全文获取类型
收费全文 | 3916篇 |
免费 | 119篇 |
国内免费 | 16篇 |
专业分类
化学 | 2443篇 |
晶体学 | 20篇 |
力学 | 217篇 |
数学 | 764篇 |
物理学 | 607篇 |
出版年
2023年 | 17篇 |
2022年 | 53篇 |
2021年 | 106篇 |
2020年 | 78篇 |
2019年 | 59篇 |
2018年 | 70篇 |
2017年 | 63篇 |
2016年 | 124篇 |
2015年 | 121篇 |
2014年 | 91篇 |
2013年 | 207篇 |
2012年 | 223篇 |
2011年 | 275篇 |
2010年 | 158篇 |
2009年 | 136篇 |
2008年 | 239篇 |
2007年 | 244篇 |
2006年 | 182篇 |
2005年 | 167篇 |
2004年 | 162篇 |
2003年 | 126篇 |
2002年 | 104篇 |
2001年 | 48篇 |
2000年 | 30篇 |
1999年 | 46篇 |
1998年 | 43篇 |
1997年 | 39篇 |
1996年 | 42篇 |
1995年 | 35篇 |
1994年 | 37篇 |
1993年 | 57篇 |
1992年 | 23篇 |
1991年 | 29篇 |
1990年 | 33篇 |
1989年 | 22篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 25篇 |
1985年 | 56篇 |
1984年 | 46篇 |
1983年 | 26篇 |
1982年 | 31篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 27篇 |
1977年 | 23篇 |
1976年 | 22篇 |
1975年 | 15篇 |
1974年 | 21篇 |
排序方式: 共有4051条查询结果,搜索用时 15 毫秒
961.
Ambrosi G Formica M Fusi V Giorgi L Guerri A Lucarini S Micheloni M Paoli P Rossi P Zappia G 《Inorganic chemistry》2005,44(9):3249-3260
The synthesis and characterization of new polydentate ligand 2-(N),2'-(N')-bis[2-(3-hydroxy-2-oxo-2H-pyridin-1-yl)acetamido]-1(N'),2(N),2'(N')-trimethyl-2,2'-diaminodiethylamine (L3) is reported. The coordination properties of L3 and of two analogous macrocyclic ligands (L1 and L2) toward Cu(II) and Zn(II) metal ions are reported. All three ligands show the 3-hydroxy-2(1H)-pyridinone (HPO) groups attached as sidearms to a polyaza fragment, which is a macrocyclic framework in the case of L1 and L2 while it is an open chain in the case of L3. The role of the polyaza fragments in preorganizing the two sidearms was investigated. The basicity of L3 and the binding properties of L1-L3 were determined by means of potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, I = 0.15 mol dm(-3)). UV-vis spectra as well 1H and 13C NMR experiments were used to understand the role of the HPO and of the polyaza fragments in the stabilization of the cations. While L1 forms stable mono- and dinuclear complexes, L2 and L3 can form only mononuclear species with each of the metal ions investigated. In the main mononuclear species of L2 and L3, the two HPO moieties stabilize the M(II) in a square planar geometry due to the two oxygen atoms of each HPO. The coordination sphere of the metal is completed by adding a secondary ligand such as water molecules in the case of Cu(II) systems or OH- in the Zn(II) systems. These results are confirmed by the crystal structures of the [CuH(-1)L2]+ and [CuH(-1)L3]+ species reported herein. Two conformations of L1 can be hypothesized in the formation of the dinuclear species, as suggested by NMR experiments on the [ZnH(-2)L1] species, which shows two conformers slowly interchanging on the NMR time scale, one of which was found to be more insoluble. 相似文献
962.
