全文获取类型
收费全文 | 1814篇 |
免费 | 43篇 |
国内免费 | 2篇 |
专业分类
化学 | 1274篇 |
晶体学 | 9篇 |
力学 | 67篇 |
数学 | 175篇 |
物理学 | 334篇 |
出版年
2021年 | 21篇 |
2019年 | 18篇 |
2018年 | 20篇 |
2016年 | 32篇 |
2015年 | 28篇 |
2014年 | 36篇 |
2013年 | 85篇 |
2012年 | 57篇 |
2011年 | 74篇 |
2010年 | 60篇 |
2009年 | 40篇 |
2008年 | 68篇 |
2007年 | 69篇 |
2006年 | 73篇 |
2005年 | 76篇 |
2004年 | 53篇 |
2003年 | 67篇 |
2002年 | 41篇 |
2001年 | 41篇 |
2000年 | 34篇 |
1999年 | 18篇 |
1998年 | 21篇 |
1997年 | 11篇 |
1996年 | 35篇 |
1995年 | 33篇 |
1994年 | 33篇 |
1993年 | 36篇 |
1992年 | 27篇 |
1991年 | 26篇 |
1990年 | 25篇 |
1989年 | 24篇 |
1988年 | 19篇 |
1987年 | 22篇 |
1986年 | 16篇 |
1985年 | 30篇 |
1984年 | 24篇 |
1983年 | 19篇 |
1982年 | 24篇 |
1981年 | 16篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1978年 | 16篇 |
1977年 | 19篇 |
1975年 | 12篇 |
1974年 | 11篇 |
1970年 | 10篇 |
1967年 | 16篇 |
1966年 | 20篇 |
1965年 | 39篇 |
1964年 | 45篇 |
排序方式: 共有1859条查询结果,搜索用时 237 毫秒
71.
Corsi DM Vander Elst L Muller RN van Bekkum H Peters JA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(7):1383-1389
We conducted relaxometric and water exchange studies of the cationic [Gd((S,S,S,S)-THP)(H2O)]3+ complex (THP 1,4,7,10-tetrakis(2-hydroxy-propyl)-1,4,7,10-tetraazacyclododecane). While the NMRD profiles obtained are typical for DOTA-like complexes (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), variable-temperature 7O NMR investigations revealed a relatively high water exchange rate (k(298)(ex) = 1.89 x 10(7) s(-1)). These results differ from those reported for other cationic tetraamide macrocyclic Gd(III) complexes, which exhibit characteristically low exchange rates. Since the low exchange rates are attributed partially to the geometry of the M isomer (square antiprismatic) in the tetraamide derivatives, the atypical water exchange rate observed in [Gd((S,S,S,S)-THP-(H2O)]3+ may result from a twisted square antiprismatic structure in this complex and from the relatively high steric strain at the water coordination site as a result of the presence of methyl groups at the alpha-position with respect to the Gd(III)-bound O atoms of THP. 相似文献
72.
73.
74.
75.
In order to study the kinetic mechanism of the e-beam pumped Ar/Xe laser, the temporal profiles of individual laser lines during multiline oscillation have been measured as a function of power deposition (1–12MW/cm3) and gas laser pressure (2–14 bar) using a short pulse (30 ns) coaxial electron beam as excitation source. It was found that the optimum output energy at each pressure was obtained at the same specific power deposition.Strong line competition has been observed between the 2.65 and 1.73 m transitions. In order to explain our results we suggest that besides electron collision mixing (ECM) between the 5d and 6p levels of Xe, there is also a redistribution between all 6p levels which strongly favours the lower levels at higher pumping densities. 相似文献
76.
77.
The pH dependent stability of esters of boric acid and borate with glycol, glycolic acid, oxalic acid and glyceric acid as dihydroxy compounds has been studied. 11B NMR provides a suitable analytical tool for the quantitative determination and structure elucidation of the various esters in aqueous medium. The pH dependent stability of esters of boric acid and borate is formulated in a general rule of thumb: esters of boric acid and borate with dihydroxy compounds in aqueous medium show the highest stability at that pH where the sum of the charges of the free esterifying species is equal to the charge of the ester. 相似文献
78.
Banks CE Kruusma J Moore RR Tomcík P Peters J Davis J Komorsky-Lovrić S Compton RG 《Talanta》2005,65(2):423-429
Three different electroanalytical techniques for the detection of manganese in marine sediments are evaluated. The anodic stripping voltammetry of manganese at an in situ bismuth-film-modified boron-doped diamond electrode and cathodic stripping voltammetry at a carbon paste electrode are shown to lack the required sensitivity and reproducibility whereas cathodic stripping voltammetry at a bare boron-doped diamond electrode is shown to be reliable and selective with a limit of detection, from applying a 60 s accumulation period of 7.4 × 10−7 M and a sensitivity of 0.24 A M−1. The method was used to evaluate the manganese content of marine sediments taken from Šibenik, Croatia. 相似文献
79.
2-Nitrophenylhydrazine (2-NPH) is widely used for the derivatization of carboxylic acids, aldehydes and ketones, in industrial and biological samples. These compounds react with 2-NPH to form derivatives, which are separated by high-performance liquid chromatography (HPLC) and detected with diode array detection (DAD). The UV spectra give information about the functionality of the compounds: carboxylic acid or ketone/aldehyde. Most of the eluting compounds in "known" samples are well characterised by the retention time (comparison with those of standards) of the 2-NPH derivative and their UV spectrum. The identification of different unknown 2-NPH derivatives of carboxylic acids, ketones and/or aldehydes, in industrial or biological samples, based on retention time and/or UV spectrum is not sufficient. These unknown 2-NPH compounds can be identified with on-line atmospheric pressure chemical ionisation mass spectrometry (APCI-MS) based on the molecular mass or/and the fragmentation of the derivative. A novel and specific on-line HPLC-DAD-APCI(-)-MS method is described for the determination of carboxylic acids, ketones and aldehydes, after on-line pre-column derivatization with 2-NHP. The fragmentation of different 2-NPH derivatives were investigated and the possibilities of APCI(-)-MS detection were demonstrated by the on-line identification of an unknown derivative, which turned out to be a side product between 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 2-NPH in the presence of high concentrations of a cyclic amide in the sample solution. 相似文献
80.
19F NMR Spectroscopic Evidence and Calculation of the Statistical Formation of Mixed Cluster Anions [(Mo6Br Cl )F ]2?, n = 0 – 8 The complete system of the innersphere mixed clusters (Mo6BrCl)4+ is formed by exchange of innersphere bound Cli against outersphere bound Bra on tempering the solid [(Mo6Cl)Br] at 500°C for 16 h. After conversion with conc. HCl into (H3O)2[(Mo6BrCl)Cl] and precipitation of the outer Cla with AgBF4 in ethanol, treatment with tetrabutylammonium(TBA)fluoride yields (TBA)2 [(Mo6BrCl)F], a mixture of 22 different species. According to the sets of chemical equivalent fluorine atoms in total 55 19F nmr signals are expected, which are really observed in the high resolution 1D-19F-nmr spectrum. Using increments of chemical shifts, peak intensities and multiplet structures as well as the 2D-19F/19F-COSY spectrum the complete and unambiguous assignment of all resonances is achieved. From the measured integral intensities the distribution of the different compounds is determined, revealing statistical formation of the geometrical isomers. 相似文献