全文获取类型
收费全文 | 2428篇 |
免费 | 42篇 |
国内免费 | 5篇 |
专业分类
化学 | 1591篇 |
晶体学 | 5篇 |
力学 | 28篇 |
数学 | 504篇 |
物理学 | 347篇 |
出版年
2021年 | 15篇 |
2019年 | 17篇 |
2018年 | 19篇 |
2016年 | 36篇 |
2015年 | 33篇 |
2014年 | 36篇 |
2013年 | 65篇 |
2012年 | 65篇 |
2011年 | 85篇 |
2010年 | 52篇 |
2009年 | 49篇 |
2008年 | 68篇 |
2007年 | 73篇 |
2006年 | 68篇 |
2005年 | 54篇 |
2004年 | 70篇 |
2003年 | 33篇 |
2002年 | 46篇 |
2001年 | 49篇 |
2000年 | 33篇 |
1999年 | 25篇 |
1998年 | 26篇 |
1997年 | 31篇 |
1996年 | 39篇 |
1995年 | 36篇 |
1994年 | 38篇 |
1993年 | 29篇 |
1992年 | 29篇 |
1991年 | 21篇 |
1990年 | 35篇 |
1989年 | 23篇 |
1988年 | 27篇 |
1987年 | 31篇 |
1986年 | 17篇 |
1985年 | 53篇 |
1984年 | 43篇 |
1983年 | 40篇 |
1982年 | 19篇 |
1981年 | 43篇 |
1980年 | 39篇 |
1979年 | 26篇 |
1978年 | 39篇 |
1977年 | 36篇 |
1976年 | 26篇 |
1975年 | 35篇 |
1974年 | 32篇 |
1973年 | 22篇 |
1972年 | 17篇 |
1960年 | 19篇 |
1885年 | 15篇 |
排序方式: 共有2475条查询结果,搜索用时 375 毫秒
991.
The reaction of cis‐(2,2′‐bipyridine) Pt(CCPh)2 cis‐(4,4′‐dimethyl‐2,2′‐bipyridine) Pt‐(CCPh)2 and trans‐(Ph3P)2Pt(CCPh)2 towards different group 11 transition‐metal salts [M′X] (M′ = Cu, Ag; X = inorganic ligand) to give heterobimetallic or linear oligomeric and polymeric transition metal complexes is described. Different coordination modes for M′, PhCC, PPh3, and X were found in these species. The structural aspects as well as the preference for one coordination mode over another is discussed. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:521–533, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10097 相似文献
992.
The B‐(triphenylgermyl)borazines 4 a and 4 b , the 1,2‐bis(dimethylamino)‐1,2‐bis(triphenylgermyl)‐diborane(4), 5 , and the (2,2,6,6‐tetramethylpiperidino)(triphenylgermyl)‐boranes 6 and 7 were prepared by allowing LiGePh3 to react with the corresponding B‐bromoborazines and aminochloroboranes, respectively. BH3 dissolved in thf readily adds to LiGePh3 generating Li(H3BGePh3), 8 a , in thf solution. Addition of N‐bases to the solution of 8 a produced (tmen · thf)Li(H3BGePh3), 8 b , and dimeric (py)2Li(H3BGePh3), 8 c . The borazine ring in 4 b is distorted into a boat shape. In 5 the NBGe planes are twisted against each other by 85°. Comparison with analogous (triphenylstannyl)boranes points to a more pronounced steric effect of the Ph3Ge group over the Ph3Sn group due to the shorter B–Ge bond. A fairly short B–Ge bond is found for the (triphenylgermyl)trihydroborates. The molecular structure of (Et2O)3LiGePh3 shows compressed C–Ge–C bond angles. Its molecular parameters fit well into the series LnLiEPh3 (E = Si, Sn, Pb). 相似文献
993.
Sven Boomgaarden Wolfgang Saak Manfred Weidenbruch Heinrich Marsmann 《无机化学与普通化学杂志》2001,627(3):349-352
Treatment of hexakis(2,4,6‐triisopropylphenyl)tetrasilabuta‐1,3‐diene ( 1 ), R2Si=SiR–SiR=SiR2, with ammonia and chlorine furnishes the correspondingly substituted 1,4‐diaminotetrasilane ( 3 ) and 1,2,3,4‐tetrachlorotetrasilane ( 6 ). While product 6 crystallizes as a racemate, 3 forms a conglomerate of enantiomerically pure crystals. The change of colors during the formation of 6 indicates a stepwise reaction sequence. 相似文献
994.
New stannylboranes were prepared from tetramethylpiperidino dichloroborane or B‐bromo‐pentamethylborazine with lithium triorganylstannides LiSnR3. Only double stannylation was possible with tmpBCl2 and LiSnMe3, while tmpBCl(SnPh3) was obtained by employing LiSnPh3. This chloride reacted with LiGePh3 to the stannyl germyl borane tmpB(GePh3)(SnPh3). On the other hand, PhMeNBCl2 and an excess of LiSnMe3 gave the borate Li[B(NMePh)(SnMe3)3], which was isolated as a solvate with 4 molecules of THF. The compound is present in the solid state as a solvent separated ion pair. The borate Li(H3BSnMe3) · 2 THF is dimeric in the solid state. Dimerization occurs via two single Li–H–B bridges and a Li–H(B)–Li bridge. The B–Sn bonds in the borates are practically of the same lengths as those in the boranes. In solution all BH bonds of this trihydridoborate are equivalent. 相似文献
995.
