全文获取类型
收费全文 | 19674篇 |
免费 | 629篇 |
国内免费 | 113篇 |
专业分类
化学 | 14117篇 |
晶体学 | 189篇 |
力学 | 378篇 |
综合类 | 1篇 |
数学 | 2955篇 |
物理学 | 2776篇 |
出版年
2022年 | 149篇 |
2021年 | 197篇 |
2020年 | 294篇 |
2019年 | 290篇 |
2018年 | 186篇 |
2017年 | 182篇 |
2016年 | 406篇 |
2015年 | 357篇 |
2014年 | 451篇 |
2013年 | 930篇 |
2012年 | 977篇 |
2011年 | 1270篇 |
2010年 | 619篇 |
2009年 | 524篇 |
2008年 | 1014篇 |
2007年 | 1093篇 |
2006年 | 1073篇 |
2005年 | 1038篇 |
2004年 | 881篇 |
2003年 | 746篇 |
2002年 | 733篇 |
2001年 | 250篇 |
2000年 | 254篇 |
1999年 | 231篇 |
1998年 | 223篇 |
1997年 | 259篇 |
1996年 | 284篇 |
1995年 | 227篇 |
1994年 | 212篇 |
1993年 | 244篇 |
1992年 | 203篇 |
1991年 | 185篇 |
1990年 | 163篇 |
1989年 | 141篇 |
1988年 | 140篇 |
1987年 | 158篇 |
1986年 | 135篇 |
1985年 | 202篇 |
1984年 | 251篇 |
1983年 | 178篇 |
1982年 | 220篇 |
1981年 | 241篇 |
1980年 | 180篇 |
1979年 | 183篇 |
1978年 | 215篇 |
1977年 | 163篇 |
1976年 | 157篇 |
1975年 | 150篇 |
1974年 | 165篇 |
1973年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
231.
Kevin Burgess Brian F.G. Johnson Jack Lewis Paul R. Raithby 《Journal of organometallic chemistry》1982,224(4):C40-C44
Organic azides [N3R] react with [Os3(CO)11(NCMe)] and with [Os3(μ-H)2(CO)10] to form [Os3(CO)10(NCMe)(N3COR)] (R Ph) and [Os3(μ-H)(CO)10(HN3R)] (R Ph, n-Bu, CH2Ph, cyclo-C6H11), respectively; the latter may be converted to [Os3(μ-H)2(CO)9(μ3-NR)] by thermolysis; the molecular structure of the phenyl derivative of each class of compound has been confirmed by x-ray analysis. 相似文献
232.
Chemical reactions occurring at the mineral–water interface are controlled by an interfacial layer, nanometers thick, whose properties may deviate from those of the respective bulk mineral and water phases. The molecular-scale structure of this interfacial layer, however, is poorly constrained, and correlations between macroscopic phenomena and molecular-scale processes remain speculative. The application of high-resolution X-ray scattering techniques has begun to provide substantial new insights into the molecular-scale structure of the mineral–water interface. In this review, we describe the characteristics of synchrotron-based X-ray scattering techniques that make them uniquely powerful probes of mineral–water interfacial structures and discuss the new insights that have been derived from their application. In particular, we focus on efforts to understand the structure and distribution of interfacial water as well as their dependence on substrate properties for major mineral classes including oxides, carbonates, sulfates, phosphates, silicates, halides and chromates. We compare these X-ray scattering results with those from other structural and spectroscopic techniques and integrate these to provide a conceptual framework upon which to base an understanding of the systematic variation of mineral–water interfacial structures. 相似文献
233.
A thin-layer chromatographic procedure is reported for the separation of serotonin from epinephrine and norepinephrine. The method involves the use of ethylenediaminetetraacetic acid-impregnated silica gel G plates and n-butanol-ethanol-acetic acid-water (8:2:1:3) as developer. Serotonin is well separated from both epinephrine and norepinephrine. Epinephrine and norepinephrine overlapped slightly but were both detectable. The procedure is thus applicable to the separation of all three biogenic amines in spite of the absence of an absolutely clear separation of epinephrine from norepinephrine. 相似文献
234.
Studies of the vibrational spectra of matrix-isolated M+NO
3
–
ion pairs have been extended to glassy aprotic solvents. The deuterated form of the solvents DMSO, THF, and ACN have windows through the 7- nitrate ionv
3(e) mode infrared region, so it was possible to clearly observe the splitting of the degeneracy of this mode,v
3, produced by the contacting, but solvated, alkali metal cation. Primary attention has been directed to the extent to which this splitting is reduced relative to the argon matrix values. This reduction, which reflects electron-density transfer from the solvating molecules to the ion pairs, is comparable to that observed for H2O and NH3 matrices as the splitting is reduced to 20–35% of the argon-matrix values. The extent of reduction ofv
3 for the different solvents has been related to Gutmann's donicity number scale with the correlation holding well for solvent molecules of comparable size, DMSO, THF and DMF, but breaking down for the smaller linear ACN, apparently because of more molecules in the cation solvation sphere. The matrix data have also been used, through comparison with spectra for saturated liquid solutions of Li+NO
3
–
, to show that the contact ion pair is the dominant species in liquid THF and ACN, whereas the ions are largely solvent separated in DMSO. 相似文献
235.
