全文获取类型
收费全文 | 2989篇 |
免费 | 55篇 |
国内免费 | 9篇 |
专业分类
化学 | 1932篇 |
晶体学 | 18篇 |
力学 | 82篇 |
数学 | 305篇 |
物理学 | 716篇 |
出版年
2021年 | 19篇 |
2020年 | 26篇 |
2019年 | 27篇 |
2018年 | 22篇 |
2017年 | 20篇 |
2016年 | 38篇 |
2015年 | 35篇 |
2014年 | 34篇 |
2013年 | 93篇 |
2012年 | 129篇 |
2011年 | 159篇 |
2010年 | 76篇 |
2009年 | 48篇 |
2008年 | 129篇 |
2007年 | 132篇 |
2006年 | 158篇 |
2005年 | 139篇 |
2004年 | 135篇 |
2003年 | 94篇 |
2002年 | 110篇 |
2001年 | 76篇 |
2000年 | 96篇 |
1999年 | 45篇 |
1998年 | 45篇 |
1997年 | 31篇 |
1996年 | 68篇 |
1995年 | 40篇 |
1994年 | 55篇 |
1993年 | 77篇 |
1992年 | 58篇 |
1991年 | 35篇 |
1990年 | 35篇 |
1989年 | 37篇 |
1988年 | 31篇 |
1987年 | 39篇 |
1986年 | 29篇 |
1985年 | 48篇 |
1984年 | 30篇 |
1983年 | 23篇 |
1982年 | 49篇 |
1981年 | 31篇 |
1980年 | 46篇 |
1979年 | 29篇 |
1978年 | 44篇 |
1977年 | 36篇 |
1976年 | 27篇 |
1975年 | 43篇 |
1974年 | 29篇 |
1973年 | 35篇 |
1972年 | 19篇 |
排序方式: 共有3053条查询结果,搜索用时 15 毫秒
151.
Graham Smith Urs D. Wermuth Peter C. Healy Jonathan M. White 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(4):o203-o207
The structures of two pseudopolymorphic hydrates of brucine, C23H26N2O4·4H2O, (I), and C23H26N2O4·5.25H2O, (II), have been determined at 130 K. In both (I) and (II) (which has two independent brucine molecules together with 10.5 water molecules of solvation in the asymmetric unit), the brucine molecules form head‐to‐tail sheet substructures, which associate with the water molecules in the interstitial cavities through hydrogen‐bonding associations and, together with water–water associations, give three‐dimensional framework structures. 相似文献
152.
Reactions between the Os(VI)-nitrido complexes cis- and trans-[Os(VI)(tpy)(Cl)2(N)]+ (tpy is 2,2':6',2"-terpyridine) and triphenylphosphine sulfide, SPPh3, give the corresponding Os(IV)-phosphoraniminato, [Os(IV)(tpy)(Cl)2(NPPh3)]+, and Os(II)-thionitrosyl, [Os(II)(tpy)(Cl)2(NS)]+, complexes as products. The Os-N bond length and Os-N-P angle in cis-[Os(IV)(tpy)(Cl)2(NPPh3)](PF6) are 2.077(6) A and 138.4(4) degrees. The rate law for formation of cis- and trans-[Os(IV)(tpy)(Cl)2(NPPh3)]+ is first order in both [Os(VI)(tpy)(Cl)2(N)]+ and SPPh3 with ktrans(25 degrees C, CH3CN) = 24.6 +/- 0.6 M(-1) s(-1) and kcis(25 degrees C, CH3CN) = 0.84 +/- 0.09 M(-1) s(-1). As found earlier for [Os(II)(tpm)(Cl)2(NS)]+, both cis- and trans-[Os(II)(tpy)(Cl)2(NS)]+ react with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3. For both complexes, the reaction is first order in each reagent with ktrans(25 degrees C, CH3CN) = (6.79 +/- 0.08) x 10(2) M(-1) s(-1) and kcis(25 degrees C, CH3CN) = (2.30 +/- 0.07) x 10(2) M(-1) s(-1). The fact that both reactions occur rules out mechanisms involving S atom transfer. These results can be explained by invoking a common intermediate, [Os(IV)(tpy)(Cl)2(NSPPh3)]+, which undergoes further reaction with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3 or with [Os(VI)(tpy)(Cl)2(N)]+ to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and [Os(II)(tpy)(Cl)2(NS)]+. 相似文献
153.
