全文获取类型
收费全文 | 9292篇 |
免费 | 462篇 |
国内免费 | 76篇 |
专业分类
化学 | 7717篇 |
晶体学 | 44篇 |
力学 | 169篇 |
数学 | 837篇 |
物理学 | 1063篇 |
出版年
2023年 | 72篇 |
2022年 | 91篇 |
2021年 | 142篇 |
2020年 | 213篇 |
2019年 | 230篇 |
2018年 | 129篇 |
2017年 | 141篇 |
2016年 | 294篇 |
2015年 | 281篇 |
2014年 | 289篇 |
2013年 | 444篇 |
2012年 | 684篇 |
2011年 | 867篇 |
2010年 | 404篇 |
2009年 | 312篇 |
2008年 | 717篇 |
2007年 | 641篇 |
2006年 | 653篇 |
2005年 | 604篇 |
2004年 | 541篇 |
2003年 | 440篇 |
2002年 | 412篇 |
2001年 | 132篇 |
2000年 | 85篇 |
1999年 | 62篇 |
1998年 | 68篇 |
1997年 | 75篇 |
1996年 | 84篇 |
1995年 | 65篇 |
1994年 | 36篇 |
1993年 | 47篇 |
1992年 | 37篇 |
1991年 | 41篇 |
1990年 | 27篇 |
1989年 | 28篇 |
1988年 | 20篇 |
1987年 | 24篇 |
1986年 | 18篇 |
1985年 | 50篇 |
1984年 | 30篇 |
1983年 | 30篇 |
1982年 | 39篇 |
1981年 | 35篇 |
1980年 | 32篇 |
1979年 | 29篇 |
1978年 | 18篇 |
1977年 | 17篇 |
1976年 | 20篇 |
1975年 | 19篇 |
1974年 | 14篇 |
排序方式: 共有9830条查询结果,搜索用时 46 毫秒
71.
Armando J.L. Pombeiro Christopher J. Pickett Raymond L. Richards 《Journal of organometallic chemistry》1982,224(3):285-294
The isocyanide complexes trans-[ReCl(CNR)(dppe)2] (R Me, But, C6H4CH3-4, C6H4CH3-2, C6H4Cl-4, C6H4OCH3-4 and C6H3Cl2-2,6; dppe Ph2PCH2CH2PPh2) have been prepared by isocyanide displacement of dinitrogen from the parent complex trans-[ReCl(N2)(ddpe)2]. Their redox properties have been studied by cyclic voltammetry and are interpreted on the basis of the electronic properties and the geometry of the ligating isocyanides which are believed to be bent in these complexes, appearing to exhibit ligand parameter (PL) values ca. +0.3 V higher than those which would be expected for linear geometry. A very high polarisability (B ? 3.4) is observed for the {ReCl(dppe)2} site. 相似文献
72.
Triolo A Russina O Hardacre C Nieuwenhuyzen M Gonzalez MA Grimm H 《The journal of physical chemistry. B》2005,109(46):22061-22066
A detailed investigation on the nature of the relaxation processes occurring in a typical room temperature ionic liquid (RTIL), namely, 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF(6)]), is reported. The study was conducted using both elastic and inelastic neutron scattering over a wide temperature range from 10 to 400 K, accessing the dynamic features of both the liquid and glassy amorphous states. In this study, the inelastic fixed energy scan technique has been applied for the first time to this class of materials. Using this technique, the existence of two relaxation processes below the glass transition and a further diffusive process occurring above the glass-liquid transition are observed. The low temperature processes are associated with methyl group rotation and butyl chain relaxation in the glassy state and have been modeled in terms of two Debye-like, Arrhenius activated processes. The high temperature process has been modeled in terms of a Kohlraush-Williams-Watts relaxation, with a distinct Vogel-Fulcher-Tamman temperature dependence. These results provide novel information that will be useful in rationalizing the observed structural and dynamical behavior of RTILs in the amorphous state. 相似文献
73.
Oxidative demetalation of cyclohexadienyl ruthenium(II) complexes: a net Ru-mediated dearomatization
[reaction: see text] An experimentally simple method for the demetalation of spirocyclic cyclohexadienylruthenium(II) complexes has been developed. Treatment of an alkoxy-substituted cyclohexadienyl complex with CuCl(2) affords either azaspiro[4.5]decane derivatives or heavily functionalized tetrahydroisoquinolines. The former reaction manifold completes a net Ru-mediated dearomatization as the organometallic starting materials are prepared from (eta(6)-arene)Ru(II) precursors. Both of these heterocyclic products are well suited for further synthetic elaboration. 相似文献
74.
Molecular dynamics simulations of double-helical DNA oligomers have been performed to investigate differences in the structure, dynamics, and hydration of F-F and T-T mispairs. Hexamers containing F-F pairs were found to be more dynamic, especially in the region of the mispair itself. This dynamic variability derives from greater flexibility of F-F pairs. The T-T mispairs, on the other hand, were found to be comparatively tightly bound as wobble pairs. The major and minor groove edges of the T-T pairs were observed to be solvated at exposed carbonyl positions by at least one water molecule, while F-F pairs lacked solvating waters. Stacking interactions were nearly identical for T-T and F-F pairs, leading to similar average structures, even though F stacking was more dynamically variable. Solvation differences between F-F and T-T therefore support the steric exclusion model for nucleotide incorporation in DNA replication. Large differences in the orientation of minor groove functional groups, in addition to differences in solvation, further rationalize why F bases present during DNA extension events induce stalls. Two novel nucleotides are proposed to further elucidate minor groove interactions of DNA with polymerase molecules.Electronic Supplementary Material This Material consists of equilibration protocol, plots of center-of-mass stacking, water radial distribution functions, helical parameter dynamics, and dynamics data for a control AT sequence. Supplementary material is available in the online version of this article at Contribution to the Jacopo Tomasi Honorary Issue 相似文献
75.
