全文获取类型
收费全文 | 4634篇 |
免费 | 148篇 |
国内免费 | 29篇 |
专业分类
化学 | 3055篇 |
晶体学 | 15篇 |
力学 | 216篇 |
数学 | 781篇 |
物理学 | 744篇 |
出版年
2023年 | 21篇 |
2022年 | 73篇 |
2021年 | 74篇 |
2020年 | 61篇 |
2019年 | 71篇 |
2018年 | 56篇 |
2017年 | 59篇 |
2016年 | 128篇 |
2015年 | 115篇 |
2014年 | 146篇 |
2013年 | 254篇 |
2012年 | 325篇 |
2011年 | 385篇 |
2010年 | 202篇 |
2009年 | 211篇 |
2008年 | 270篇 |
2007年 | 277篇 |
2006年 | 296篇 |
2005年 | 281篇 |
2004年 | 224篇 |
2003年 | 218篇 |
2002年 | 200篇 |
2001年 | 66篇 |
2000年 | 71篇 |
1999年 | 45篇 |
1998年 | 60篇 |
1997年 | 65篇 |
1996年 | 62篇 |
1995年 | 44篇 |
1994年 | 52篇 |
1993年 | 34篇 |
1992年 | 27篇 |
1991年 | 24篇 |
1990年 | 22篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1986年 | 11篇 |
1985年 | 25篇 |
1984年 | 19篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 21篇 |
1979年 | 22篇 |
1978年 | 19篇 |
1977年 | 11篇 |
1976年 | 15篇 |
1975年 | 12篇 |
1974年 | 9篇 |
排序方式: 共有4811条查询结果,搜索用时 46 毫秒
81.
Dolain C Jiang H Léger JM Guionneau P Huc I 《Journal of the American Chemical Society》2005,127(37):12943-12951
Chiral groups attached to the end of quinoline-derived oligoamide foldamers give rise to chiral helical induction in solution. Using various chiral groups, diastereomeric excesses ranging from 9% to 83% could be measured by NMR and circular dichroism. Despite these relatively weak values and the fact that diastereomeric helices coexist and interconvert in solution, the right-handed or left-handed helical sense favored by the terminal chiral group could be determined unambiguously using X-ray crystallography. Assignment of chiral induction was performed in an original way using the strong tendency of racemates to cocrystallize, and taking advantage of slow helix inversion rates, which allowed one to establish that the stereomers observed in the crystals do correspond to the major stereomers in solution. The sense of chiral helical induction was rationalized on the basis of sterics. Upon assigning an Rs or Ss chirality to the stereogenic center using a nomenclature where the four substituents are ranked according to decreasing sizes, it is observed that Rs chirality always favors left-handed helicity and Ss chirality favors right-handed helicity (P). X-ray structures shed some light on the role of sterics in the mechanism of chiral induction. The preferred conformation at the stereocenter is apparently one where the bulkiest group should preferentially point away from the helix, the second largest group should be aligned with the helix backbone, and the smallest should point to the helix. 相似文献
82.
Two polyesters containing rigid biphenyl and hydroquinone bisbenzoate groups and presenting flexible thioether moieties with different lengths (14 elements in the flexible group) have been carried out using a reaction of the Michaël type. The properties of these polyesters have been compared to those of polyesters of the same type presenting shorter flexible groups (11 elements). All these polymers present thermotropic properties: the biphenyl ones are smectic and the bisbenzoate ones are nematic. The biphenyl polyesters present two types of dielectric relaxations: α and β. The bisbenzoate ones show three relaxations, α and two β (β and β′). The lengthening of the flexible group increases significantly the flexibility of the molecular chains. 相似文献
83.
Capillary electrophoresis - mass spectrometry: survey on developments and applications 2003-2004 总被引:1,自引:0,他引:1
The major developments and applications related to CE-MS over the last two years (2003-2004) and most of the reviews and applications found in the ISI Web of Science and publisher data bases are presented in a tabulated way. This article complements our previous review "Capillary electrophoresis - mass spectrometry: 15 years of developments and applications", Electrophoresis, 2003, 24, 3837-3867 for the last two years 2003-2004. All cited articles were analyzed in a way to illustrate (i) in which journals CE-MS-related papers were mostly found over the last decades and (ii) which commercial CE-, MS-instrumentations or CE-MS combinations were mostly used in the European, Asian, and American continent. Additionally, like it was done in our last review, the reader will rapidly find applications classified as forensics, environment, bioanalytics, pharmaceutics, and metabolites. 相似文献
84.
Bocquet ML Michaelides A Loffreda D Sautet P Alavi A King DA 《Journal of the American Chemical Society》2003,125(19):5620-5621
Reaction mechanisms and activation energies for the complete conversion of ethene to ethene epoxide on two recently characterized oxidized Ag{111} surfaces have been determined from density functional theory. On both surfaces, epoxidation proceeds through a two-step nonconcerted mechanism via an oxametallacycle intermediate. The key implications are that both surfaces are active and that epoxidation can take place over a wide O coverage regime. 相似文献
85.