Mass spectrometric analysis of ceramide perturbations in brain and fibroblasts of mice and human patients with peroxisomal disorders 总被引:2,自引:0,他引:2
Pettus BJ Baes M Busman M Hannun YA Van Veldhoven PP 《Rapid communications in mass spectrometry : RCM》2004,18(14):1569-1574
In this study, the levels and composition of ceramides in brains of newborn mice lacking peroxisomes (Pex5-/-, Zellweger mice) were analyzed using normal-phase high-performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (HPLC/APCI-MS). Total ceramide compositions were found to be comparable to that of control animals. However, a minor ceramide species, containing hexacosanoic/hexacosenoic acid as the amide fatty acid, was 9-fold increased. Also, in the sphingomyelin-derived ceramides this species was elevated. Subsequent analysis of extracts from fibroblasts of Pex5-/- mice and mice with a defective peroxisomal beta-oxidation (lacking D-specific multifunctional protein 2 (MFP2)), revealed, again, a similar rise in this particular ceramide. Further, this ceramide was elevated in human X-ALD fibroblasts as well. Whether C26:1/0-ceramide is linked to some of the pathology seen in Zellweger syndrome remains to be investigated. However, an increase in this sphingolipid can be considered as a diagnostic criterion for diseases caused by defects in peroxisome biogenesis or peroxisomal beta-oxidation. 相似文献
236.
Highly luminescent and photostable CdS:Mn/ZnS core/shell quantum dots are not water soluble because of their hydrophobicity. To create water-soluble quantum dots by an appropriate surface functionalization, CdS:Mn/ZnS quantum dots synthesized in a water-in-oil (W/O) microemulsion system (reverse micelles) were consecutively overcoated with a very thin silica layer ( approximately 2.5 nm thick) within the same reverse micellar system. The water droplet serves as a nanosized reactor for the controlled hydrolysis and condensation of a silica precursor, tetraethyl orthosilicate (TEOS), using an ammonium hydroxide (NH4OH) catalyst. Structural characterizations with transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) indicate that the silica-quantum dot nanocomposites consist of a layered structure. Owing to the amorphous, porous nature of a silica layer, the optical and photophysical properties of silica-overcoated CdS:Mn/ZnS quantum dots are found to remain close to those of uncoated counterparts. 相似文献
237.
Secondary or tertiary amines may be prepared from primary alcohols and primary or secondary amines by treating triphenylphosphine with N-bromosuccinimide (NBS) in the presence of the alcohol at low temperature, followed by addition of the amine and heating for about 1 h. The yield of amine is good to fair, decreasing sharply with sterically congested alcohols and starting amines. 相似文献
238.
Ram Parkash S. C. Ahluwalia S. C. Sud R. N. Sahni R. C. Paul 《Monatshefte für Chemie / Chemical Monthly》1981,112(6-7):707-711
Heats solution of someLewis acids and bases in acetic anhydride have been determined and the following order of their relative strengths is proposed: SbCl5>SO3>SnCl4>TiCl4>AsCl3 and piperidine>
n-butylamine>potassium acetate>sodium acetate -picoline>quinoline. Heats neutralization of theseLewis acids and bases in acetic anhydride suggest that the major enthalpy change in these neutralization reactions is due to the combination of a proton and the (CH2COOCOCH3)-ion, resulting in the formation of acetic anhydride.
Lösungs- und Neutralisationswärmen von Lewis-Säuren und-Basen in Essigsäureanhydrid
Zusammenfassung Es wurden die Lösungswärmen einigerLewis-Säuren in Essigsäureanhydrid bestimmt und folgende Reihung nach ihrer relativen Stärke vorgeschlagen: SbCl5>SO3>SnCl4>TiCl4>AsCl3 und Piperidin> n-Butylamin>KAc>NaAc-Picolin>Chinolin. Die Neutralisationswärmen dieserLewis-Säuren und-Basen legen nahe, daß der Hauptanteil daran auf die Reaktion eines Protons mit (CH2–COOCOCH3)-zurückzuführen ist.相似文献
239.
Burford N Ragogna PJ McDonald R Ferguson MJ 《Chemical communications (Cambridge, England)》2003,(16):2066-2067
Polyphosphorus dications have been synthesised and comprehensively characterised as diphosphine (dppe, dmpe and dpph) linked Ar2P+ Lewis acids (Ar = C6H5); this application of unique homoatomic coordination chemistry provides important building blocks for extended systems. 相似文献
240.
Zareen Ahkter Andrew J. Edwards Scott L. Ingham Jack Lewis Ana M. Martin Castro Paul R. Raithby Gregory P. Shields 《Journal of Cluster Science》2000,11(1):217-226
The reaction of the di-gold cation [Au2(dppx)]2+ with the heptanuclear cluster dianion [Os7(CO)20]2– affords the mixed metal cluster [Os7(CO)20{Au2(dppx)}] (x=m (1), e (2), b (3)). On standing, in solution, this complex undergoes decarbonylation to give the cluster [Os7(CO)19{Au2(dppx)}] (x=m (4), e (5), b (6)). The complexes have been characterised spectroscopically, and an X-ray structure determination of the dppm derivative shows that it contains a metal core based on an Os7 edge-bridged bicapped tetrahedron with the two
3-Au atoms capping adjacent triangular Os3 faces of the central tetrahedron. In an analogous reaction, the carbido anion [Os7(H)C(CO)19]– affords the neutral cluster [Os7C(CO)19{Au2(dppm)}] (7) when treated with [Au2(dppm)]2+ in the presence of base. 相似文献