全文获取类型
收费全文 | 703篇 |
免费 | 6篇 |
国内免费 | 1篇 |
专业分类
化学 | 358篇 |
晶体学 | 5篇 |
力学 | 8篇 |
数学 | 40篇 |
物理学 | 299篇 |
出版年
2020年 | 5篇 |
2018年 | 6篇 |
2017年 | 10篇 |
2016年 | 9篇 |
2014年 | 11篇 |
2013年 | 63篇 |
2012年 | 45篇 |
2011年 | 35篇 |
2010年 | 20篇 |
2009年 | 24篇 |
2008年 | 36篇 |
2007年 | 27篇 |
2006年 | 30篇 |
2005年 | 20篇 |
2004年 | 21篇 |
2003年 | 15篇 |
2002年 | 20篇 |
2001年 | 13篇 |
2000年 | 16篇 |
1999年 | 13篇 |
1998年 | 4篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1978年 | 4篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1970年 | 4篇 |
1966年 | 6篇 |
1963年 | 4篇 |
1962年 | 9篇 |
1961年 | 7篇 |
1960年 | 10篇 |
1959年 | 13篇 |
1958年 | 24篇 |
1957年 | 8篇 |
1953年 | 4篇 |
1930年 | 3篇 |
排序方式: 共有710条查询结果,搜索用时 31 毫秒
21.
Krishna C. Majumdar Uday K. Kundu Swapan K. Samanta Nirmal K. Jana 《Monatshefte für Chemie / Chemical Monthly》2003,134(8):1137-1143
Summary. Two hitherto unreported pyrimidine annelated heterocycles were synthesized from 6-cyclopent-2-enyl-5-hydroxy-1,3-dimethylpyrimidine-2,4(1H,3H)-dione and 5-cyclopent-2-enyl-6-hydroxy-1,3-dimethylpyrimidine-2,4(1H,3H)-dione by reaction with pyridine hydrotribromide or hexamethylenetetramine hydrotribromide. The first one was also obtained by reaction with concentrated sulfuric acid.Received October 28, 2002; accepted October 30, 2002
Published online June 2, 2003 相似文献
22.
The reagent 5,6-diphenyl-2,3-dihydro(asym)triazine-3-thione produces a green complex with perrhenate in hydrochloric acid medium in presence of stannous chloride. The complex on extraction in chloroform shows a peak at 685 nm. The system obeys Beer's law from 2–10 p.p.m. of rhenium and its molar absorptivity is 1.53-l0-4. It tolerates the presence of a large number of ions, including MoXXX and WXXX. Job's and molar ratio methods suggest that rhenium and the reagent are present in the ratio 1:2 and the isolated complex is of the same composition, i.e. ReO(C25H10N3S)2. 相似文献
23.
Zusammenfassung Es wurde festgestellt, daß die Mehrzahl der Amidoxime mit folgenden Ionen reagiert: Hg2+, Cu2+, Fe3+, Co2+, UO2
2+, Cd2+, Ag+;, Pb2+, Pd2+, Ni2+. Von den Malonsäure-bis-amidoximen wurden hergestellt die Silber-, Blei-, Cadmium-, Kupfer- und zwei Quecksilbersalze, von den Amidoximen der Nicotin- und Isonicotinsäure Palladium-, Kupfer-, Cadmium- und Bleisalze. Die Salzbildungsfähigkeit der Amidoximgruppe ist vom Radikal, von den Substituenten an der Amidgruppe und vom Reagens-medium abhängig. Die braunrote Färbung mit Fe3+-Ionen ist für alle Amidoxime typisch, diese Reaktion kann zum Nachweis der am Amidstickstoff nichtsubstituierten Amidoxime angewandt werden.IX. Mitteilung: Collect. czechoslov. chem. Commun. 24, 1720 (1959). — X. Mitteilung: Collect. czechoslov. chem. Commun. (im Druck). 相似文献
24.
Summary Like -amino-picolinic acid, quinolinic acid behaves as a highly selective reagent for palladium with which it forms a complex of composition Pd(C7H4O4N)2. The complex being very stable is precipitated from solutions with an acidity of 0.25 N to a maximum pH of 2.1 and thus is separated from all the common ions and other platinum metals except copper which, however, can be kept in solution in presence of EDTA. The palladium complex is either weighed after drying or dissolved in a standard potassium cyanide solution which is later back titrated with silver nitrate solution. 相似文献
25.
