全文获取类型
收费全文 | 4821篇 |
免费 | 253篇 |
国内免费 | 17篇 |
专业分类
化学 | 3740篇 |
晶体学 | 20篇 |
力学 | 86篇 |
数学 | 480篇 |
物理学 | 765篇 |
出版年
2023年 | 43篇 |
2022年 | 111篇 |
2021年 | 185篇 |
2020年 | 146篇 |
2019年 | 127篇 |
2018年 | 62篇 |
2017年 | 64篇 |
2016年 | 192篇 |
2015年 | 191篇 |
2014年 | 173篇 |
2013年 | 258篇 |
2012年 | 366篇 |
2011年 | 418篇 |
2010年 | 234篇 |
2009年 | 193篇 |
2008年 | 319篇 |
2007年 | 296篇 |
2006年 | 277篇 |
2005年 | 204篇 |
2004年 | 224篇 |
2003年 | 175篇 |
2002年 | 164篇 |
2001年 | 37篇 |
2000年 | 44篇 |
1999年 | 34篇 |
1998年 | 29篇 |
1997年 | 26篇 |
1996年 | 31篇 |
1995年 | 32篇 |
1994年 | 28篇 |
1993年 | 11篇 |
1992年 | 20篇 |
1991年 | 17篇 |
1990年 | 19篇 |
1989年 | 25篇 |
1988年 | 13篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 31篇 |
1984年 | 24篇 |
1983年 | 22篇 |
1982年 | 22篇 |
1981年 | 28篇 |
1980年 | 19篇 |
1979年 | 12篇 |
1978年 | 12篇 |
1977年 | 16篇 |
1976年 | 12篇 |
1975年 | 16篇 |
1974年 | 11篇 |
排序方式: 共有5091条查询结果,搜索用时 15 毫秒
101.
[reaction: see text] Condensation of N-substituted glycines with chiral 3-allyl-2-formyl perhydro-1,3-benzoxazines forms an azomethine ylide that cyclizes to give octahydropyrrolo[3,4-b]pyrrole derivatives. The [3 + 2] dipolar cycloadditions are stereoespecific leading to a single diastereoisomer. The chemical yields are dependent on the reaction temperature and the presence or absence of a base. 相似文献
102.
The redox catalysis of oxygen reduction was performed on a platinum rotating disk electrode. The Fe(III)/Fe(II)/H2SO4 system at different pH's was used as a MEDIATOR. The catalytic effect of mediator was directly related to the solvation sphere of Fe(III) and Fe(II). Only the redox couple FeHSO
4
2+
/FeHSO
4
+
(pH<0) showed a catalytic effect on oxygen reduction. 相似文献
103.
[reactions: see text] Two complementary and efficient strategies have been developed for the regiospecific synthesis of unsymmetrical indolopyrrolocarbazoles (IPCs) mono-N-substituted with a pentacycle. A halogen in position 2 of the intermediate bisindolylmaleimides 3a-e allows a selective Mitsunobu coupling by exploiting the increased acidity of the 2-chloro-substituted indole nitrogen. It also promotes an easier cyclization of bisindolylmaleimides 4a-e and 7b-e to IPCs. Alkylation of the 2-unsubstituted indole-3-carboxamides 2a,b and further processing to the corresponding IPCs gives access to the opposite regioisomers. 相似文献
104.
The Sm+O chemiionization reaction has been investigated theoretically using a method that allows for correlation and relativistic effects. Potential energy curves have been calculated for several electronic states of SmO and SmO+. Comparison with available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemiionization reaction Sm+O is proposed. The importance of spin-orbit coupling in the excited states of SmO, in allowing this chemiionization reaction to take place, has been revealed by these calculations. This paper shows the metal-plus-oxidant chemiionization reaction. 相似文献
105.
106.
107.
108.
Katja Schulz R. Klaus Mueller Werner Engewald Adelgunde Graefe Jan Dreßler 《Chromatographia》2007,66(11-12):879-886
Some aroma compounds found in alcoholic beverages are characteristic of a certain beverage (i.e. 2,4-decadienoic acid ethyl ester is characteristic of pear spirit and 5-butyltetrahydro-4-methylfuran-2-on “whiskey lactone” is characteristic of aged spirits like whiskey). These substances were detectable in beverages but not in blood samples. The aim of this investigation was to find a sensitive sampling technique for aroma compounds in whole blood samples. This technique may be used in forensic toxicology for examination of drinking claims. The method comprises dynamic headspace sampling using a purge and trap concentrator, followed by quantitative gas chromatography–mass spectrometry (dynamic HS–GC–MS). The influence of sample preparation, trap adsorbents and sample temperature as well as desorption time and purge time on the quality of the analytical results were investigated. The following optimal parameters were determined: stirred and diluted whole blood sample without salt addition, use of Carbotrap C as trap material, sample temperature at 80 °C, desorption time 20 min and purge time 30 min. These optimal parameters were used for the determination of detection limits (LOD). The LOD of aroma compounds by means of dynamic headspace sampling were compared with the results of conventional sampling: the static headspace technique. Limits of detection for the aroma compounds with conventional static headspace GC are in the range 400–10,000 μg L?1. Dynamic headspace–GC was found to be a more sensitive sampling technique for most of the aroma compounds investigated (e.g. C4–C8 ethyl esters, benzoic acid ethyl ester, linalool oxide and 4-ethylguaiacol) with detection limits between 1 and 50 μg L?1, but there were also limits to the sampling of substances with lower volatility like decanoic acid ethyl ester, 2,4-decadienoic acid ethyl ester, eugenol and whiskey lactone with detection limits of about 1,000 μg L?1. 相似文献
109.
An azaspiro[5.5]undecadiene corresponding to a subunit of the shellfish toxin gymnodimine was synthesized by Diels-Alder cycloaddition. One member of the pair of stereoisomeric adducts was transformed to a spiroimine, which will serve as the core around which the macrocyclic portion of the toxin will be assembled. [structure: see text] 相似文献
110.
Laura Gagliardi Giorgio Orlandi Fernando Bernardi Alessandro Cembran Marco Garavelli 《Theoretical chemistry accounts》2004,111(2-6):363-372
In the present paper we report the results of a multiconfigurational computational study on potential-energy curves of azobenzene along the NN twisting to clarify the role of this coordinate in the decay of the S2(*) and S1(n*) states. We have found that there is a singlet state, S3 at the trans geometry, on the basis of the doubly excited configuration n2*2, that has a deep minimum at about 90° of twisting, where it is the lowest excited singlet state. The existence of this state provides an explanation for the short lifetime of S2(*) and for the wavelength-dependence of azobenzene photochemistry. We have characterized the S1(n*) state by calculating its vibrational frequencies, which are found to correspond to the recently observed transient Raman spectrum. We have also computed the potential-energy curve for the triplet T1(n*) at the density functional theory B3LYP level, which indicates that in this state the isomerization occurs along the twisting coordinate.Acknowledgement The financial support from MIUR (project Modellistica delle proprietà spettroscopiche di sistemi molecolari complessi funds ex 60% and project Dinamiche molecolari in sistemi di interesse chimico funds ex 40%), from the University of Bologna (Funds for Selected Research Topics) is gratefully acknowledged.Contribution to the Jacopo Tomasi Honorary Issue 相似文献