154.
Jerry E. White Robert E. Singler 《Journal of polymer science. Part A, Polymer chemistry》1977,15(5):1169-1178
A series of polyarylaminophosphazene copolymers have been prepared and are described. In each preparation, two arylamines were allowed to competitively react with polydichlorophosphazene, and the substituent ratios of the resulting copolymers, determined by high-temperature 1H-NMR spectroscopy, generally reflect expected steric and electronic effects on the relative reactivities of the arylamines studied. The copolymers have intrinsic viscosities of 0.64–1.25 dl/g, glass transition temperatures of 75–104°C, and decomposition temperatures of 220–260°C. These properties, which are comparable to those of polyarylaminophosphazene homopolymers, are discussed in regard to what they may reflect about the structure of the polymers. 相似文献
155.
I. L. Brunetti E. J. H. Bechara G. Cilento E. H. White 《Photochemistry and photobiology》1982,36(2):245-249
Abstract In Colchicum autumnale L., colchicine is accompanied by its photoproducts, mainly β-lumicholchicine, even in parts not exposed to light such as the corm of plants grown in the dark. The transformation of colchicine into lumicolchicines can be efficiently accomplished in vitro by energy transfer from enzyme-generated triplet species to colchicine, thus providing additional evidence for'photobiochemistry without light'. 相似文献
156.
Notti A Occhipinti S Pappalardo S Parisi MF Pisagatti I White AJ Williams DJ 《The Journal of organic chemistry》2002,67(21):7569-7572
Synthesis and single-crystal X-ray structures of mixed triple and double calixarenes 6 and 7, obtained from the base-catalyzed condensation of calix[5]arene 1 with cone pertosylated calix[4]arene 2, are reported. VT-NMR studies on 7 are consistent with a molecular motion arising from the anti-gauche conformational interconversion of its ethylene linkages. 相似文献
157.
A differential pulse voltammetric study of complexes of Cd(II) and Pb(II) with crown ethers is reported. Measured log K(1) values for Cd(II) with 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane), 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane), and 12-crown-4 (1,4,7,10-tetraoxacyclododecane) are respectively 2.53 (+/-0.06), 1.97 (+/-0.07), and 1.72 (+/-0.08) and for Pb(II) with 18-crown-6 is 4.17 (+/-0.03), all at 25 degrees C in 0.1 M LiNO(3). Cd(II) is smaller than is usually associated with strong bonding with crown ethers. The high log K(1) values for Cd(2+) with crown ethers found here are discussed in terms of distortion of Cd(II) by relativistic effects. The resulting plasticity of the coordination geometry of the Cd(II) ion allows it to meet the metal ion size requirements of all the crown ethers, allowing high log K(1) values to occur. Crystal structures for [Cd(bz-18-crown-6)(SCN)(2)] (1) (bz-18-crown-6 = benzo-1,4,7,10,13,16-hexaoxacyclooctadecane) and [K(18-crown-6)][Cd(SCN)(3)] (2) are reported. 1 was triclinic, space group P1, a = 8.5413(2), b = 10.0389(2), and c = 13.4644(2) A, alpha = 94.424(1), beta = 102.286(1), and gamma = 93.236(1) degrees, Z = 2, and final R = 0.023. 2 was orthorhombic, space group Cmc2(1), a = 14.7309(3), b = 15.1647(3), and c = 10.6154(2) A, Z = 4, and final R = 0.020. In 1, the Cd occupies the cavity of the bz-18-crown-6 with long average Cd-O bond lengths of 2.65 A and is N-bonded to the thiocyanates with short average Cd-N bonds of 2.12 A. In [Cd(bz-18-crown-6)(SCN)(2)], the linear coordination involving the Cd and the two N-bonded thiocyanate groups in 1 is discussed in terms of the role of relativistic effects in the tendency to linear coordination geometry in group 12 metal ions. In 2 Cd forms a polymeric structure involving thiocyanate bridges between Cd atoms and K(+) occupies the cavity of the crown ether. 2 highlights the fact that cadmium is almost never S-bonded to thiocyanate except in bridging thiocyanates. 相似文献
158.