The major flavonoids present in the leaves and flowers of the cannabinoid-free cannabis (Cannabis sativa subsp. sativa L.) cultivars Felina and Futura are orientin (1), vitexin (2), luteolin-7-O-beta-D-glucuronide (3), and apigenin-7-O-beta-D-glucuronide (4), while prenylated flavonoids, to which the potent estrogenicity of hops (Humilus lupulus L.) is associated, are absent. The different composition of flavonoids has chemotaxonomic value. 相似文献
86.
Information
Congresses, Conferences, Symposia, Meetings, and Seminars in the Field of Chemical Sciences held in 1995–1996 相似文献87.
Dowding PJ Atkin R Vincent B Bouillot P 《Langmuir : the ACS journal of surfaces and colloids》2004,20(26):11374-11379
Microcapsules with an oil core surrounded by a polymeric shell have been prepared by the controlled phase separation of polymer dissolved within the oil droplets of an oil-in-water emulsion. The dispersed oil phase consists of the shell polymer (polystyrene), a good solvent for the polymer (dichloromethane), and a poor solvent for the polymer (typically hexadecane). Removal of the good solvent results in phase separation of the polymer within the oil droplets. If the three interfacial tensions between the core oil, the shell-forming polymer, and the continuous phase are of the required relative magnitudes, a polymer shell forms surrounding the poor solvent. A UV-responsive organic molecule was added to the oil phase, prior to emulsification, to investigate the release of a model active ingredient from the microcapsules. This molecule should be soluble in the organic core but also have some water solubility to provide a driving force for release into the continuous aqueous phase. As the release rate of the active ingredient is a function of the thickness of the polymeric shell, for controlled release applications, it is necessary to control this parameter. For the preparative method described here, the thickness of the shell formed is directly related to the mass of polymer dissolved in the oil phase. The rate of volatile solvent removal influences the porosity of the polymer shell. Rapid evaporation leads to cracks in the shell and a relatively fast release rate of the active ingredient. If a more gentle evaporation method is employed, the porosity of the polymer shell is decreased, resulting in a reduction in release rate. Cross-linking the polymer shell after capsule formation was also found to decrease both the release rate and the yield of the active ingredient. The nature of the oil core also affected the release yield. 相似文献
88.
Antoine R Broyer M Dugourd P Breaux G Hagemeister FC Pippen D Hudgins RR Jarrold MF 《Journal of the American Chemical Society》2003,125(30):8996-8997
Molecular beam electric deflection measurements have been used to determine electric susceptibilities for small unsolvated alanine-based peptides. The electric susceptibility provides information about the charge distribution within the peptide and can be used to distinguish between zwitterionic and canonical forms. Measured electric susceptibilities for WAn peptides (n = 1-5) are similar to those for capped Ac-WAn-NH2 peptides (which cannot form zwitterions). Susceptibilities calculated using a simulated tempering-based approach are substantially larger for the zwitterionic form than for the canonical form. The measured susceptibilities are in good agreement with those calculated for the canonical form. For the larger peptides, the lowest potential energy structure found in the simulations is hairpin-like, while the lowest free energy structure found at room temperature is extended. The zwitterionic form is constrained by intramolecular interactions which make it entropically unfavorable. 相似文献
89.
Bytchenkoff D Pelupessy P Bodenhausen G 《Journal of the American Chemical Society》2005,127(14):5180-5185
A new method has been developed to obtain dynamic and structural information about peptide planes in proteins by a combination of measurements of weak short-range cross-correlation rates R(H(N)N/NC') that are due to concerted fluctuations of the H(N)-N and N-C' dipole-dipole interactions and stronger long-range cross-correlation rates R(C'H(N)/H(N)N) and R(NH(N)/H(N)C(alpha)). The rates were interpreted using the axially symmetric Gaussian axial fluctuation model (GAF). The oscillation amplitudes as well as the positions of H(N) atoms with respect to peptide planes in ubiquitin were determined. Most N-H(N) bonds were found not to lie exactly along the bisector of the N-C' and N-C(alpha) bonds but to be slightly tilted toward the carbon-terminal side of the peptide. 相似文献
90.
The bicyclic monoselenoacetal 7 , easily obtained from (±)-7-oxabicyclo[2.2.1]hept-5-en-2-one ( 6 ) via a radical addition-acyl migration sequence, was converted to racemic 12-epiprostaglandins 3 and 4 . The key intermediate was the all-cis-formyllactone 2b related to Corey lactone (see 12 ; Scheme 1). The presence of a (tert-butyl)-dimethylsilyl protective group for the 11-OH substituent (prostaglandin numbering) was found to be crucial in avoidingβ -elimination and epimerization during the Wittig-Horner reaction (Scheme 2). Epimerization at C(12) at the formyllactone stage (see 2b ) was also possible and gave the known precursor 1b of naturally occurring prostaglandins and analogs. 相似文献