全文获取类型
收费全文 | 1818篇 |
免费 | 48篇 |
国内免费 | 10篇 |
专业分类
化学 | 1078篇 |
晶体学 | 14篇 |
力学 | 44篇 |
数学 | 432篇 |
物理学 | 308篇 |
出版年
2020年 | 16篇 |
2019年 | 18篇 |
2018年 | 12篇 |
2017年 | 16篇 |
2016年 | 33篇 |
2015年 | 32篇 |
2014年 | 33篇 |
2013年 | 92篇 |
2012年 | 80篇 |
2011年 | 94篇 |
2010年 | 53篇 |
2009年 | 44篇 |
2008年 | 85篇 |
2007年 | 81篇 |
2006年 | 97篇 |
2005年 | 93篇 |
2004年 | 66篇 |
2003年 | 66篇 |
2002年 | 84篇 |
2001年 | 20篇 |
2000年 | 32篇 |
1999年 | 27篇 |
1998年 | 28篇 |
1997年 | 33篇 |
1996年 | 27篇 |
1995年 | 15篇 |
1994年 | 31篇 |
1993年 | 27篇 |
1992年 | 21篇 |
1991年 | 22篇 |
1990年 | 19篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 18篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 30篇 |
1981年 | 33篇 |
1980年 | 28篇 |
1979年 | 23篇 |
1978年 | 30篇 |
1977年 | 23篇 |
1976年 | 20篇 |
1975年 | 18篇 |
1974年 | 21篇 |
1973年 | 19篇 |
1972年 | 14篇 |
1971年 | 13篇 |
排序方式: 共有1876条查询结果,搜索用时 0 毫秒
41.
Eight differently substituted title dye compounds have been investigated regarding intersystem crossing, triplet state, fluorescence and singlet excited state pKa properties. In general, non-halogenated oxazines and thiazines as well as a mono bromooxazine show very low triplet quantum yields, phi tau (less than 0.03) and relatively long triplet lifetimes (approximately 40 microseconds) in acidic methanol. The phi tau data correlate well with known singlet oxygen yields. In basic methanol no triplet transient is observed but a significant yield of a ground state transient protonated (base dye) form is produced with a short lifetime, approximately 400 ns. Fluorescence can be seen simultaneously from both the excited base and the protonated base dye forms in basic methanol. For iodinated oxazine or thiazines, the triplet yield increases and can be as high as 0.5 (diiodo case) in acidic methanol. The triplet lifetimes are further shortened to approximately 10 microseconds compared to the non-iodinated derivatives above. The triplet yields of the iodo compounds are higher or equal to known singlet oxygen yields. In basic methanol triplet yields up to 0.2 can be seen, the triplet lifetime are shortened still further to 1 microsecond but no observable protonated form is produced (in distinction to the non-iodinated cases). Consideration is given to the correlation of triplet and singlet oxygen yields, ground and excited pKa properties, spin-orbit coupling and internal conversion properties, solvent effects, and phototherapeutic activity of these dyes. 相似文献
42.
Ralph McKenzie 《Algebra Universalis》1983,17(1):143-149
It is proved for any varietyG of groups that if the subdirectly irreducible groups inG form a set, and if the subdirectly irreducible representation algebras of groups inG form a set, then every finite group inG is Abelian. The result is essential for the characterization of residually finite varieties of semigroups. 相似文献
43.
Tan L Yasuda N Yoshikawa N Hartner FW Eng KK Leonard WR Tsay FR Volante RP Tillyer RD 《The Journal of organic chemistry》2005,70(20):8027-8034
[Chemical reaction: See text] A Et3Al mediated intramolecular epoxide opening, cyclopropanation reaction is described. The transformation provided highly functionalized bicyclo[3.1.0]hexane systems in high efficiency and with perfect H or F endo selectivity. Application of this reaction to the synthesis of mGluR2/3 agonist 1 (43% overall yield) and a few intermediates suitable for the synthesis of other bicyclo[3.1.0]hexane mGluR2/3 agonists is discussed. 相似文献
44.
45.
Abstract— Exploratory experiments using simulated conditions indicate that the terpene, (+)-limonene can serve to detect O2 (1 Δg ) in polluted atmospheres. 1 O2 attack on limonene results in the formation of specific oxidation products in a specific distribution; quantitative identification of these products is a highly specific test for 1 O2 . 相似文献
46.
Li2H4I2O10, the First Tetrahydrogendimesoperiodate Li2H4I2O10 has been obtained as an intermediate during the dehydration of LiH4IO6 · H2O to LiIO4, for the first time. According to the results of an X-ray structure determination (monoclinic, P21/n, a = 533.98(4), b = 471.85(4), c = 1431.48(10) pm, β = 91.614(7)°, Z = 2, 726 diffractometer data, R = 0.056), Li2H4I2O10 contains the previously unknown tetrahydrogendimesoperiodate ion H4I2O102?, consisting of two edge-shared IO6 octahedra. They are connected with LiO6 octahedra via common edges and vertices. The crystals are non-merohedrally twinned along (100). 相似文献
47.
Abstract— A systematic comprehensive consideration of the emission spectroscopy, emission lifetimes, transient spectroscopy, as well as quantum yields of fluorescence, triplet occupation and photoisomeriz-ation is given for a wide variety of polyene derivatives including retinyl and longer, as well as shorter chainlength homologs. Absorption spectral properties and the results and significance of theoretical calculations are also included. Chainlength, solvent and temperature effects on state order and photophysical as well as photoisomerization properties are evaluated. The mechanism for the primary light step in vision is considered in light of photophysical and photoisomerization data on model visual pigments and rhodopsin. 相似文献
48.
The fitting of highly accurate potential energy points and of the diagonal adiabatic coupling for H3+ using different functional forms is presented. A recently derived analytical potential based on 69 points has been extended to give a highly reliable form of the topology of the surface far beyond the barrier to linearity. Rovibrational frequencies have been derived and are compared with experiment. Detailed information about the experimentally observed rovibrational transitions near 1.25 microm will be given. The computed transition frequencies reproduce experimental transitions within a tenth up to a few hundredths of a wavenumber, if a simulation of non-adiabatic effects is taken into account. 相似文献
49.
[reaction: see text] The palladium-catalyzed coupling of a range of enol triflates with amides, carbamates, and sulfonamides has been developed. This offers a simple and widely applicable synthesis of enamides, which may not be readily available by other means. 相似文献
50.