全文获取类型
收费全文 | 1241篇 |
免费 | 35篇 |
国内免费 | 16篇 |
专业分类
化学 | 714篇 |
晶体学 | 8篇 |
力学 | 87篇 |
综合类 | 3篇 |
数学 | 181篇 |
物理学 | 299篇 |
出版年
2021年 | 9篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 14篇 |
2015年 | 12篇 |
2014年 | 25篇 |
2013年 | 94篇 |
2012年 | 51篇 |
2011年 | 56篇 |
2010年 | 26篇 |
2009年 | 30篇 |
2008年 | 49篇 |
2007年 | 69篇 |
2006年 | 60篇 |
2005年 | 51篇 |
2004年 | 30篇 |
2003年 | 40篇 |
2002年 | 44篇 |
2001年 | 24篇 |
2000年 | 35篇 |
1999年 | 17篇 |
1998年 | 17篇 |
1997年 | 26篇 |
1996年 | 27篇 |
1995年 | 18篇 |
1994年 | 19篇 |
1993年 | 24篇 |
1992年 | 28篇 |
1991年 | 25篇 |
1990年 | 18篇 |
1989年 | 12篇 |
1988年 | 21篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 12篇 |
1983年 | 19篇 |
1981年 | 14篇 |
1980年 | 20篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 28篇 |
1976年 | 9篇 |
1975年 | 17篇 |
1974年 | 15篇 |
1973年 | 12篇 |
1972年 | 11篇 |
1969年 | 7篇 |
1968年 | 9篇 |
排序方式: 共有1292条查询结果,搜索用时 15 毫秒
21.
Molecular structures of three emissive annular digold compounds [Au2(dmpm)(dtc)]Cl (dmpm = Me2PCH2PMe2,dtc = S2CNEt2), 1 , [Au2(dppm)(dtc)]PF6, (dppm = Ph2PCH2PPh2),2,and [Au2(dppe)(dtc)]-(PF2) (dppe = Ph2P(CH2)2PPh2), 3, were determined. All three compounds are dimetallacycles having two gold atoms bridged by a dithiolate ligand and a diphosphine ligand, the geometry around each gold atom being almost linear. All the dimetal lacy die rings are slightly distorted from planarity with intramolecular Au-Au distances shorter than 3.0 Å. Compound 1 forms a polymeric chain through intermolecular Au-Au contacts (3.061 ? 3.135 Å). Compound 2 forms a tetramer through intermolecular Au-Au interactions (Au-Au distances ranging from 3.086 to 3.222 ). Compound 3 is monomeric. All of the compounds luminesce at 77 K in the solid state. Emissions originating from 3LMCT from dtc ligand to Au excited states are assigned. The emission maxima of 1 ? 3 are at 541,535 and 520 nm respectively and are blue shifted as the number of Au-Au interactions is decreased. 相似文献
22.
McClean S Robinson RC Shaw C Smyth WF 《Rapid communications in mass spectrometry : RCM》2002,16(5):346-354
The characterisation of selected indole alkaloids in a quadrupole ion trap mass spectrometer is presented. Fragmentation profiles for tryptamine, 5-hydroxytryptamine (5-HT), N'-methyl 5-hydroxytryptamine (N'-methyl 5-HT), N',N'-dimethyl 5-hydroxytryptamine (bufotenine), N',N',N'-trimethyl 5-hydroxytryptamine (5-HTQ), and N',N'-dimethyl 5-methoxytryptamine (5-MeODMT) are presented with proposed structures given for each product ion observed. Such MS(n) experiments can be used to differentiate the isobaric molecular ions of the compounds 5-HTQ (M(+)) and 5-MeODMT (MH(+)). The quantitative determination of certain indole alkaloids in the skin secretions of the Australian Golden Bell frog, Litoria aurea, by LC/ESI-ion trap MS is also presented. The concentrations of 5-HT, N'-methyl 5-HT and 5-HTQ were found to be 2.68, 0.26 and 0.54 microg per mg of skin secretion, respectively. 相似文献
23.
