全文获取类型
收费全文 | 316篇 |
免费 | 4篇 |
国内免费 | 2篇 |
专业分类
化学 | 183篇 |
晶体学 | 2篇 |
力学 | 5篇 |
数学 | 39篇 |
物理学 | 93篇 |
出版年
2020年 | 1篇 |
2019年 | 1篇 |
2017年 | 4篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 8篇 |
2013年 | 4篇 |
2012年 | 7篇 |
2011年 | 11篇 |
2010年 | 8篇 |
2009年 | 6篇 |
2008年 | 11篇 |
2007年 | 16篇 |
2006年 | 13篇 |
2005年 | 14篇 |
2004年 | 17篇 |
2003年 | 15篇 |
2002年 | 19篇 |
2001年 | 18篇 |
2000年 | 13篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1996年 | 13篇 |
1995年 | 6篇 |
1994年 | 9篇 |
1993年 | 11篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1931年 | 2篇 |
1928年 | 1篇 |
1927年 | 1篇 |
1914年 | 2篇 |
1912年 | 2篇 |
1909年 | 2篇 |
排序方式: 共有322条查询结果,搜索用时 15 毫秒
11.
A strategy for the stereoselective synthesis of all the possible diastereoisomers of the 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane (furofuran) lignans from a single dihydrofuran precursor is described. The key steps involve a diastereocontrolled templated cationic cyclization followed by stereoselective reduction of the resulting methyl glycoside. 相似文献
12.
Katritzky AR Wang Z Hall CD Akhmedov NG Shestopalov AA Steel PJ 《The Journal of organic chemistry》2003,68(23):9093-9099
Reactions of oximes 9, 17, and 19 with electrophiles 15a-f and 24 in the presence of anhydrous potassium carbonate or triethylamine give 2-substituted condensed ring oxazoles 10, 16a-c, 18a-d, 20a-c, and 25 in a new general route to these compounds. 相似文献
13.
Alberti G Brunet E Dionigi C Juanes O de La Mata MJ Rodríguez-Ubis JC Vivani R 《Angewandte Chemie (International ed. in English)》1999,38(22):3351-3353
The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space. 相似文献
14.
Merle A. Battiste James M. Coxon Gregory W. Simpson Peter J. Steel Alan J. Jones 《Tetrahedron》1984,40(16):3137-3144
The reaction of endo-tricyclo[3.2.1.02,4]oct-6-ene 1 with methanol in the presence of catalytic amounts of toluene-p-sulphonic acid has been shown to give 2-exo- and endo-methoxybicyclo[3.2.1]oct-3-ene (2c) and (2d) and 2-endo-methoxybicyclo[3.2.1]oct-6-ene (13). The formation of 2-exo- methoxybicyclo[3.2.1]oct-3-ene (2c), the major product of reaction, has been probed by deuterium labelling experiments and a series of 6-exo-7-exo- dideuterobicyclo[3.2.1]oct-3-enes synthesised for 2H, 1H and 13C NMR spectral analysis in order unambiguously to determine the stereochemistry of proton attack on endo-tricyclo[3.2.1 02,4]oct-6-ene (1). The formation of 2-exo-methoxybicyclo[3.2.1]oct-3-ene (2c) has been determined to involve corner protonation of the cyclopropyl moiety and skeletal rearrangement to an allylic cation with a small but measurable memory effect 相似文献
15.
16.
17.
The variation in the lifetime of flash-excited gaseous benzophenone with pressure and temperature indicates that (1) self-quenching is a relatively inefficient process for the long-lived emission, ksq = 9 × 105 M?1 s?1 (estimated from solution data) at 25°C and 1.2 × 107 M?1 s?1 at 170°C and (2) the lifetime decreases with increasing temperature as a result of photochemical and photophysical decay pathways which have significant activation energies. The importance of diffusion to the walls on lifetime measurements is discussed. 相似文献
18.
2-Aminopyridines 1a-c and 1-aminoisoquinoline with 1-chloromethylbenzotriazole give 2-amino-1-[alpha-benzotriazol-1-ylmethyl]pyridinium chlorides 2a-c and 1-amino-2-(alpha-benzotriazol-1-ylmethyl)isoquinolinium++ + chloride 12, respectively. Compounds 2a-c and 12 react with aryl aldehydes 3a-h to afford imidazolo[1,2-a]pyridines 7a-k and imidazolo[2, 1-a]isoquinolines 13a,b in good yields. 相似文献
19.
Fischer CJ Gafni A Steel DG Schauerte JA 《Journal of the American Chemical Society》2002,124(35):10359-10366
The interpretation of room temperature phosphorescence studies of proteins requires an understanding of the mechanisms governing the tryptophan triplet-state lifetimes of residues fully exposed to solvent and those deeply buried in the hydrophobic core of proteins. Since solvents exposed tryptophans are expected to behave similarly to indole free in solution, it is important to have an accurate measure of the triplet state lifetime of indole in aqueous solution. Using photon counting techniques and low optical fluence (J/cm(2)), we observed the triplet-state lifetime of aqueous, deoxygenated indole and several indole derivatives to be approximately 40 micros, closely matching the previous reports by Bent and Hayon based on flash photolysis (12 micros; Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2612-2619) but much shorter than the 1.2 ms lifetime observed more recently (Strambini, G. B.; Gonnelli, M. J. Am. Chem. Soc. 1995, 117, 7646-7651). However, we have now been able to reproduce the long lifetime reported by the latter workers for aqueous indole solutions and show that it likely arises from geminate recombination of the indole radical cation and solvated electron, a conclusion based on studies of the indole radical cation in water (Bent and Hayon, 1975). The evidence for this comes from a fast rise in the phosphorescence emission and measurements of a corresponding enhanced quantum yield in unbuffered solutions. This species can be readily quenched, and the corresponding fast rise disappears, leaving a monoexponential 40 micros decay, which we argue is the true indole triplet lifetime. The work is put in the context of room temperature phosphorescence studies of proteins. 相似文献
20.