全文获取类型
收费全文 | 41156篇 |
免费 | 1166篇 |
国内免费 | 153篇 |
专业分类
化学 | 26343篇 |
晶体学 | 263篇 |
力学 | 1035篇 |
综合类 | 9篇 |
数学 | 5826篇 |
物理学 | 8999篇 |
出版年
2023年 | 223篇 |
2022年 | 548篇 |
2021年 | 909篇 |
2020年 | 703篇 |
2019年 | 695篇 |
2018年 | 731篇 |
2017年 | 647篇 |
2016年 | 1190篇 |
2015年 | 1081篇 |
2014年 | 1126篇 |
2013年 | 2060篇 |
2012年 | 2381篇 |
2011年 | 2636篇 |
2010年 | 1518篇 |
2009年 | 1475篇 |
2008年 | 2244篇 |
2007年 | 2176篇 |
2006年 | 1935篇 |
2005年 | 2295篇 |
2004年 | 2170篇 |
2003年 | 1625篇 |
2002年 | 1261篇 |
2001年 | 823篇 |
2000年 | 732篇 |
1999年 | 489篇 |
1998年 | 433篇 |
1997年 | 417篇 |
1996年 | 535篇 |
1995年 | 425篇 |
1994年 | 418篇 |
1993年 | 429篇 |
1992年 | 406篇 |
1991年 | 352篇 |
1990年 | 287篇 |
1989年 | 290篇 |
1988年 | 247篇 |
1987年 | 230篇 |
1986年 | 236篇 |
1985年 | 302篇 |
1984年 | 306篇 |
1983年 | 233篇 |
1982年 | 261篇 |
1981年 | 248篇 |
1980年 | 230篇 |
1979年 | 201篇 |
1978年 | 237篇 |
1977年 | 182篇 |
1976年 | 173篇 |
1974年 | 150篇 |
1973年 | 144篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
991.
Hans-Jürgen?BuschmannEmail author Lucia?Mutihac Eckhard?Schollmeyer 《Journal of inclusion phenomena and macrocyclic chemistry》2005,51(1-2):53-57
The formation of complexes between α-cyclodextrin and n-alkylamines and their hydrochlorides has been studied in aqueous solution using calorimetric titrations. All alkylamines form stronger complexes than the corresponding hydrochlorides. The values of the reaction enthalpies are smaller for the alkylamine hydrochlorides compared with the alkylamines. By increasing the number of methylene groups, these differences become smaller. In addition, the reaction enthalpies for protonation of the alkylamines and their complexes with α-cyclodextrin have been measured. The heat of protonation of these complexes is always smaller compared with the alkylamines. Due to the protonation and the formation of a strong solvation shell around the ammonium group the interactions with α-cyclodextrins are weakened. From a thermodynamic cycle using all measured reactions, it can be concluded that the aggregation of the alkylamines with long alkyl chains (heptyl-, octyl-, and nonylamine) has an influence on the values of the reaction enthalpies and entropies for the protonated form only.This revised version was published online in July 2005 with a corrected issue number. 相似文献
992.
Characterisation of fatty acids in biological oil samples using comprehensive multidimensional gas chromatography 总被引:1,自引:0,他引:1
Comprehensive multidimensional gas chromatography can adequately resolve very complex mixtures of analytes such as the fatty acid mixtures which are contained in, e.g., fish and vegetable oils. Well-ordered patterns are obtained in the two-dimensional separation plane which can be used to tentatively identify peaks when no standard is available. The technique which can also be used for quantification, i.e., quantitative ratio analysis, should be especially useful for fingerprinting purposes. Unravelling the composition of complex mixtures such as fish oils appears to be highly rewarding. 相似文献
993.
Katrin?Anhalt Ines?Sprung Klaus?SchulzeEmail author 《Monatshefte für Chemie / Chemical Monthly》2003,134(12):1593-1606
Summary. Optically active dimethylcyclohexenones, potential building blocks for enantioselective syntheses of various naturally active substances, were prepared. These compounds were obtained by oxidation with KMnO4/Pb(OAc)4 or ozonolysis of the corresponding cyclopentenic precursors, followed by aldol condensation. During the course of the preparation intermediate diols and chiral polyfunctional carbonyl derivatives were separated and identified analytically. 相似文献
994.
A mathematical method for the calculation of the dead time (tm) in HPLC was evaluated using a computer simulation approach, in which artificial perturbations were introduced to Simulated homolog retention times. The calculation was based on a modified and extended Grobler and Bálisz (GB) method. Investigated wav how the precision of the calculated tM is affected by: statistical fluctuations in retention times and which, and how many homolog retention times are used. Based on these simulations a two-step procedure for the tM calculation is proposed: In the first step the linearity of log tR, n vs carbon number n is checked using as many homolog retention times as possible. The slope value bo of the first linear regression in the GB method is used for the selection of homolog retention times in the final tM calculation. In the second step the optimal selection of homologs is made and the final tM calculation is carried out. Guidelines for homolog selection are given. 相似文献
995.
