全文获取类型
收费全文 | 1525篇 |
免费 | 37篇 |
国内免费 | 10篇 |
专业分类
化学 | 981篇 |
晶体学 | 5篇 |
力学 | 31篇 |
数学 | 217篇 |
物理学 | 338篇 |
出版年
2021年 | 14篇 |
2019年 | 19篇 |
2018年 | 17篇 |
2017年 | 13篇 |
2016年 | 26篇 |
2015年 | 27篇 |
2014年 | 19篇 |
2013年 | 47篇 |
2012年 | 64篇 |
2011年 | 60篇 |
2010年 | 51篇 |
2009年 | 45篇 |
2008年 | 83篇 |
2007年 | 82篇 |
2006年 | 64篇 |
2005年 | 67篇 |
2004年 | 45篇 |
2003年 | 63篇 |
2002年 | 41篇 |
2001年 | 29篇 |
2000年 | 23篇 |
1999年 | 23篇 |
1998年 | 25篇 |
1997年 | 18篇 |
1996年 | 27篇 |
1995年 | 20篇 |
1994年 | 15篇 |
1993年 | 25篇 |
1992年 | 16篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 19篇 |
1986年 | 21篇 |
1985年 | 32篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1980年 | 18篇 |
1979年 | 21篇 |
1978年 | 15篇 |
1977年 | 22篇 |
1976年 | 20篇 |
1975年 | 19篇 |
1974年 | 23篇 |
1973年 | 15篇 |
1972年 | 15篇 |
1971年 | 13篇 |
1970年 | 11篇 |
1967年 | 11篇 |
1885年 | 16篇 |
排序方式: 共有1572条查询结果,搜索用时 719 毫秒
61.
Claus Harzdorf und Ottmar Steinhauser 《Fresenius' Journal of Analytical Chemistry》1966,218(2):96-104
Zusammenfassung PbClF und PbBrF werden bezüglich ihrer Fällungsbedingungen und Eignung zur Fluoridbestimmung verglichen.Für reine Lösungen sind bei Einhaltung bestimmter Fällungsbedingungen beide Methoden gleich gut geeignet. Bei der Fällung aus silicathaltigen Probelösungen, wie sie bei der Fluorbestimmung in technischen Fluoriden anfallen, liefert die PbClF-Fällung positive Fehler, während man über PbBrF sehr gute Werte erhält.Eine Arbeitsvorschrift für die Fluorbestimmung in Aluminiumfluorid und Kyrolith nach der Bleibromidfluoridmethode ist angegeben.
Summary PbClF and PbBrF have been compared with respect to their precipitation conditions and their suitability for the determination of fluoride.For pure solutions, if certain precipitation conditions are adhered to, both methods are equally well suitable. When precipitating from silicate-containing sample solutions, as they are obtained when fluorine is determined in technical fluorides, the PbClF precipitations yields positive errors, while very good values are obtained with PbBrF.A working procedure is given for the determination of fluorine in aluminium fluoride and cryolite by the lead bromide fluoride method.相似文献
62.
Improved full ab initio optimizations of the molecular structure of biphenyl in twisted minimum energy, coplanar, and perpendicular conformations by use of Poles's GAUSSIAN 82 program have been performed in the 6-31G basis set. These lead to geometries and energies of much higher reliability than our earlier STO-3G results. The torsional angle Φmin obtained now is 45.41° in close agreement with the recent experimental value of 44.4° ± 1.2°. Calculated CC distances may be converted to experimental ED rg-values by means of independently determined linear regression correlations with very high statistical confidence, although they agree better with experimental x ray data for coplanar biphenyl without this correction. Calculated intramolecular angles are very similar for both STO-3G and 6-31G basis sets. The calculated torsional energy barrier towards Φ = 90° (ΔE90) is 6.76 kJ/mol in close agreement with the experimental-31G value of 6.5 ± 2.0 kJ/mol. For coplanar biphenyl with D2h-symmetry the calculated torsional energy barrier ΔE0 is 13.26 kJ/mol which is surprisingly much higher than the experimental value of 6.0 ± 2.1 kJ/mol. This discrepancy could not be resolved by optimizations assumed for two kinds of distortions of planarity of orthohydrogens from the molecular plane of the coplanar carbon atoms. But for the twisted minimum energy conformation asymmetric bending of ortho-H atoms lead to a torsional angle Φmin = 44.74° together with a dihedral angle towards ortho-H of 1.22°, and consequently even to an increase of torsional energy barriers to ΔE0 = 13.51 and ΔE90 = 6.91 kJ/mol. 相似文献
63.
A. A. El‐Barbary M. A. Sakran A. M. El‐Madani Claus Nielsen 《Journal of heterocyclic chemistry》2005,42(5):935-941
4‐Amino‐6‐methyl‐3‐(2H)‐thioxo‐5‐(4H)‐oxo‐1,2,4‐triazine ( 1 ) was condensed with 2‐methyl (or phenyl)‐4H‐3,1‐benzoxazin‐4‐one ( 5a,b ) in boiling acetic acid to give compounds 8‐11 . Reacting 1 with chloroacetyl chloride afforded the corresponding chloroacetamido and triazinothiadiazine derivatives 12 and 13 . Condensing 2 with succinic anhydride and/or phthalic anhydride yielded compounds 14 and 15 . Benzoylation of 4‐amino‐6‐methyl‐3‐(2H)‐thioxo‐5‐(4H)‐oxo‐2‐(2,3,4,5‐tetra‐O‐acetyl‐α‐D‐glucopyra‐nosyl)‐1,2,4‐triazine ( 19 ) afforded the corresponding 4‐N,N‐dibenzoyl derivative 20 . Deblocking of the N‐2 glycoside 21 and the S‐glycoside 22 by methanolic ammonia gave compounds 23 and 24 . Acetylation of 4‐amino glycoside 25a afforded the corresponding 4‐mono‐ and 4‐diacetyl derivatives 26 and 27 . Deamination of 25a,b yielded compounds 28a,b . Methylation of compound 28b afforded the corresponding N4‐ and S‐methyl derivatives 29 and 30 . 相似文献
64.