A.J. White 《Journal of organometallic chemistry》1979,168(2):197-202
The compound (C5H5)2Fe2(CO)3PPh3, previously obtained by the photolysis of (C5H5)2Fe2(CO)4 with PPh3, may also be obtained by reflluxing these same reactants in benzene. The compound was isolated in pure form by means of low temperature column chromatography. It is unstable in solution in the absence of added PPh3. Solid samples also are unstable over long periods of time. Decomposition in solution is complete within one hour at 80° yielding a mixture of (C5H5)2Fe2(CO)4 and (C5H5)4Fe4(CO)4. This reaction is suppressed by excess PPh3. Heating a mixture of (C5H5)2Fe2(CO)3PPh3 and P(OEt)3 gives a nearly quantitative yield of (C5H5)2Fe2(CO)3P(OEt)3. Refluxing a xylene solution of (C5H5)2Fe2(CO)4 containing a slight molarr excess of PPh3 for 7 h results in the isolation of (C5H5)4Fe4(CO)4 in 56% yield, making this reaction by far the most convenient method for the preparation, in gram quantities, of this transition metal cluster. 相似文献
159.
Ron S. Dickson Rhonda J. Nesbit Helen Pateras Jennifer M. Patrick Allan H. White 《Journal of organometallic chemistry》1984,265(2):c25-c28
Complexes of formula (η-C5H52Rh2{CF3C2CF3 · RNCO} have been prepared by three methods, from reactions between organic isocyanates and (η-C5H5)2Rh2(CO)(CF3C2CF3) or (η-C5H5)2Rh2(CO)2(CF3C2CF3); by treatment of (η-C5H5)2Rh2(CO)(CF3C2CF3) with organic azides; and by oxidation with Me3NO of the organic isocyanide in (η-C5H5)2Rh2(CO)(CNR)(CF3C2CF3). The crystal and molecular structure of the complex (η-C5H5)2Rh2{CF3C2CF3 · RNCO} with R = Ph has been determined from single crystal X-ray diffraction data. This reveals that the isocyanate has condensed with the hexafluorobut-2-yne to form an amide ligand of the form C(CF3)C(CF3)C(=O)N(R); this bridges the two rhodium atoms in a μ2η3-manner. 相似文献
160.
Juan M. Kok Kevin C. Lim Brian W. Skelton Allan H. White 《Journal of Cluster Science》2004,15(3):377-386
The structural characterizations of some copper(I) halide (CuX) adducts with norbornadiene (nbd) are recorded. CuCl : nbd (1:1)4 (a redetermination), (2:1)2(|), are systems both based around Cu4Cl4 cubane-type cluster arrays. CuBr : nbd (7:3)(|)( 0.5 MeOH), a complex polymer with 3-symmetry, is believed to be the complex previously described as an adduct of 2:1 stoichiometry. Attempts to obtain an iodide counterpart have resulted in the definition of an ephemeral adduct CuI : MeCN (3:2)(|). 0.5 C7H8 in which, remarkably, the nbd is uncoordinated; the complex is a polymer, related to the [AgX(quinoline)](|) (X = Cl, Br) saddle polymer. 相似文献