全文获取类型
收费全文 | 4755篇 |
免费 | 128篇 |
国内免费 | 56篇 |
专业分类
化学 | 3427篇 |
晶体学 | 17篇 |
力学 | 86篇 |
数学 | 728篇 |
物理学 | 681篇 |
出版年
2021年 | 53篇 |
2020年 | 60篇 |
2019年 | 45篇 |
2018年 | 42篇 |
2017年 | 40篇 |
2016年 | 106篇 |
2015年 | 97篇 |
2014年 | 116篇 |
2013年 | 250篇 |
2012年 | 195篇 |
2011年 | 287篇 |
2010年 | 158篇 |
2009年 | 130篇 |
2008年 | 221篇 |
2007年 | 227篇 |
2006年 | 212篇 |
2005年 | 235篇 |
2004年 | 178篇 |
2003年 | 147篇 |
2002年 | 132篇 |
2001年 | 38篇 |
2000年 | 40篇 |
1999年 | 41篇 |
1998年 | 43篇 |
1997年 | 48篇 |
1996年 | 55篇 |
1995年 | 49篇 |
1994年 | 45篇 |
1993年 | 34篇 |
1992年 | 43篇 |
1991年 | 45篇 |
1990年 | 48篇 |
1989年 | 26篇 |
1988年 | 52篇 |
1987年 | 64篇 |
1986年 | 49篇 |
1985年 | 52篇 |
1984年 | 56篇 |
1983年 | 45篇 |
1982年 | 44篇 |
1981年 | 50篇 |
1980年 | 47篇 |
1979年 | 49篇 |
1978年 | 75篇 |
1977年 | 58篇 |
1976年 | 62篇 |
1975年 | 28篇 |
1974年 | 43篇 |
1973年 | 42篇 |
1884年 | 26篇 |
排序方式: 共有4939条查询结果,搜索用时 15 毫秒
101.
The present paper describes constructional details and evaluations of an at-column injector for capillary GC. Injections were made via a sample loop on a 0.32 mm i.d. capillary column. Two rotary valves were employed to allow a wash of the sample loop and a backflush of the transfer line. Repeatability, calculated from absolute area counts for n-alkanes was between 0.3–1% RSD, for injected sample volumes between 5 and 100 μl. Promising results were also obtained with syringe-based injections on narrow bore (100 μ i.d.) columns. Repeatability on the basis of normalized area counts was in the order of 0.1–0.2% RSD, while solvent tailing was practically absent. 相似文献
102.
p-Carborane has been vinylated on the 2-B-atom in high yields using the Heck reaction. Thus, the reaction between 2-iodo-p-carborane and various styrenes [4-H-, 4-C(6)H(4)-, 4-Cl-, 4-Br-, 4-NO(2)-, 4-CH(3)O-, and 4-CH(3)-] resulted in the production of the corresponding trans-beta-(2-B-p-carboranyl)styrene in DMF solution when reacted in the presence of silver phosphate and the palladacycle Herrmann's catalyst. 相似文献
103.
104.
Peter C. Burgers J. L. Holmes F. P. Lossing Frank R. Povel Johan K. Terlouw 《Journal of mass spectrometry : JMS》1983,18(8):335-339
The structures of the m/z 87, [C4H7O2]+, ions generated by dissociative ionization of CH3CGXCOOCH3 and XCH2CH2COOCH3 (X = CH3, Cl, Br, and I) have been investigated via their unimolecular and collisionally activated fragmentations and by apperance energy measurements. For both precursors loss of X = CH3 produced, via H atom transfer, ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_2 = CH}\mathop {\rm C}\limits^{\rm + } \left({{\rm OH}} \right){\rm OCH}_{\rm 3} $\end{document} (a), ΔHf = 386 kj mol?1. In marked contrast, loss of I˙ from ionized CH3CHICOOCH3 and ICH2CH2COOCH3 proceeded without rearrangement to yield respectively ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_3}\mathop {\rm C}\limits^{\rm + } {{\rm HCOOCH_3}} $\end{document} (b), ΔHf = 480 kJ mol?1 and (c), ΔHf = 450 kJ mol?1. These different fragmentation behaviours are explained via photoelectron spectra which show that the formal charge site in the precursor ion is at the carbonyl oxygen when X = CH3 but at the halogen atom when X = I. The precursor molecules X = Cl and Br display both of the above characteristics, CH3CHXCOOCH3 yielding mixtures of a and b and XCH2CH2COOCH3 producing a and c ions. 相似文献
105.
Hjelm J Handel RW Hagfeldt A Constable EC Housecroft CE Forster RJ 《Inorganic chemistry》2005,44(4):1073-1081
The electropolymerization of a series of Ru and Os bis-terpyridine complexes that form rodlike polymers with bithienyl, quaterthienyl, or hexathienyl bridges has been studied. Absorption spectroscopy, scanning electron microscopy, and cyclic voltammetry have been used to characterize the monomers and resulting polymer films. The absolute dc conductivity of the quaterthienyl-bridged {Ru(tpy)2} and {Os(tpy)2} polymers is unusually large and independent of the identity of the metal center at 1.6 x 10(-3) S cm(-1). The maximum conductivity occurs at the formal potential of each redox process, which typically is observed for systems where redox conduction is the dominant charge transport mechanism. Significantly, the dc conductivity of the metal-based redox couple observed in these polymers is 2 orders of magnitude higher thanthat of a comparable nonconjugated system. 相似文献
106.
Negative-ion photoelectron spectroscopy is applied to the PH-, PH2-, P2H-, P2H2-, and P2H3-molecular anions. Franck-Condon simulations of the photoelectron spectra are used to analyze the spectra and to identify various P2H(n)- species. The simulations employ density-functional theory calculations of molecular geometries and vibrational frequencies and normal modes, and coupled-cluster theory calculations of electron affinities. The following electron affinities are obtained: EA0(PH) = 1.027 +/- 0.006 eV, EA0(PH2) = 1.263 +/- 0.006 eV, and EA0(P2H) = 1.514 +/- 0.010 eV. A band is identified as a mixture of trans-HPPH- and cis-HPPH-. Although the trans and cis bands cannot be definitively assigned from experimental information, using theory as a guide we obtain EA0(trans-HPPH)= 1.00 +/- 0.01 eV and EA0(cis-HPPH) = 1.03 +/- 0.01 eV. A weak feature tentatively assigned to P2H3- has a vertical detachment energy of 1.74 eV. The derived gas-phase acidity of phosphine is delta(acid)G298(PH3) < or = 1509.7 +/- 2.1 kJ mo1(-1). 相似文献
107.
Surowiec I Baena JR Frank J Laurell T Nilsson J Trojanowicz M Lendl B 《Journal of chromatography. A》2005,1080(2):132-139
A flow-through microdispenser has been coupled to a micro HPLC separation system and used as a solvent elimination interface for Fourier transform infrared (FTIR) and Raman spectroscopic detection of the separated compounds. Using the microdispenser picoliter sized droplets can be generated and deposited on an appropriate target placed on a computerized x, y-stage. Evaporation of volatile solvent and buffer is rapid and allows analysis of the obtained dry deposits by various techniques. Due to the destruction free character of Raman and FTIR spectroscopy they can be applied sequentially to interrogate the same deposit. In the reported application five phenolic acids typically present in wine have been separated on a C-18 column technique using a mixture of water, methanol and acetic acid as mobile phase. For spectrum acquisition infrared and Raman microscopes have been used. The spectra recorded from the dried deposits of the separated compounds agreed well with the reference spectra of corresponding components. 相似文献
108.
109.
110.