全文获取类型
收费全文 | 1194篇 |
免费 | 23篇 |
国内免费 | 6篇 |
专业分类
化学 | 609篇 |
晶体学 | 5篇 |
力学 | 19篇 |
数学 | 132篇 |
物理学 | 458篇 |
出版年
2020年 | 9篇 |
2019年 | 11篇 |
2018年 | 12篇 |
2016年 | 31篇 |
2015年 | 16篇 |
2014年 | 20篇 |
2013年 | 50篇 |
2012年 | 44篇 |
2011年 | 57篇 |
2010年 | 24篇 |
2009年 | 33篇 |
2008年 | 68篇 |
2007年 | 54篇 |
2006年 | 35篇 |
2005年 | 32篇 |
2004年 | 38篇 |
2003年 | 31篇 |
2002年 | 33篇 |
2001年 | 29篇 |
2000年 | 30篇 |
1999年 | 13篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 17篇 |
1995年 | 24篇 |
1994年 | 10篇 |
1993年 | 22篇 |
1992年 | 24篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 21篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1985年 | 19篇 |
1984年 | 24篇 |
1983年 | 18篇 |
1982年 | 19篇 |
1981年 | 22篇 |
1980年 | 16篇 |
1979年 | 24篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 23篇 |
1975年 | 18篇 |
1974年 | 20篇 |
1973年 | 14篇 |
1972年 | 12篇 |
1971年 | 12篇 |
1970年 | 10篇 |
1969年 | 9篇 |
排序方式: 共有1223条查询结果,搜索用时 390 毫秒
961.
Phosphines are traditionally considered as Lewis bases or ligands in transition metal and main group complexes. Despite their electron-rich (lone pair-bearing) nature, an extensive coordination chemistry for Lewis acidic phosphorus centers is being developed; such chemistry provides a new synthetic approach for phosphorus-element bond formation, leading to new types of structures and modes of bonding. Complexes of Ph2P+ with a variety of donor elements (P, N, C) give experimentally short donor-acceptor bond lengths, when compared to other cationic phosphorus Lewis acid complexes. We have calculated that the energy of the lowest unoccupied molecular orbital (LUMO) in Ph2P+ is lower than that of (Me2N)2P+, which partially explains the greater exothermicity of reactions of donors with the diaryl acceptor. Furthermore, the energies required to distort the diphenylphosphenium cation from its ground-state geometry are significantly smaller than those of the diamido cations and, thus, enhance the exothermicity of donor coordination. These computational data, in conjunction with evidence from experimental solid-state structures, indicate that Ph2P+ is a significantly better Lewis acid relative to the more common diaminophosphenium analogues (R2N)2P+ and are used to elucidate the nature of the bonding in donor-phosphenium complexes. 相似文献
962.
Burke A Ellis D Ferrer D Ormsby DL Rosair GM Welch AJ 《Dalton transactions (Cambridge, England : 2003)》2005,(9):1716-1721
Reduction of 1,2-closo-C2B10H12 followed by treatment with [RuCl2(p-cymene)]2(p-cymene = C6H4MeiPr-1,4) affords the 13-vertex ruthenacarborane 4-(p-cymene)-4,1,6-closo-RuC2B10H12, characterised both spectroscopically and, in two crystalline forms, crystallographically. Although asymmetric in the solid state, having a docosahedral cage architecture with cage C atoms at vertices 1 and 6, this species clearly has Cs symmetry on the NMR timescale at room temperature. However, the fluctional process in operation can be arrested at low temperature, and an activation energy of 43.1 kJ mol(-1) is estimated. A computational study of the related species 4-(eta-C6H6)-4,1,6-closo-RuC2B10H12 reveals that the fluctionality is due to a double diamond-square-diamond process, first suggested by Hawthorne et al for the analogous CpCo species. These calculations yield an activation energy of 40.4 kJ mol(-1), in excellent agreement with that derived from experiment. Reduction of 1,2-Ph(2)-1,2-closo-C2B10H10 followed by treatment with [RuCl2(eta-C6H6)]2 or [RuCl2(p-cymene)]2 yields the analogous species 1,6-Ph2-4-(eta-C6H6)-4,1,6-closo-RuC2B10H10 and 1,6-Ph2-4-(p-cymene)-4,1,6-closo-RuC2B10H10, respectively. These C,C-diphenyl compounds were again studied spectroscopically and crystallographically, the p-cymene species again showing two crystalline modifications. In contrast to their CpCo and Cp*Co analogues all three ruthenacarboranes do not undergo isomerisation in refluxing toluene. 相似文献
963.