Complexes of the type syn-(R,S)-Me(3)dienPtL (Me(3)dien = N,N',N' '-trimethyldiethylenetriamine; L = guanine or hypoxanthine derivative) have two rotamers, a feature useful for assessing hydrogen-bond interactions between a Me(3)dien NH group and either the O6 or the phosphate group of the coordinated L. The two rotamers are defined as endo and exo for the rotamer with the six-membered ring of the purine on the same side and on the opposite side, respectively, of the coordination plane as the N-Me's. For L = 5'-GMP and 5'-IMP the endo rotamer is the exclusive form (at neutral and basic pH) or is present at 90% and more (low pH where 5'-phosphate group is protonated). A 5'-phosphate group can be positioned to form a direct H-bond with a Me(3)dien NH group only in the endo form; such an H-bond explains this high endo preference. Such a direct phosphate-NH H-bond is not possible for other complexes used in this study because either L has no phosphate group (9-EtG, Guo) or the phosphate is at the 3'-position (3'-GMP and 3'-IMP), too far for H-bonding. Nevertheless, a preference for the endo rotamer was observed for these L also. This result is opposite to that expected both from potential steric repulsion of the L O6 with the N-Me groups and also from the lack of a potential favorable H-bond interaction between L O6 and a Me(3)dien NH. For the 9-EtG adduct, the temperature dependence of the endo/exo equilibrium and the activation parameters for endo/exo interconversion suggest that the preference for the endo rotamer arises from the hydration of the Me(3)dien NH groups; such hydration is favorable in the endo rotamer. At basic pH, N1H deprotonation increases the H-bond capacity of O6, and the exo rotamer increases in stability, becoming the dominant rotamer for the 9-EtG and Guo adducts. For L = 3'-GMP and 3'-IMP, stabilization of the endo form upon phosphate deprotonation at neutral pH was observed. This result is attributed to an H-bonding network involving water, the 3'-phosphate, and the Me(3)dien NH groups. 相似文献
963.
Paolo Ganis Donatella Furlani Daniele Marton Giuseppe Tagliavini Giovanni Valle 《Journal of organometallic chemistry》1985,293(2):207-212
The crystal structure of (C6H5)3SnCH2CHCH2 has been determined. The asymmetric unit consists of two molecules of triphenylallyltin having virtually the same conformation. Their geometrical parameters exclude pπ-dπ bonding between the metal and the allyl double bond, and seem to be in a better agreement with a σ-π conjugation effect. 相似文献
964.
Bianchi L Dell'Erba C Maccagno M Petrillo G Rizzato E Sancassan F Severi E Tavani C 《The Journal of organic chemistry》2005,70(22):8734-8738
[reaction: see text] In prosecution of previous work on the thermal cyclization of 1-aryl-4-methanesulfonyl-2-nitro-3-phenylsulfonyl-1,3-butadienes (7), the 3-unsubstituted derivatives 8, deriving from the initial ring opening of 3-nitrothiophene (2), have been likewise found herein to undergo cyclization, followed by aromatization, in analogous mild experimental conditions, leading to the ring-fused homo- or heteroaromatic nitro derivatives 10. The concerted electrocyclic nature of the process is strongly supported by the outcome of tests based on the variation of the polarity of the solvent or of the electron density on the aryl of 8. Thus, the successful application of the process to the non-phenylsulfonyl-activated 8 significantly widens the scope of a synthetically valuable overall ring-opening/ring-closing procedure from nitrothiophenes. Support to the recently renewed interest in thermal 6pi electrocyclizations as a tool for the construction of the benzene ring is furthermore provided. 相似文献
965.
Summary The crystal structure of bis(thiourea)iron(II) thiocyanate, Fe[SC(NH2)2]2(NCS)2, has been determined and refined by three-dimensional x-ray analysis. The complex crystallizes in the triclinic system, space group P –1, witha = 10.114(6),b = 7.662(8),c = 3.897(10) Å, = 105.0(2)°, = 81.8(3)°, = 96.4(3)° andz = 1. It is isostructural with M(tu)2(NCS)2, where M = Mn, Co or Ni and to = thiourea. The iron atom is located on a centre of symmetry and is coordinated to four planar sulfur atoms of the thiourea ligands (Fe-S 2.61 and 2.65 Å) and two nitrogen atoms of the thiocyanate groups (Fe-N 2.05 Å) in a distorted octahedral way. These octahedra are linked in chains with each thionilic sulfur atom shared by two adjacent iron atoms; magnetic measurements, Mössbauer and spectroscopic data are consistent with this polymeric structure. 相似文献
966.
