全文获取类型
收费全文 | 199篇 |
免费 | 5篇 |
国内免费 | 1篇 |
专业分类
化学 | 151篇 |
力学 | 2篇 |
数学 | 22篇 |
物理学 | 30篇 |
出版年
2021年 | 2篇 |
2017年 | 1篇 |
2016年 | 4篇 |
2015年 | 5篇 |
2014年 | 2篇 |
2013年 | 1篇 |
2012年 | 5篇 |
2011年 | 5篇 |
2010年 | 12篇 |
2009年 | 5篇 |
2008年 | 13篇 |
2007年 | 16篇 |
2006年 | 11篇 |
2005年 | 13篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 10篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1966年 | 1篇 |
排序方式: 共有205条查询结果,搜索用时 15 毫秒
31.
32.
Abstract— Site-specific DNA damage caused by 334-nm radiation in the presence of the rare Escherichia coli base 4-thiouridine was investigated in vitro by detecting the sites of the termination of DNA synthesis with irradiated M13 phage DNA used as a template. Single-strand breakage was also examined. The results indicate that 334-nm radiation at very low fluences in the presence of 4-thiouridine induces termination of strand synthesis at thymine base sites and at the base immediately prior to thymine. Termination at these sites was diminished by treatment with hot piperidine. Strand cleavage by piperidine treatments was observed preferentially at the guanine site, but only after irradiation at much larger fluences. It is hypothesized that at low fluences 4-thiouridine forms photoadducts with thymine that block DNA synthesis, while at high fluences the guanine site is damaged via oxygen species. 相似文献
33.
Abstract—Kinetics of inactivation of two separate leucine transport systems (leucine specific and LIV) in E. coli by seven wavelengths of monochromatic light have been studied. Loss of leucine uptake is not due to generalized membrane damage causing non-specific leucine leakage. Inactivations are usually exponential but some wavelengths show shoulders at low doses. Two-component spectra between 254 and 435 nm occur for both transport systems. Inactivation is most efficient at 290 nm and a second peak occurs at 365 nm. Both leucine transport systems are inactivated similarly. 相似文献
34.
A double-strand polymeric complex, which suppresses electroosmotic flow relative to fused-silica, is described. The polymeric complex contains a strand polyaniline (PAN) with the second strand containing polyacrylic acid (PAA) and methacrylate (MA) groups. The complex is referred to as PAN:P(AAMA). This polymeric complex has pH-controlled electroactive and hydrophobic characteristics and can be easily coated onto fused-silica. Enhanced separations of theophylline, theobromine, caffeine and adenine, thymine, uracil and cytosine were obtained by the use of the coated capillary in the micellar electrokinetic capillary electrophoresis (MEKC) system. The purine and pyrimdine bases were separated on the coated capillary with a 20 mM, pH 7 phosphate buffer which contains 0.05 M sodium dodecyl sulfate (SDS) as an additive. 相似文献
35.
Bernardi F Bottoni A Olivucci M Robb MA Schlegel HB Tonachini G 《Journal of the American Chemical Society》1988,110(18):5993-5995
36.
García-Expósito E Bearpark MJ Ortuño RM Robb MA Branchadell V 《The Journal of organic chemistry》2002,67(17):6070-6077
The ground and first triplet excited-state potential energy surfaces of the [2 + 2]-cycloadditions of 2-cyclohexenone, methyl acrylate, and methyl crotonate to ethylene have been studied by means of CASSCF and DFT-B3LYP calculations. The attack of ethylene to the (3)(pi-pi) alpha,beta-unsaturated carbonyl compound leads to the formation of a triplet 1,4-biradical intermediate that evolves to the ground-state potential energy surface. The outcome of the reaction is governed by the competition between the deactivation of the (3)(pi-pi) alpha,beta-unsaturated carbonyl compound itself and its reaction with ethylene to form the triplet 1,4-biradical. For 2-cyclohexenone, the potential energy barrier corresponding to the formation of the biradical intermediate is lower than for the acyclic systems. On the other hand, the energy necessary to reach the crossing point between the (3)(pi-pi) and the ground-state potential energy surfaces is lower for the acyclic systems than for 2-cyclohexenone. For methyl acrylate and methyl crotonate, the decay of the (3)(pi-pi) state of the isolated molecule is therefore expected to be faster than the formation of the 1,4-biradical, so that the [2 + 2]-cycloaddition will not take place. However, for 2-cyclohexenone the formation of the triplet 1,4-biradical is favorable, and the process will lead to the formation of the corresponding cyclobutane derivative. 相似文献
37.
Robb Damon B. Blades Michael W. 《Journal of the American Society for Mass Spectrometry》2005,16(8):1275-1290
In this paper, the effects of solvent flow, dopant flow, and lamp power on proton transfer ionization in dopant-assisted (DA)
atmospheric pressure photoionization (APPI) are investigated. A broad theoretical framework is presented, describing the primary
photoionization process, the formation of protonated-solvent cluster ions, and the balance between analyte ion creation via
proton transfer and loss via recombination. The principal experimental test system utilized methanol as the solvent, toluene
as the dopant, and acridine as the analyte. Comparisons are made between acridine and a less basic compound, 9-methylanthracene
(9-MA). Experimental determinations of the trends in the analyte MH+ signal and the total ion current (TIC) with variations in the subject parameters are provided. Experimental results and theory
demonstrate that both the analyte signal and the TIC approach asymptotic limits with increases in dopant flow and/or lamp
current (two factors which dictate the rate of photoion generation). The data show that these limits are lowered at higher
solvent flow rates. These results are attributed to the recombination loss process, the rate of which increases with the second
power of ion concentration. We deduce that the recombination rate constant increases with solvent flow rate, a consequence
of the growth of ion-solvent clusters. Cluster growth is also believed to be a factor in the dramatic loss of sensitivity
for 9-MA that occurs as the solvent flow is raised, because larger protonated-solvent cluster ions have greater solvation
energies and may be unreactive with compounds having low gas-phase basicity and/or low solvation energy. 相似文献
38.
39.
40.