全文获取类型
收费全文 | 4935篇 |
免费 | 195篇 |
国内免费 | 12篇 |
专业分类
化学 | 3804篇 |
晶体学 | 20篇 |
力学 | 55篇 |
数学 | 515篇 |
物理学 | 748篇 |
出版年
2022年 | 37篇 |
2021年 | 45篇 |
2020年 | 70篇 |
2019年 | 62篇 |
2018年 | 42篇 |
2016年 | 140篇 |
2015年 | 133篇 |
2014年 | 114篇 |
2013年 | 209篇 |
2012年 | 264篇 |
2011年 | 309篇 |
2010年 | 182篇 |
2009年 | 129篇 |
2008年 | 253篇 |
2007年 | 228篇 |
2006年 | 219篇 |
2005年 | 216篇 |
2004年 | 202篇 |
2003年 | 168篇 |
2002年 | 152篇 |
2001年 | 114篇 |
2000年 | 93篇 |
1999年 | 60篇 |
1998年 | 55篇 |
1997年 | 47篇 |
1996年 | 73篇 |
1995年 | 83篇 |
1994年 | 49篇 |
1993年 | 66篇 |
1992年 | 64篇 |
1991年 | 43篇 |
1990年 | 53篇 |
1989年 | 46篇 |
1988年 | 49篇 |
1987年 | 43篇 |
1986年 | 37篇 |
1984年 | 51篇 |
1982年 | 43篇 |
1981年 | 56篇 |
1980年 | 34篇 |
1979年 | 41篇 |
1978年 | 39篇 |
1977年 | 36篇 |
1976年 | 35篇 |
1974年 | 41篇 |
1973年 | 33篇 |
1967年 | 35篇 |
1966年 | 43篇 |
1964年 | 40篇 |
1963年 | 36篇 |
排序方式: 共有5142条查询结果,搜索用时 15 毫秒
51.
The electron-impact fragmentation of S-p-tolylester of nitrothiobenzoic acid leads to a previously unknown rearrangement in the case of the ortho compound. This consists of an intra molecular redoxreaction with oxygen transfer from the o-nitro group onto the sulphur of the thiol ester group. The position of the nitro group is not only an essential influence on the fragmentation pattern, but also of dramatic importance in determining the lifetime of the molecular ion. 相似文献
52.
Cooperative conformational transitions of proteins and nucleic acids are of decisive importance to many processes of molecular biology, and particularly to their regulation. They proceed via numerous interdependent elementary processes, and their kinetics are therefore often complicated. They are frequently also very fast. However, kinetic analyses can be carried out by chemical relaxation methods. The theoretical interpretation is comparatively simple in the case of linear biopolymers. When the linear Ising model extended for kinetics was applied to model peptides and polynucleotides, it provided an insight into the fundamental principles of cooperative transformations. 相似文献
53.
The dialysis potentials of different collodion membranes with graded pore sizes and electrochemical activities have been measured in dilute aqueous KCl solutions as functions of concentration. It is possible to predict the value of the diffusion potential within a few millivolts on the basis of electrical conductivity data obtained with the same membranes. In general, the measured values are lower than those calculated. It is assumed that this difference is caused by the membranes having a distribution of pore sizes. 相似文献
54.
The adsorption of Mo from dilute aqueous solutions (10(-3) to 3x10(-2) M) is effected on three samples of titania, two are anatase and the third is P25, which is composed of rutile and anatase. The adsorption isotherms at 298, 318, and 338 K are analyzed using a Langmuir linear equation. The isotherms on P25 showed a distinct inflection point that is reproduced by two linear portions, indicating different adsorption regimes. This adsorption behavior is explained as follows: with low amount adsorbed the adsorption is initiated by protonation of the basic hydroxyls on which the negatively charged MoO(4)(2-')s are adsorbed, and this is accompanied by an increase in the pH of the impregnating solution. At higher adsorption the coordinatively unsaturated Ti(4+) sites participate in the process, leading to a decline in the initial increase in pH. In the case of the two anatase samples the low surface area resulted in poor distribution of adsorption sites; consequently, the distinction between the two modes of adsorption was not entirely clear. The higher adsorption site density in the case of anatase is accompanied by a lower surface coverage, θ, than that for P25. The heat of adsorption, Q, on the three titania samples showed a linear increase with θ, which is represented by the regression equation: -Q=95.77θ-4.25 (R(2)=0.993). Copyright 2001 Academic Press. 相似文献
55.
Mass spectroscopic investigations of 2H- and 13C-labelled derivatives of e.g. 1-, 2-chloro-methyl-naphthalene and 1-chloro-phenyl(5)-pentene-(2)-in-(4) show that the [C11H9]+ ion, which gives [C9H7]+ by acetylene elimination, has the structure of a benztropylium cation. Model considerations show that the formation of this cation [C11H9]+ through a common transition state with a three-membered ring is very probable. 相似文献
56.