Giovanni Piccardi Francesco Pergola Maria Luisa Foresti Rolando Guidelli 《Journal of Electroanalytical Chemistry》1977,84(2):235-253
Polarographic current-potential characteristics and current-time curves for the reduction of methylene blue (MB) to methylene blue leucoform (MBL) in a pH 7.9 aqueous phosphate buffer have been examined in detail over a wide concentration range. It has thus been shown that the so called “normal” or “main” reduction wave of MB actually consists of two separate steps, the former with a half-wave potential practically coinciding with the formal potential E0=?0.250 V/SCE of the MB/MBL couple and the latter with a half-wave potential of about ?0.310 V/SCE. As soon as the well-known MB adsorption prewave has attained its maximum height (which occurs at a MB bulk concentration c0*?5×10?5M), a further slight increase in c0* causes the appearance of the wave with E1/2=?0.310 V. The height of the linear potential-sweep voltammetric peak corresponding to the latter wave increases proportionally to the sweep rate, thus revealing the “adsorption” nature of this wave. A comparison with a previous chronocoulometric investigation of the MB/MBL system has permitted us to conclude that the wave with E1/2=?0.310 V is due to reduction of the MB molecules which, after having reached the surface of the dropping electrode by diffusion, are adsorbed at the top of the adsorbed monolayer of MBL in direct contact with the electrode and remain in this adsorbed state after reduction. For c0*>7×10?5M the wave with E1/2?E0=?0.250 V starts to develop. This wave is due to the electroformation of MBL molecules which diffuse back into the solution. The shape of polarographic current-potential characteristics and current-time curves has been accounted for semiquantitatively through an approximate solution of the corresponding diffusional problem. 相似文献
967.
Giovanni Carini 《Rendiconti del Circolo Matematico di Palermo》1987,36(3):423-433
We show that the Euler equation of classical ideal fluid dynamics can be deduced either from the Lagrange-D' Alembert principle or from the Hamilton principle by means of only one method, which is valid both to ideal compressible and to incompressible fluids. Futhermore, making use of some thermodynamical relations we determine the conditions under which the motion of the medium must occur in order to be possible to introduce the Hamilton's action A and the Lagrangian L of the motion. 相似文献
968.
969.
Summary The Ni2(NCS)4(4-Methylpyridine)6 complex has been prepared crystalline from methanolic solutions of Ni(NCS)2(4-Methylpyridine)4. Three dimensional x-ray diffraction data were collected by means of an automated single crystal diffractometer. The structure was solved by Patterson methods and refined to a final R value of 0.075, based on 2422 observed_ reflections. The compound crystallizes in space group P1 with one dimeric molecule in a unit cell of dimensions a = 10.57(1), 6 = 14.06(1), c = 8.66(1) Å, = 105.2(1), = 114.8(1), y = 95.6(1)°. The Ni atoms have distorted octahedral coordination with one sulfur and five nitrogen atoms - two of the four thiocyanate groups form a double bridge between the neighbouring Ni atoms.4-MePy = 4-Methylpyridine 相似文献
970.
Spezia S Bocca B Forte G Gatti A Mincione G Ronchi A Bavazzano P Alimonti A Minoia C 《Rapid communications in mass spectrometry : RCM》2005,19(11):1551-1556
In recent years the increasing use of platinum (Pt) both in medical and in industrial applications has caused its growing anthropogenic emission and spread in the environment. Pt is released into the atmosphere by exhaust catalytic converters, and Pt compounds are often used in antitumour therapies. As a consequence, significant amounts of Pt can be detected in hospital wastewaters. This can lead to an increase in the exposure levels to Pt, especially in urban areas. It is therefore necessary to determine Pt reference values in the general population, by using suitable procedures able to achieve adequate analytical performances. Several measurements of Pt in biological fluids have been reported, but the analytical methods used for the determination of Pt often lack information about the uncertainty of the results, especially for low concentrations of urinary Pt in non-occupationally exposed subjects. The present paper considers the measurement of urinary Pt levels in a general population group from central Italy, by both quadrupole (Q) and sector field (SF) inductively coupled plasma mass spectrometry (ICP-MS). The two procedures were validated and their expanded uncertainties were evaluated. The limits of detection (LODs), calculated taking into account dilution factors, were 0.18 and 0.05 ng L(-1) of Pt for the Q and SF procedures, respectively. The median value observed was 4.13 ng L(-1) of Pt in urine, while the relative combined uncertainty at 5 ng L(-1) was below 20% with both ICP-MS techniques. These data are in good agreement with those reported in the literature for similar studies. 相似文献