全文获取类型
收费全文 | 509篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
化学 | 336篇 |
晶体学 | 8篇 |
力学 | 5篇 |
数学 | 56篇 |
物理学 | 118篇 |
出版年
2023年 | 4篇 |
2022年 | 6篇 |
2021年 | 9篇 |
2020年 | 5篇 |
2019年 | 11篇 |
2018年 | 5篇 |
2017年 | 5篇 |
2015年 | 4篇 |
2014年 | 17篇 |
2013年 | 26篇 |
2012年 | 22篇 |
2011年 | 31篇 |
2010年 | 13篇 |
2009年 | 12篇 |
2008年 | 21篇 |
2007年 | 30篇 |
2006年 | 24篇 |
2005年 | 25篇 |
2004年 | 18篇 |
2003年 | 16篇 |
2002年 | 16篇 |
2001年 | 9篇 |
2000年 | 12篇 |
1999年 | 10篇 |
1998年 | 5篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1989年 | 4篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1972年 | 3篇 |
1971年 | 7篇 |
1966年 | 3篇 |
排序方式: 共有523条查询结果,搜索用时 15 毫秒
131.
Smalley JF Finklea HO Chidsey CE Linford MR Creager SE Ferraris JP Chalfant K Zawodzinsk T Feldberg SW Newton MD 《Journal of the American Chemical Society》2003,125(7):2004-2013
The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here. 相似文献
132.
Löwdin's canonical orthonormalization procedure is non-unique if the overlap matrix has degenerate eigenvalues. A recent attempt by Roby to justify neglect of differential overlap is re-examined taking proper account of this lack of uniqueness. We conclude that his central result, eq. (6) in our paper, is formally correct in the limit of complete basis sets, but Roby's presentation lends a misleading interpretation to this equation. Consequently, some conclusions which are claimed to follow are, in fact, not valid. We briefly investigate the Roby approximation for finite basis sets and indicate the strengths and weaknesses when used for practical computations. 相似文献
133.
Rosokha SV Lü JM Newton MD Kochi JK 《Journal of the American Chemical Society》2005,127(20):7411-7420
Definitive X-ray structures of "separated" versus "contact" ion pairs, together with their spectral (UV-NIR, ESR) characterizations, provide the quantitative basis for evaluating the complex equilibria and intrinsic (self-exchange) electron-transfer rates for the potassium salts of p-dinitrobenzene radical anion (DNB(-)). Three principal types of ion pairs, K(L)(+)DNB(-), are designated as Classes S, M, and C via the specific ligation of K(+) with different macrocyclic polyether ligands (L). For Class S, the self-exchange rate constant for the separated ion pair (SIP) is essentially the same as that of the "free" anion, and we conclude that dinitrobenzenide reactivity is unaffected when the interionic distance in the separated ion pair is r(SIP) > or =6 Angstroms. For Class M, the dynamic equilibrium between the contact ion pair (with r(CIP) = 2.7 Angstroms) and its separated ion pair is quantitatively evaluated, and the rather minor fraction of SIP is nonetheless the principal contributor to the overall electron-transfer kinetics. For Class C, the SIP rate is limited by the slow rate of CIP right arrow over left arrow SIP interconversion, and the self-exchange proceeds via the contact ion pair by default. Theoretically, the electron-transfer rate constant for the separated ion pair is well-accommodated by the Marcus/Sutin two-state formulation when the precursor in Scheme 2 is identified as the "separated" inner-sphere complex (IS(SIP)) of cofacial DNB(-)/DNB dyads. By contrast, the significantly slower rate of self-exchange via the contact ion pair requires an associative mechanism (Scheme 3) in which the electron-transfer rate is strongly governed by cationic mobility of K(L)(+) within the "contact" precursor complex (IS(CIP)) according to the kinetics in Scheme 4. 相似文献
134.
Gilbert BC Smith JR Newton MS Oakes J Pons i Prats R 《Organic & biomolecular chemistry》2003,1(9):1568-1577
A kinetic and mechanistic study is reported of the oxidation of a number of azonaphthol dyes with hydrogen peroxide in aqueous solution, catalysed by some mono and dinuclear manganese(IV) complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN). The results of UV-Vis investigations, augmented by EPR and ESI-MS studies, are described for a series of experiments in which concentrations, pH and ionic strength have been varied. The reactions are characterised by an induction period followed by a relatively rapid oxidation. For the dinuclear manganese complex 2, these are consistent with an initial perhydrolysis of the manganese complex involving both the dye anion and HO2-, to give mononuclear manganese species and the operation of a catalytic cycle incorporating MnIIIL(OH)3, O = MnVL(OH)2 and MnIVL(OH)3 (L = Me3TACN) (cf. the reactions of peroxidase enzymes). ESI-MS results provide evidence for the formation and reaction (with the dye) of MnIVL(OH)3. With the mononuclear manganese complex MnIVL(OMe)3, there is a short lag-phase attributed to perhydrolysis by HO2- followed by the same catalytic cycle. 相似文献
135.