Starting from 2-isopropylidene-1,3-dithiane, various ketene-dithioacetal-S-imides have been prepared and characterized by1H- and13C-NMR spectra.
Ketendithioacetal-S-imideKurze Mitteilung相似文献
65.
66.
1,3-Dithiane-1-N-p-chlorophenylimides (1,4-9) were prepared and their configuration and conformation was determined by 1H and 13C NMR. The compounds were rearranged to the corresponding 2-(2'-amino-5'-chlorophenyl)-1, 3-dithianes (1U,4U,9U). The rearrangement reactions took place with ?95% stereospecifity. The mechanism of the reaction was investigated with the aid of analogs specifically deuterated at C-2. 相似文献
67.
Rhodium-Katalysierte Isomerisierungen von 2-Methylidenglutarsäureestern zu Methylglutaconsäureestern
The Rhodium(III)-catalyzed thermal isomerization of 2-methylidenglutaric acid esters affords predominantly mixtures of cis/trans-2-methylglutaconic acid esters ( 2, 3 ) which upon slow distillation isomerize completely into the cis-esters ( 2 ). Saponification of 2 yields trans-2-methylglutaconic acid ( 5 ). Attempts to prepare the acid chloride of 5 produces 6-chloro-5-methylpyrone-2 ( 9 ) or 6-chloro-3-methylpyrone-2 ( 10 ) which react with anilines to N-substituted derivatives of trans-4-methylglutaconic acid amides ( 11 ). The thermal isomerizations of the respective esters are discussed in terms of 1,5-hydrogen shifts in their ester enol structures. 相似文献
68.
The thermal decomposition of Co(NO3)2·6H2O (1) as well as that one of NO[Co(NO3)3] (Co(NO3)2·N2O4) (2) was followed by thermogravimetric (TG) measurements, X-ray recording and Raman and IR spectra. The stepwise decomposition reactions of 1 and 2 leading to anhydrous cobalt(II)nitrate (3) were established. In N2 atmosphere, cobalt oxides are finally formed whereas in H2/N2 (10% H2) cobalt metal is produced. Rapid heating of cobalt(II)nitrate hexahydrate causes melting (formation of a hydrate melt) and therefore side reactions in the hydrate melt by incoupled reactions and evolution/evaporation of different species as, e.g., HNO3, NO2, etc. In case of larger amounts in dense packing in the sample container, the formation of oxo(hydoxo)nitrates is possible at higher temperature. For 2, its thermal decomposition to 3 was followed and its decomposition mechanism is proposed. 相似文献
69.
Dalia R. Imam Ahmed A. El-Barbary Claus Nielsen Erik B. Pedersen 《Monatshefte für Chemie / Chemical Monthly》2002,133(5):723-734
Summary. 5-Isopropyl-6-naphthyl uracil and 5-isopropyl-6-naphthyl-2-thiouracil were alkylated to give N-1-(ethoxymethyl and methylthiomethyl)
uracil and S2-cyclohexyl-thiouracil, respectively. 5-Ethyl-6-naphthyl uracil and 5-ethyl-6-naphthyl-2-thiouracil afforded N-1-(ethoxymethyl,
methoxy-methyl, methylthiomethyl, acetoxyethoxy methyl and hydroxyethoxy methyl) uracil and S2-((2,2- diethoxyethyl), methoxycarbonylmethyl, ethoxycarbonylpropyl, methylthiomethyl, ethoxymethyl, methyl and cyclohexyl)-thiouracil
upon alkylation.
Received September 25, 2001. Accepted (revised) December 3, 2001 相似文献
70.
Srivastava SK Ojha AK Kiefer W Asthana BP 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(13-14):2832-2839
Raman spectra of propylamine (C3H7NH2) and its binary mixtures, C3H7NH2 + CH3OH with varying mole fractions of the reference system, C3H7NH2, C were recorded in two widely apart wavenumber regions, 3100-3600 cm(-1) and 1225-1325 cm(-1). In the former region, the two Raman bands at approximately 3305 and approximately 3326 cm(-1), obtained after the line shape analysis, which were assigned to symmetric nu(N-H) and anti-symmetric nu(N-H) stretching modes, respectively, show a downshift upon dilution. However, whereas the nu(N-H) anti-symmetric mode shows a shift of 18.6 cm(-1), the nu(N-H) symmetric mode shows a much smaller shift (5.7 cm(-1)) between neat liquid and high dilution, C = 0.1. This aspect has been explained using the optimized geometries calculated employing ab initio theory (MP2 level) for the neat C3H7NH2 and its different hydrogen-bonded complexes. The linewidth versus concentration plot for the nu(N-H) anti-symmetric stretching mode, however exhibits a distinct maxima at C = 0.4, which has been explained as a slight departure from the concentration fluctuation model. In the latter region, a symmetric peak is observed, which corresponds to nu(C-N) stretching mode, which shows an upshift upon dilution and an almost linear concentration dependence. This has also been explained in terms of the parameters obtained from the optimized geometries of the different hydrogen-bonded complexes. 相似文献