相似文献
174.
DC Muddiman AP Null JC Hannis 《Rapid communications in mass spectrometry : RCM》1999,13(12):1201-1204
Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) has been used to determine the mass of a double-stranded 500 base-pair (bp) polymerase chain reaction (PCR) product with an average theoretical mass of the blunt-ended (i.e. unadenylated) species of 308 859.35 Da. The PCR product was generated from the linearized bacteriophage Lambda genome which is a double-stranded template. Utilization of ethanol precipitation in tandem with a rapid microdialysis step to purify and desalt the PCR product was crucial to obtain a precise mass measurement. The PCR product (0.8 pmol/μL) was electrosprayed from a solution containing 75% acetonitrile, 25 mM piperidine, and 25 mM imidazole and was infused at a rate of 200 nL/min. The average molecular mass and the corresponding precision were determined using the charge-states ranging from 172 to 235 net negative charges. The experimental mass and corresponding precision (reported as the 95% confidence interval of the mean) was 309 406 +/- 27 Da (87 ppm). The mass accuracy was compromised due to the fact that the PCR generates multiple products when using Taq polymerase due to the non-template directed 3'-adenylation. This results in a mixture of three PCR products with nearly identical mass (i.e. blunt-ended, mono-adenylated and di-adenylated) with unknown relative abundances that were not resolved in the spectrum. Thus, the experimental mass will be a weighted average of the three species which, under our experimental conditions, reflects a nearly equal concentration of the mono- and di-adenylated species. This report demonstrates that precise mass measurements of PCR products up to 309 kDa (500 bp) can be routinely obtained by ESI-FTICR requiring low femtomole amounts. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
175.
176.
Xiangjun Chen James A. Carroll Ronald C. Beavis 《Journal of the American Society for Mass Spectrometry》1998,9(9):885-891
The matrix-assisted laser desorption/ionization (MALDI) effect for the ablation of polypeptide ions was studied as a function of wavelength in the range 360–450 nm, using a tunable titanium:sapphire laser and the matrices (trans)α-cyano-4-hydroxycinnamic acid, (trans)3,5-dimethoxy-4-hydroxycinnamic acid, and (trans)β-indoleacrylic acid. The results demonstrated that the qualitative aspects of the MALDI effect remained largely unchanged as a function of wavelength until the matrix crystal’s absorption coefficient approached 6 × 104 cm?1. Wavelengths at which the absorption coefficient was lower than this value did not produce observable polypeptide ions. In the range of wavelengths where polypeptide ions could be observed, the effect was generated only when the average energy absorbed in the 10-nm layer immediately below the crystal surface was >8 kJ/cm3 regardless of the total amount of energy absorbed in the remainder of the crystal. It was demonstrated that on the basis of the other results in this paper and those in the literature that the mechanism that generates polypeptide ions cannot be either photochemically induced or the same mechanism that gives rise to matrix ions. A model for ionization is proposed that decouples matrix and protein ionization in a simple manner that has direct analogy to the behavior of ionic solutes in polar solvents. The model satisfies the currently available experimental evidence and unifies the results obtained by MALDI at all wavelengths. 相似文献
177.
Engle JM Lakshminarayanan PS Carroll CN Zakharov LN Haley MM Johnson DW 《Crystal growth & design》2011,11(11):5144-5152
The conformations of 2,6-bis(2-anilinoethynyl)pyridine-based urea receptors were studied by single crystal X-ray diffraction methods and revealed a rich conformational flexibility influenced by solvents. Whereas receptor L(1) in DMSO prefers an "S" conformation, receptor L(1) crystallizes in an "O" conformation from DMSO/CH(3)OH binary solvent system, and a "W" conformation in the ternary solvent mixture DMSO/toluene/1,4-dioxane. In the case of L(2), the molecule adopts an "S" conformation where water molecules are sandwiched between two molecules of L(2) to form a dimer. Similar to L(2), L(3) also forms a dimer where water molecules are sandwiched between L(3) molecules, which are capped with two molecules of DMSO. Such a capping DMSO solvate is lacking in the case of L(2). Taken together, these results demonstrate that the conformation of 2,6-bis(2-anilinoethynyl) pyridine-based urea receptors can be dramatically manipulated and tuned by the choice of crystallization solvents. 相似文献
178.
B. Dromey D. Adams S. Kar C. Bellei D. C. Carroll R. J. Clarke J. S. Green S. Kneip K. Markey S. R. Nagel P. T. Simpson L. Willingale P. McKenna D. Neely Z. Najmudin K. Krushelnick P. A. Norreys M. Zepf 《The European physical journal. Special topics》2009,175(1):57-60
X-ray harmonic radiation extending to 3.3 Å, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to ~3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy hν > 1 keV is found to be into a cone angle < 4°, significantly less than that of the incident laser cone (20°). 相似文献
179.
Ondrej Nikel Ann M. Anderson Mary K. Carroll William D. Keat 《Journal of Non》2011,357(16-17):3176-3183
Aerogels are unique materials offering a combination of remarkable properties that make them useful in a wide range of applications. However, aerogel materials can be difficult to work with because they are fragile. The intent of the work presented here was to study the relationship between axial loading and pore structure in aerogel material. Silica aerogel samples with a bulk density of 0.1 g/mL were compressed by uni-axial force loads from 1 to 5 kN which resulted in stress levels up to 23 MPa. The resulting change in the pore distribution was observed using nitrogen desorption analysis and scanning electron microscopy. Uncompressed aerogel samples exhibit peak pore volume at diameters of about 20 nm. As the aerogels are subjected to increased loading, the location of the peak volume moves to smaller diameters with a reduced volume of pores occurring above this diameter. The peak diameter, the average pore diameter and pore volume all decrease and scale with increasing maximum stress while the surface area of the aerogel samples remains unaffected at about 520 m2/g. When combined with data from the literature, the relation between maximum pore diameter and applied stress suggests a failure mechanism dominated by bending induced fracture. 相似文献
180.
Teena Carroll David Galvin Prasad Tetali 《Journal of Combinatorial Theory, Series A》2009,116(7):1219-1227
We use an entropy based method to study two graph maximization problems. We upper bound the number of matchings of fixed size ℓ in a d-regular graph on N vertices. For bounded away from 0 and 1, the logarithm of the bound we obtain agrees in its leading term with the logarithm of the number of matchings of size ℓ in the graph consisting of disjoint copies of Kd,d. This provides asymptotic evidence for a conjecture of S. Friedland et al. We also obtain an analogous result for independent sets of a fixed size in regular graphs, giving asymptotic evidence for a conjecture of J. Kahn. Our bounds on the number of matchings and independent sets of a fixed size are derived from bounds on the partition function (or generating polynomial) for matchings and independent sets. 相似文献