Boddy CN Hotta K Tse ML Watts RE Khosla C 《Journal of the American Chemical Society》2004,126(24):7436-7437
Engineered biosynthetic pathways provide a powerful method for generating complex molecules. Precursor-directed biosynthesis, which combines chemical synthesis and enzymatic transformations, allows non-native starting materials to be incorporated into biosynthetic pathways. Using this approach, we achieved the production of the anticancer agent epothilone C in Escherichia coli. An E. coli strain was engineered to express the last three modules of the epothilone biosynthetic pathway (epoD-M6, epoE, and epoF) and the substrate required to complement the biosynthetic enzymes was obtained by chemical synthesis. Under high-density cell culture conditions, the E. coli strain processed exogenously fed synthetic substrate into epothilone C at levels comparable to the native host (1 mg/L) and at higher levels than other heterologous hosts. Importantly, this precursor-directed approach will allow chemical modifications to be introduced into the polyketide backbone and may ultimately provide access to epothilone analogues with improved pharmacological properties in quantities sufficient for clinical development. 相似文献
76.
Jensen BP Smith CJ Bailey CJ Rodgers C Wilson ID Nicholson JK 《Rapid communications in mass spectrometry : RCM》2005,19(4):519-524
Inductively coupled plasma mass spectrometry (ICPMS) has been used to determine the rate and routes of excretion of bromine following the intraperitoneal administration (50 mg kg(-1)) of 2-, 3- and 4-bromobenzoic acids to male bile-duct-cannulated rats. Analysis of urine and bile for (79/81)Br using ICPMS showed that all three bromobenzoic acids were rapidly excreted (82-98%) within 48 h of dosing, primarily via the urine. High-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICPMS) was then used to obtain metabolite profiles for bile and urine. These profiles revealed that extensive metabolism had taken place, with the unchanged bromobenzoic acids forming a minor part of the total of compound-related material detected. Concomitant MS studies, supplemented by alkaline hydrolysis, enabled the identification of the major metabolite of all three of the bromobenzoic acids as a glycine conjugate. Ester glucuronide conjugates were also identified, but formed only a small proportion of total. 相似文献
77.
Simple 2H-chromenes and 2H-thiochromenes form the [2+2]-adducts, tetrahydro[1]benzo(thio)pyrano[3,4-c] [1,2]diazeto[1,2-a][1,2,4]triazoles, with triazolinediones, whereas their 3- and 4-bromo and the corresponding cycloalkylamino derivatives undergo an overall etectrophilic substitution sequence. 相似文献
78.
Christopher J. Chandler Leslie W. Deady James A. Reiss Vasilios Tzimos 《Journal of heterocyclic chemistry》1982,19(5):1017-1019
The preparation of a series of macrocycles, formed by reaction of HO-(-CH2-CH2-O-)nH with 1,10-phenanthroline-2,9-dicarbonyl chloride (n = 2,3,4) and 1,8-naphthyridine-2,7-dicarbonyl chloride (n = 3,4), is described. An improved synthetic route to 2,7-dimethyl-1,8-naphthyridine 9 is also reported. 相似文献
79.
Biswajit Pradhan Christopher Engelhard Sebastiaan Van Mulken Xueyan Miao Gerard W. Canters Michel Orrit 《Chemical science》2020,11(3):763
Monitoring the fluorescence of single-dye-labeled azurin molecules, we observed the reaction of azurin with hexacyanoferrate under controlled redox potential yielding data on the timing of individual (forward and backward) electron transfer (ET) events. Change-point analysis of the time traces demonstrates significant fluctuations of ET rates and of mid-point potential E0. These fluctuations are a signature of dynamical heterogeneity, here observed on a 14 kDa protein, the smallest to date. By correlating changes in forward and backward reaction rates we found that 6% of the observed change events could be explained by a change in midpoint potential, while for 25% a change of the donor–acceptor coupling could explain the data. The remaining 69% are driven by variations in complex association constants or structural changes that cause forward and back ET rates to vary independently. Thus, the observed spread in individual ET rates could be related in a unique way to variations in molecular parameters. The relevance for the understanding of metabolic processes is briefly discussed.Observing electron transfer events in individual azurin molecules, we relate the spread in transfer rates in a unique way to variations in molecular parameters. 相似文献
80.
Christopher Imrie Phumelele Kleyi Thomas I.A. Gerber Jennifer Look 《Journal of organometallic chemistry》2007,692(16):3443-3453
Grinding of 1,1′-ferrocenedicarboxaldehyde with a 2.2 molar equivalent of an aromatic amine in a solvent-free environment provided excellent yields of 1,1′-ferrocenyldiimines. After mixing the aldehyde and amines, a gum or melt formed which eventually solidified to the product. An analytically pure sample of the product was obtained by cold recrystallization. Grinding of ferrocenecarboxaldehyde and 4-substituted phenylacetonitriles under solvent-free conditions provided good yields of the corresponding ferrocenylacrylonitriles. The yield in this reaction was very low when the substituent group para to the acetonitrile group was electron-donating. 相似文献