Geometries and energy separations of the various low-lying electronic states of Nb(n) and Nb(n) (-) (n=4,5) clusters with various structural arrangements have been investigated. The complete active space multiconfiguration self-consistent field method followed by multireference singles and doubles configuration interaction (MRSDCI) calculations that included up to 52x10(6) configuration spin functions have been used to compute several electronic states of these clusters. The ground states of both Nb(4) ((1)A('), pyramidal) and Nb(4) (-) ((2)B(3g), rhombus) are low-spin states at the MRSDCI level. The ground state of Nb(5) cluster is a doublet with a distorted trigonal bipyramid (DTB) structure. The anionic cluster of Nb(5) has two competitive ground states with singlet and triplet multiplicities (DTB). The low-lying electronic states of these clusters have been found to be distorted due to Jahn-Teller effect. On the basis of the energy separations of our computed electronic states of Nb(4) and Nb(5), we have assigned the observed photoelectron spectrum of Nb(n) (-) (n=4,5) clusters. We have also compared our MRSDCI results with density functional calculations. The electron affinity, ionization potential, dissociation and atomization energies of Nb(4) and Nb(5) have been calculated and the results have been found to be in excellent agreement with the experiment. 相似文献
26.
Summary The estimation of bismuth by the reagent Bismuthiol II is studied critically. The effect of acidity, reagent concentration and interfering ions are given in detail. The maximum acidity that may be tolerated for the complete precipitation of bismuth is 0.3 N in nitric acid, 0.5 N in hydrochloric acid and 1N in sulphuric acid. Higher acidity than 0.1 N decomposes the reagent present in excess. In 0.1 N nitric acid bismuth has been separated from a number of ions like Al3+, Cr3+, Th4+, rare earths, Zr4+, Ti4+, UO2
2+, Be2+, Mn2+, Co2+, Ni2+, Mg, alkalis and alkaline earths, SO4
2–, Cl–, C2O4
2–- and from Fe2+ and Ce3+ in 0.1 N hydrochloric acid. In presence of a citrate or a tartrate it can be separated from As3+, Ce4+, MoO4
2–- and WO4
2–-at pH 1.5 to 2.5. When Hg2+, Pb2+, Pd2+, Cd2+, Cu2+, Ag+ and Tl+ are present they are first precipitated by the reagent at pH 6 to 8 in presence of a citrate or a tratrate and the bismuth is estimated gravimetrically in the acidified filtrate. Ions as F– and PO4
3– that form insoluble compounds with bismuth, Sb3+ and Sn2+ that form less soluble compounds with the reagent and Fe3+, VO3
–, CrO4
2–, AsO4
3– that act as oxidising agents, interfere. 相似文献
27.
N-(omega-Bromoalkyl)-amino acid derivatives, readily prepared from natural alpha-amino acids, gave cyclic amino acids with a quaternary stereocenter by treatment with potassium hexamethyldisilazaide in DMF. The chirality of parent amino acids was almost completely preserved during an enolate-formation and cyclization process, giving aza-cyclic amino acids in up to 98% ee in retention of configuration. This method is applicable to the asymmetric synthesis of azetidine, pyrrolidine, piperidine, and azepane derivatives. The asymmetric cyclization seems to proceed via an axially chiral enolate intermediate and not through a concerted SEi process. 相似文献
28.
Benzohydroxamic acid (I) or phenylacetylhydroxamic acid (II) is suggested for the quantitative separation of tantalum from niobium in an oxalate solution. The tantalum precipitate must be ignited for weighing; niobium is determined in the filtrate with another reagent. The pH range for complete separation is 4.0–6.4 for I and 4.5–6.2 for II. Single precipitation is satistactory for Nb: Ta ratios of 18 : 1 to 1 : 20 for I, and 8 : 1 to 1 : 23 for II. Titanium, zirconium, tartrate, citrate and a large excess of oxalate interfere. 相似文献
29.
30.
N-Benzoyl-o-tolylhydroxylamine is shown to provide a virtually specific reagent for the spectrophotometric determination of vanadium. The reddish-violet complex formed with the reagent in 4–8 N hydrochloric acid after extraction with chloroform shows absorption maxima at 510 mm, and obeyes Beer's law from 0.5 to 10 μg with an optimum range of 2–10 μg; the percent relative error is 2.7. The sensitivity is 0.0108 μg V/cm2. The complex contains the metal and the reagent in a ratio of 1:2 and the dissociation constant is of the order of 10-9. 相似文献