A simple procedure for the determination of selenium and tellurium in electrolytic copper is described. These two elements are first separated from copper by passing an ammoniacal solution of the sample through Chelex-100 resin. Voltammetric interferences from nitrite liberated during the dissolution of the metal sample in nitric acid and from arsenic and antimony present in the metal are eliminated by addition of hydrogen peroxide. Excess of peroxide is quickly decomposed by the copper(II) ions present. As little as 0.01 μg Se g-1 and 0.02 μg Te g-1 can be determined; relative standard deviations (n = 5) are in the ranges 1.4–3.7% for selenium concentrations of 7.3–0.6 ppm in copper and 1.6—3.1% for tellurium concentrations of 4.6—0.5 ppm. 相似文献
964.
Brennessel WW Ellis JE Pomije MK Sussman VJ Urnezius E Young VG 《Journal of the American Chemical Society》2002,124(35):10258-10259
Reduction of tantalum pentachloride by 6 equiv of sodium naphthalene in 1,2-dimethoxyethane provided, after recrystallization from tetrahydrofuran, 50-55% yields of yellow, pyrophoric [Na(THF)][Ta(C10H8)3]. The product was shown by 1H and 13C NMR spectra and an X-ray study (on the corresponding [Na(crypt 2.2.2)]salt) to be tris(eta4-naphthalene)tantalate(1-), 1, the first homoleptic naphthalene complex of a third row (5d) transition metal. Salts of 1 react under mild conditions with excess CO (1 atm pressure, -60 degrees to +20 degrees C) and 3 equiv of anthracene, C14H10 (20 degrees C), to give 99 and 52% yields of yellow [Ta(CO)6]- and orange [Ta(C14H10)3]-, (2), respectively. The latter is the first homoleptic anthracene complex of a group 5 element and only the third one known, the others being Cr(eta6-C14H10)2 and [Co(eta4-C14H10)2]-. NMR spectra and X-ray structural characterization, as the [Na(crypt 2.2.2)] salt, established 2 to be [Ta(1-4-eta4-C14H10)3]- and is very similar to 1 in solution and in the solid state. Salts of 2 also undergo facile ligand substitution reactions. For example, it reacts with 1,3,5,7-cyclooctatetraene, COT, at 20 degrees in THF to give high yields of the previously known [Ta(COT)3]-, which was structurally characterized as the Na(crypt 2.2.2)salt. One particularly important feature concerning 1 and 2 is that they are the first available synthons for "naked" atomic Ta- and promise to be useful reagents for the general exploration of low-valent tantalum chemistry. Also, 1 and 2 represent the first homoleptic arene tantalum complexes to have been prepared by conventional syntheses. The only previously known substance of this class is the neutral bis(benzene)tantalum(0), which was accessed by the co-condensation of atomic tantalum and benzene vapor in a sophisticated (electron-gun furnace equipped) metal atom reactor. 相似文献
965.
Ellis V. Brown Andrew C. Plasz Stanley R. Mitchell 《Journal of heterocyclic chemistry》1970,7(3):661-665
The mass spectra of all the aminoquinolines, the 2–, 3– and 4-amino-1,5-naphthyridines, some amino-1,6-naphthyridines, and two amino-1,8-naphthyridines with methyl substituents are reported. The major fragment in the aminoquinolines is formed by the loss of HCN from the molecular ion. The most abundant fragment in the aminonaphthyridines is formed by the loss of HCN from the molecular ion except in the 2-amino-1,5-naphthyridine isomer. In both 1,8-naphthyridine isomers investigated, the loss of C2H2 is an alternate fragmentation pathway of significance. In all of the compounds investigated, the loss of the primary amino group from the molecular ion was found to be an insignificant fragmentation. 相似文献
966.
967.
J. I. Degraw J. S. Engstrom M. Ellis H. L. Johnson 《Journal of heterocyclic chemistry》1978,15(2):217-219
The synthesis of methyl 3-(4-imidazolyl)-2-pyridine ( 12 ) and piperidineearboxylates ( 13 ) is described. Hydrolysates of these esters were found to be devoid of inhibitory activity against histidine decarboxylase. 3-Bromoacetyl-2-pieoline ( 2 ) could be converted to 3-(4-imidazolyl)-2-picoline ( 6 ) by two different routes. Treatment of 6 with peroxide and acetic anhydride, followed by transesterification yielded the 2-hydroxymcthyl pyridine ( 9 ). Oxidation of 9 followed by esterification gave the imidazole pyridine acid ester ( 12 ) which after hydrogenation afforded the imidazolylpiperidine ester ( 13 ). 相似文献
968.