Fawcett NC Craven RD Zhang P Evans JA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(16):6651-6657
Yoshimoto et al. [Anal. Chem. 2002, 74, 4306-4309] reported that a quartz crystal microbalance or QCM changed its response to sucrose solutions according to its angle of immersion. The effect was tentatively attributed to gravity-caused stress on the viscous interface between the oscillator and the bulk solution. The present work reports results from QCM experiments carried out so that any effect of gravity on the interfacial region would be magnified. This permitted use of a lower-frequency, less-sensitive QCM. Molecules of DNA were tethered to a functionalized QCM surface and then extended in steps, via sandwich hybridization, to produce DNA of uniform and known length. This feature allowed both the effect of QCM immersion angle and the relationship between frequency and molecular length to be investigated simultaneously. Comparison of acoustic wave damping at 0 degrees and 180 degrees immersion angles offers compelling evidence that the interfacial region expands when the active face of the QCM is down and contracts when it is up. This is apparently a consequence of the interfacial region being more dense than the bulk solution. The results are consistent with (a) slow gravity-driven movement of molecules away from a down-facing QCM, (b) rapid hybridization-driven movement away from an up-facing QCM, and (c) a QCM frequency response that decreases according to a simple exponential function of the tethered molecules' radius of gyration. 相似文献
136.
Dudley E Lemiere F Van Dongen W Langridge JI El-Sharkawi S Games DE Esmans EL Newton RP 《Rapid communications in mass spectrometry : RCM》2003,17(11):1132-1136
Modified nucleosides in the urine have been postulated to be diagnostic indices of disease, particularly of cancer. The urine of a patient with terminal head and neck cancer has been found to contain a modified nucleoside with a protonated molecule of m/z 228. By means of high-performance liquid chromatography/ion trap mass spectrometry (HPLC/ITMS) and capillary liquid chromatography/triple quadruple mass spectrometry (CapLC/TQMS) we have identified the compound as 5'-deoxycytidine. This is the first report of 5'-deoxycytidine in man: in addition to the elucidation of its structure, its possible origins and the potential significance of its occurrence are discussed. 相似文献
137.
Dudley E Tuytten R Bond A Lemière F Brenton AG Esmans EL Newton RP 《Rapid communications in mass spectrometry : RCM》2005,19(21):3075-3085
Many nucleosides and their modified forms have been studied by mass spectrometry elaborating the detailed fragmentation pathways under MS2 and MS(n) conditions. Although the C-nucleoside pseudouridine has been fragmented and studied briefly, usually amongst many other nucleosides, it has not been investigated to the same extent as other nucleosides. In this report a number of different mass spectrometric techniques are applied to obtain a fuller picture of pseudouridine fragmentation. At the same time this study is used to compare different tandem mass spectrometric techniques, including a novel methodology utilising a quadrupole time-of-flight (Q-ToF) instrument for MS(n) analysis comparable with that available with an ion trap mass spectrometer. 相似文献
138.
Russell P. Newton Terence J. Walton Salem A. Basaif Andrea M. Jenkins A. Gareth Brenton Dipankar Ghosh Frank M. Harris 《Journal of mass spectrometry : JMS》1989,24(8):679-688
The syntheses of the dibutyryl derivatives of the 3′,5′-cyclic monophosphates of adenosine, guanosine and cytidine are described. The fast atom bombardment mass spectra of these compounds are discussed, together with the mass-analysed ion kinetic energy spectra of their protonated molecular ions and of diagnostic fragments. A protocol for the identification of the derivatives is reported which includes criteria for confirming retention of the cyclic phosphodiester moiety, substitution of both heterocyclic base and ribose ring, and butyrylation of the 2′-O-position. The origins of significant fragments in the spectra are discussed. 相似文献
139.
Balawant S. Joshi Haridutt K. Desai Sudhakarrao Bhandaru S. William Pelletier M. Gary Newton Andrew J. Kay 《Journal of chemical crystallography》1993,23(11):877-883
The crystal and molecular structure of the norditerpenoid alkaloid 1-epi-delphisine (5), C28H43NO8, M
r
521.66, has been confirmed by as X-ray diffraction study using the SIR 88 analysis program. The alkaloid crystallizes in the space groupP21 with cell parameters:a=11.853(1)Å,b=10.511(1)Å,c=11.854(1)Å,=112.58(1),V=1363.61(0)Å3,Z=2,D
calc=1.27 g/cm3, (CuK)=1.54184 Å, (CuK=7.2 cm–1,F(000)=564, temperature 23°C,R=0.052, for 2907 reflections. Ring A of 1-epi-delphisine exists in a chair conformation. By comparison, delphisine which bears a C-1 hydroxyl group, has ring A in a boat conformation stabilized by an intramolecular N----H-O hydrogen bond. Ring D of both of these alkaloids exists in a boat form. Unambiguous proton and carbon-13 nmr assignments for delphisine, 1-epi-delphisine and delphinine have been made by a detailed analysis of the DEPT, COSY, HETCOR, COLOC, DIFNOE, and selective INEPT nmr techniques.Dedicated to the memory of Dr. K. Krishna Bhandary (1946–1992). 相似文献
140.
Newton L. Dias Filho 《Mikrochimica acta》1999,130(4):233-240
The isotherms of adsorption of MeX2 (Me = Cu2+, Co2+; X = Cl–, Br–, ClO
4
–
) by silica gel chemically modified with 2-mercaptoimidazole (SiMI) were studied in acetone and ethanol solutions, at 25 °C. Covalently attached 2-mercaptoimidazole molecule to silica gel surface adsorbs MeX2 from solvent by forming a surface complex. The metal is bonded to the surface through the nitrogen atom of attached 2-mercaptoimidazole. At low loading, the electronic and ESR spectral parameters indicated that the Cu2+ complexes are in a distorted-tetragonal symmetry field. The d-d electronic transition spectra showed that for Cu(ClO4)2 complex, the peak of absorption did not change for any degree of metal loading and for Cl– and Br– complexes, the peak maxima shifted to higher energy with lower metal loading. The CoX2(X = Cl–, Br–, ClO
4
–
) analogues possess a distorted-tetrahedral field. 相似文献