S. S. Krishnamurthy P. Ramabrahmam A. R. Vasudeva Murthy R. A. Shaw M. Woods 《无机化学与普通化学杂志》1985,522(3):226-234
The reactions of pentachloro(2′, 2′, 2′-triphenylphosphazen-1′-yl)cyclotriphosphazene, N3P3Cl5(NPPh3), with primary and secondary amines have been investigated using diethyl ether, methyl cyanid or benzene as the solvent. The structures of the products obtained, N3P3Cl5minus;nRn(NPPh3) [n = 1, R = NHMe, NHBut, NMe2, NC5H10, NEt2; n = 2, R = NMe2, NC5H10, NEt2; n = 3, R = NMe2, NHBut; n = 5, R = NMe2] are elucidated by 1H and 31P NMR spectroscopy. The ? NPPh3 substituent exerts a pronounced geminal directing influence on incoming secondary amino nucleophiles; compounds containing a ≡ PCl(NPPh3) group are not formed at the bis and subsequent stages of chlorine replacement. The reactions that involve primary amines follow the pattern established for the analogous reactions of hexachlorocyclotriphosphazene. The effect of solvent and possible mechanism(s) are discussed. 相似文献
24.
We have used a micro-gel system to analyze the proteins in extracts of amphibian neurulae. Maximum band separation and resolution was achieved by using a dissociating discontinuous buffer with gels made of a discontinuous gradient of acrylamide. 相似文献
25.
Ya‐Ching Shen Yun‐Sheng Lin Shaw‐Man Hsu Ashraf Taha Khalil Shih‐Sheng Wang Ching‐Te Chien Yao‐Haur Kuo Chang‐Hung Chou 《Helvetica chimica acta》2007,90(7):1319-1329
The phytochemical investigation of the more polar fractions from the leaves and twigs of Taxus sumatrana (Taxaceae) afforded five new taxane diterpene esters, tasumatrols P–T ( 1 – 5 ) possessing an 11(15→1),11(10→9)‐diabeotaxane skeleton. Compounds 1, 4 , and 5 contain an α‐hydroxy group at C(14), while 3 has no OH group at either C(13) or C(14). Compound 2 is a natural 4,5‐acetonide derivative, while 4 has an unusual spiro‐connected 2‐hydroxy‐2‐phenyl‐1,3‐dioxolane ring. Ten known taxoids, were also isolated in the course of the chromatographic fractionation. Five additional new O‐acetyl derivatives 3a, 4a, 4b, 5a , and 5b were prepared from the taxanes 3 – 5 . The structures of all new compounds were established on the basis of their spectroscopic analyses. Compound 1 showed mild cytotoxic activity against human Hela and Daoy tumor cells. 相似文献
26.
BARB FLORIN DAN; IONESCU VLAD; KONING WILLEM DE 《IMA Journal of Mathematical Control and Information》1994,11(4):277-309
In this paper, we deal with the digital output-measurement-feedbackH control problem for Pritchard-Salamon infinite-dimensionalsystems with unbounded input and output operators. A discretePopov-theory-based solution is given in terms of the solvabilityof Kalman-Szegö-Popov-Yakubovitch systems associated withthe equivalent discrete-time time-invariant system obtainedby lifting the T-periodic continuous-time system. 相似文献
27.
28.
A response equation for conductivity detection in ion chromatography has been derived. This equation is applicable to non-suppressed ion chromatography using both fully ionised and partially ionised eluents. A prime assumption of this equation is that when partially ionised eluents are used (such as benzoic acid), both the ionised and neutral components of the eluent species contribute to the detector response of anionic analytes. Experimental evidence is provided to support this assumption in that pH changes accompanying the elution of an analyte have been measured. These pH changes are proportional to the concentration of analyte injected onto the column, in accordance with predictions from the response equation. Furthermore, it is shown that protonated eluents (such as benzoic acid) give more sensitive detection than equivalent ionised eluents (such as potassium benzoate) and the signal enhancement achieved using a protonated eluent species is in accordance with theory. 相似文献
29.
The mass spectra of isohumulones (IV, V), tetrahydroisocohumulones (VI, VII), tetrahydroisohumulones (VIII) neohydroisocohumulones (XI, XII) cohumulinic acid (III), lupuloxinic acid (XV), humulinone (XIV) and related compounds are described. Ions which appear to be diagnostic for particular structures are discussed. 相似文献
30.