During the determination of zinc in a polluted stream by atomic-absorption with electrothermal atomization, chloride was found to exercise a large negative interference (40%) when a carbon-filament atomizer was used, but not when a graphite-furnace atomizer was used. The effect on the filament method was confirmed and shown to be due to the formation of zinc chloride, and further complicated by interaction of this with iron. This interference could be overcome by the use of aqueous ammonia solution or silver nitrate added as matrix modifier. The absence of interference in the graphite-furnace method is attributed to the liberation of hydrogen and removal of chloride as hydrogen chloride. It is further suggested that these observations offer a basis for the exploration of apparently contradictory reports in the literature. 相似文献
969.
Bender R Braunstein P Dedieu A Ellis PD Huggins B Harvey PD Sappa E Tiripicchio A 《Inorganic chemistry》1996,35(5):1223-1234
Two isomers of the phosphido-bridged platinum cluster Pt(3)(&mgr;-PPh(2))(3)Ph(PPh(3))(2) (2 and 3) have been isolated, and their structures have been solved by single-crystal X-ray diffraction. Compound 2 crystallizes in the orthorhombic space group Cmc2(1) with a = 22.192(10) ?, b = 17.650(9) ?, c = 18.182(8) ?, and Z = 4. Compound 3 crystallizes with 2 molecules of dichloromethane in the monoclinic space group C2/c with a = 21.390(10) ?, b = 18.471(9) ?, c = 19.021(11) ?, beta = 105.27(5) degrees, and Z = 4. The two isomers differ essentially in their metal-metal distances and Pt-(&mgr;-PPh(2))-Pt angles. Thus 2, having an imposed C(s) symmetry, contains a bent chain of metal atoms with two short Pt-Pt distances of 2.758(3) ? and a long separation of 3.586(2) ?. In 3, which has an imposed C(2) symmetry, the metal atoms form an isosceles triangle with two Pt-Pt distances of 2.956(3) ? and one of 3.074(4) ?. These isomers can be smoothly interconverted by changing the crystallization solvents. Solution and solid-state (31)P NMR studies have been performed in order to assign the resonances of the different P nuclei and relate their chemical shifts with their structural environments. Raman spectroscopy was used to assign the nu(Pt-Pt) modes of the two structural isomers. Theoretical studies based on extended Hückel calculations and using the fragment molecular orbital approach show that the isomer with the three medium Pt-Pt distances is slightly more stable, in agreement with earlier theoretical predictions. Cluster core isomerism remains a rare phenomenon, and the present example emphasizes the role and the importance of flexible phosphido bridges in stabilizing clusters as well as the unprecedented features which can be observed in phosphine phosphido-rich metal clusters. 相似文献
970.
The reaction ofRu3(-H)(-C,N-C5H4N)(CO)10 (1) with Pt(P
i
Pr3)(nb)2 {nb = bicyclo-[2.2.1]hept-2-ene} does not afford any Ru-Pt mixed metal clusters, but gives instead the mono-substituted phosphine derivative Ru3(-H) (-C,N-C5H4N)(CO)9(P
i
Pr3) (2) as the sole isolable product. Single crystal X-ray studies have been carriedout on 1 and 2. Crystal data for 1: monoclinic, space group P21/c,a = 16.9637(10) A,b = 7.6632(5) Á,c = 17.4058(11) Á, = 117.214(5)°,V = 2009.0(2) Á3,R(R
w) = 0.022 (0.034) for 3090 independent absorption corrected data. Crystal data for 2: triclinic, space group P,a = 9.3389(5) Á,b = 11.4376(6) Á,c = 15.1781(8) Á, = 76.454(4), = 79.900(5), = 67.428(5)°,V = 1448.8(2) Á3
R(R
w
) = 0.024 (0.034) for 4564 independent absorption corrected data. In cluster 1 the Ru-Ru bonds are in the range 2.8462(4)-2.8986(4) Á. The hydride and-pyridyl ligand bridge the same Ru-Ru vector, and the Ru(-H) bridge is symmetric, with Ru-H = 1.78(4) and 1.77(4) Á. In cluster 2 the Ru-Ru distances show a greater ranger 2.7267(3)-3.0604(3) Á. The phosphine ligand is bonded to the Ru atom which is not involved in the-pyridyl bridge. In contrast to 1, the hydride and-pyridyl ligands in 2 bridge different Ru-Ru vectors and the resultant Ru(-H)Ru bridge is asymmetric, with Ru-H = 1.70(4) and 1.89(4) Á. 相似文献