W. D. Knight Walt A. de Heer Winston A. Saunders 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1986,3(2):109-114
Electronic shell structure, which was first recognized in sodium clusters, has been observed in alkali and noble metals, as well as in divalent and trivalent metals. Shell structure with modifications is expected to be broadly applicable to most metals. Features in the cluster abundance spectra and in the experimental dipole polarizabilities and ionization potentials correlate well with predictions of electronic level filling in spherical and spheroidal potential wells. The lack of precise quantitative agreement between experiment and theory for the response properties indicates necessary refinements in the self-consistent uniform background jellium model for clusters. 相似文献
996.
At room temperature, three of the six polymorphic compounds described are not present in their highest melting-point form, owing to enantiotropy. These three are triphenyltin chloride (3 modifications), 1-(4,4-dimethyl-2-oxo-3-tetrahydrofuryl)thiourea (4 modifications), and 2-aminoethyl diphenylborinate (3 modifications). In the case of 4-amino-1-(2-phenylethyl)-1,2,4-triazoline-5-thione, in addition to the absolutely stable modificationI, a further six unstable crystal forms have been observed. The three modifications of,-dimethyl-[1,1-biphenyl]-4-methanol are also monotropic in character. 2,9-Dimethyl-1,10-phenanthroline, the three polymorphic forms of which are unstable, is a special case, since it absorbs water from the air and changes into a semi-hydrate. 相似文献
997.
Series of 3-arylalkyl- or 3-alkylamino-4-(indol-1-yl)maleimides and bis(indol-1-yl)maleimides were synthesised. The cyclization of the 3-substituted 4-(indol-1-yl)maleimides under the action of acids resulted in the formation of diazepine[1,4] derivatives with indoline and maleimide nuclei annelated. These compounds readily produced the corresponding indolomaleimidodiazepines[1,4] after dehydrogenation. 相似文献
998.
Quanwei?Yu Jens?Fr?mmel Thomas?WolffEmail author Karel?Procházka 《Colloid and polymer science》2004,282(9):1039-1043
Solid stoichiometric complexes of [3,12]-ionene and dodecyl sulfate form upon reaction of the bromide of the ionene and the silver salt of dodecyl sulfate in methanol. IR, DSC, and TG investigations indicated that the solid complexes are stable between 30 and 120 °C. TG and DSC also showed that the complexes easily take up water at ambient conditions. These samples are optically isotropic. When exposed to an increased humidity they exhibit optical anisotropy, i.e., birefringence, which is caused by the formation of a hexagonal mesogenic phase. Mesogenicity is necessarily accompanied by a further uptake of water (4–5 H2O molecules per ionic unit), which is dependent on the relative humidity. The phase behavior as a function of temperature and controlled relative humidity was studied using birefringence measurements and polarizing microscopy. 相似文献
999.
Ghirardi Maria L. Togasaki Robert K. Seibert Michael 《Applied biochemistry and biotechnology》1997,63(1):141-151
Applied Biochemistry and Biotechnology - Photoproduction of H2 by green algae utilizes electrons originating from the photosynthetic oxidation of water and does not require metabolic intermediates.... 相似文献
1000.
Marialuisa Russo Gabriella Poggi Maria Luisa Navacchia Mila D'Angelantonio Salvatore S Emmi 《Research on Chemical Intermediates》2006,32(2):153-170
Chemical radiolytic oxidation induced by OH addition on 1-(2-furan-2-yl-5-hydroxy-6-hydroxymethyl-[1,3]dioxan-4-yl)-ethan-1,2-diol
(sorbitylfurfural, SF) leads, in the presence of controlled amounts of oxygen, to a permanent functional modification of the
target molecule. The yield of conversion reaches 60% of the starting material. LC-MS analysis allowed the identification,
as final products, of carboxylic acids, butenal and hydroxy-furan derivatives in which the sugar chain remains unbroken, while
the furanic ring is attacked first by OH and then by oxygen, giving in succession an intra-/inter-molecular rearrangement
of the allylperoxyl radicals thus formed. The proposed oxidation of the furanic ring envisages the peroxyl intermediates undergoing
mono- and/or bi-molecular reactions; a reaction path has been outlined and is reported here. The presence of unsaturated bonds
in the final products could provide a further site for radical scavenger activity. Therefore, the fast reaction with O2 and the rearrangement of the produced peroxyl radicals to species, which are likely to be effective OH-capturers, reinforces
the antioxidant ability of SF. 相似文献