Non‐systematic Zinc Chemistry: Accidental Formation and Structure of Zn6(SC6F5)10(BPPAE)2 The reaction between pyridine‐2‐carbaldehyde and ammonia‐contaminated zinc‐bis(pentafluorothiophenolate) yielded, beside the expected 1 : 1 adduct, small amounts of the hexanuclear cluster Zn6(SC6F5)10(BPPAE)2. Its ligand BPPAE (1,2‐bis‐pyridine‐2‐yl‐2[(pyridine‐2‐ylmethylene)‐amino]‐ethenolate) results from condensation of three molecules of pyridine‐2‐carbaldehyde with one molecule of ammonia. The Zn6 cluster consists of two centrosymmetrically arranged Zn3 units which contain three differently coordinated zinc ions in ZnS4, ZnS2ON and ZnSON3 environments. 相似文献
996.
Gottfried Mrkl Robert Ehrl Heinrich Sauer Peter Kreitmeier Thomas Burgemeister 《Helvetica chimica acta》1999,82(1):59-84
Configurational and Conformational Isomeric Paratopic, Rotational Dynamics Tetraepoxy[30]annulenes(6.2.6.2) and Diatropic Tetraoxa[30]porphyrin(6.2.6.2) Dications: Detection of a Tetraepoxy[31]annulene(6.2.6.2)Radical Cation The synthesis of tetraepoxy[32]annulenes(6.2.6.2) ( 4 ) by a cyclizing twofold Wittig reaction of (E,E,E)-5,5′-(hexa-1,3,5-triene-1,6-diyl)bis[furan-2-carbaldehyde] ( 6 ) and the corresponding bis-phosphonium salt 7 is described (Scheme 1). Contrary to the configuration of the educts, the obtained annulenes 4a and 4b are (Z,E,E,E,Z,E,E,E)- and (E,Z,E,E,E,Z,E,E)-configurated, respectively. The 1H-NMR spectra establish the paratropic, antiaromatic character of 4 . The annulenes 4 are highly dynamic systems, the (E)-ethenediyl bridges rotate around the adjacent σ-bonds, these rotations are frozen at −80°. The McMurry condensation of dialdehyde 6 yields the (E,E,Z,E,E,E,Z)-4,5-dihydrotetraepoxy[32]annulene(6.2.6.2) ( 13a ), where the configuration of the dialdehyde 6 – beside the hydrogenated double bond – is retained. As result of an intramolecular McMurry reaction of 6 , (Z,E,Z,Z)-dioxa[16]annulene(6.2) 14 is formed. By oxidation of the [32]annulenes(6.2.6.2) 4a and 4b , a mixture of the four stereoisomeric tetraoxa[30]porphyrin(6.2.6.2) dications 5a / 5a ′/ 5b / 5c is obtained; the configuration of the isomers is determined by COSY, NOESY, and NOE experiments. The Δδ values (26.81, 25.83, and 21.11 ppm) underline the diatropic, aromatic character of the dications 5 , the Soret bands are shifted bathochromically to 550 nm, and the Q-bands are in the NIR region (896 – 1039 nm). The dihydroannulene 13a is dehydrogenated by p-chloroanil (tetrachloro-1,4-benzoquinone) to give the annulenes 4a and 4b , its oxidation with DDQ (=4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile) results in the same mixture of dications 5 . Entirely different results are obtained by reaction of the dihydroannulene 13a with DDQ. Here, the (E,E,E,Z,E,E,E,Z) tetraoxa[30]porphyrin(6.2.6.2) dication 5c – formed only in traces from 4a / 4b – is the main product. Beside 5c , a by-product (3%) can be isolated, which turns out (ESR, conductivity) to be the (E,E,E,Z,E,E,E,Z)-tetraoxa[31]porphyrin(6.2.6.2) radical cation 16 , obviously the intermediate in the oxidation sequence of the annulene to the dication. This result leads to the conclusion that the reaction of the dihydro compound 13a with p-chloroanil and DDQ follows different reaction mechanisms. For all isolated stereoisomeric tetraepoxy annulenes and tetraoxaporphyrin dications, the ΔHf values are calculated by the semiempiric AM1 method. The results are in agreement with the experimental observations. All data confirm the antiaromaticity of the tetraepoxy[32]annulenes(6.2.6.2) 4 and the aromaticity of the tetraoxa[30]porphyrin(6.2.6.2) dications. 相似文献
997.
998.
999.
Matthias Westerhausen Roland Knig Tassilo Habereder Heinrich Nth 《无机化学与普通化学杂志》1999,625(10):1740-1747
Tetrakis(tetrahydrofuran- O )calcium bis[pentacarbonyl(cyano)chromate(0)], -molybdate(0)], and -tungstate(0)] The reaction of (THF)2Ca[N(SiMe3)2]2 with hexacarbonylchromium, -molybdenum, and -tungsten yields hexamethyldisiloxane and tetrakis(tetrahydrofuran-O)calcium bis[pentacarbonyl(cyano)chromate(0)] ( 1 ), -molybdate(0)] ( 2 ), and -tungstate(0)] ( 3 ), respectively. Compound 1 crystallizes in the centrosymmetric space group P 1, whereas the other two complexes 2 and 3 are isotypic (monoclinic, P21/n). In all these compounds the calcium atom is bonded to the nitrogen atoms of the cyano ligands and bent Ca–N–C fragments with angles of 158,3° ( 1 ), 161,2° ( 2 ), and 161,3° ( 3 ) are observed. 相似文献
1000.