P. Palade G. Principi T. Spataru P. Blaha K. Schwarz V. Kuncser S. Lo Russo S. Dal Toé V. A. Yartys 《Journal of Radioanalytical and Nuclear Chemistry》2005,266(3):553-556
Summary LaNiSn and NdNiSn compounds and their deuterides have been studied by variable temperature 119Sn M?ssbauer spectroscopy. The hyperfine parameters obtained experimentally are in good agreement with those derived from
first principle calculations. The enlargement of quadrupole splitting observed for LaNiSn after deuteration confirms the lower
symmetry of electron density around tin atoms indicated by the calculation of partial Sn-p density of states (DOS). Magnetic
ordering is observed at low temperature in deuterided NdNiSn. 相似文献
57.
H. Schwarz 《Fresenius' Journal of Analytical Chemistry》1976,279(2):136-137
Ohne Zusammenfassung
Analytical importance of electron-impact induced retro-diels-alder reactions for stereochemical investigations相似文献
58.
Winfried Wagner-Redeker Karsten Levsen Helmut Schwarz Waltraud Zummack 《Journal of mass spectrometry : JMS》1981,16(8):361-371
Using the relative abundance of metastable ions, collisional activation spectra, field ionization kinetic measurements, isotopic labelling, appearance energy and kinetic energy release data, it is shown that linear alkyne radical cations with more than six carbon atoms do not isomerize to equilibrating structures prior to decomposition. At the shortest ion lifetimes the molecular ions of linear alkynes decompose mainly by simple β-bond fission which allows an unequivocal localization of the triple bond. At medium ion lifetimes fragmentation occurs predominantly via a McLafferty rearrangement, while at long ion lifetimes competing alkyl losses prevail. These alkyl losses occur via cyclic intermediates leading to thermochemically stable cycloalkenyl ions. All these reactions occur with a high specificity with respect to the carbon and hydrogen atoms involved and are preceded by little or no hydrogen exchange reactions. 相似文献
59.
[reaction: see text] Acid chlorides and aromatic aldehydes react in the presence of a stoichiometric amount of a tertiary amine and catalytic amounts of a cinchona alkaloid derivative and a Lewis acid to produce beta-lactones in high diastereo- and enantioselectivity. The sense of the diastereoselectivity depends on the substitution of the acid chloride, with the reactions of aliphatic acid chlorides giving predominantly the trans-isomer and those of alkoxyacetyl chlorides favoring formation of the cis-isomer. 相似文献
60.
The rearrangement of aminoethanol catalyzed by ethanolamine ammonia lyase is investigated by computational means employing DFT (B3LYP/6-31G) and ab initio molecular orbital theory (QCISD/cc-pVDZ). The study aims at providing a detailed account on various crucial aspects, in particular a distinction between a direct intramolecular migration of the partially protonated NH(2) group vs elimination of NH(4)(+). Three mechanistic scenarios were explored: (i) According to the calculations, irrespective of the nature of the protonating species, intramolecular migration of the NH(3) group is energetically less demanding than elimination of NH(4)(+). However, all computed activation enthalpies exceed the experimentally derived activation enthalpy (15 kcal/mol) associated with the rate-determining step, i.e., the hydrogen abstraction from the 5'-deoxyadenosine by the product radical. For example, when imidazole is used as a model system for His interacting with the NH(3) group of the substrate, the activation enthalpy for the migration process amounts to 27.4 kcal/mol. If acetic acid is employed to mimic Asp or Glu, the activation enthalpy is somewhat lower, being equal to 24.2 kcal/mol. (ii) For a partial deprotonation of the substrate 2 at the OH group, the rearrangement mechanism consists of the dissociation of an NH(2) radical from C(2) and its association at C(1) atom. For all investigated proton acceptors (i.e., OH(-), HCOO(-), CH(3)COO(-), CH(2)NH, imidazole), the activation enthalpy for the dissociation step also exceeds 15 kcal/mol. Typical data are 20.2 kcal/mol for Ac(-) and 23.8 kcal/mol for imidazole. (iii) However, in a synergistic action of partial protonation of the NH(2) group and partial deprotonation of the OH group by the two conceivable catalytic auxiliaries Asp/Glu and His, the activation enthalpy computed is compatible with the experimental data. For imidazole and acetate as model systems, the activation enthalpy is equal to 13.7 kcal/mol. This synergistic action of the two catalytic groups is expected to take place in a physiologically realistic pH range of 6-9.5, and the present computational findings may help to further characterize the yet unknown structural details of the ethanolamine ammonia lyase's active site. 相似文献