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161.
Krishna V. Ramanathan Neeraj Sinha 《Monatshefte für Chemie / Chemical Monthly》2002,133(12):1535-1548
Summary. Cross polarisation is extensively used in solid state NMR for enhancing signals of nuclei with low gyromagnetic ratio. However,
the use of the method for providing quantitative structural and dynamics information is limited. This arises due to the fact
that the mechanism which is responsible for cross polarisation namely, the dipolar interaction, has a long range and is also
anisotropic. In nematic liquid crystals these limitations are easily overcome since molecules orient in a magnetic field.
The uniaxial ordering of the molecules essentially removes problems associated with the angular dependence of the interactions
encountered in powdered solids. The molecular motion averages out intermolecular dipolar interaction, while retaining partially
averaged intramolecular interaction. In this article the use of cross polarisation for obtaining heteronuclear dipolar couplings
and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment were considered
and their utility illustrated. A method for obtaining proton–proton dipolar couplings, by utilizing cross polarisation from
the dipolar reservoir, is also presented.
Corresponding author. E-mail: kvr@sif.iisc.ernet.in
Received May 28, 2002; accepted June 19, 2002 相似文献
162.
Rafiq S Rajbongshi BK Nair NN Sen P Ramanathan G 《The journal of physical chemistry. A》2011,115(47):13733-13742
Two green fluorescent protein (GFP) chromophore analogs (4Z)-4-(N,N-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DMPI) and (4Z)-4-(N,N-diphenylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DPMPI) were investigated using femtosecond fluorescence up-conversion spectroscopy and quantum chemical calculations with the results being substantiated by HPLC and NMR measurements. The femtosecond fluorescence transients are found to be biexponential in nature and the time constants exhibit a significant dependence on solvent viscosity and polarity. A multicoordinate relaxation mechanism is proposed for the excited state relaxation behavior of the model GFP analogs. The first time component (τ(1)) was assigned to the formation of twisted intramolecular charge transfer (TICT) state along the rotational coordinate of N-substituted amine group. Time resolved intensity normalized and area normalized emission spectra (TRES and TRANES) were constructed to authenticate the occurrence of TICT state in subpicosecond time scale. Another picosecond time component (τ(2)) was attributed to internal conversion via large amplitude motion along the exomethylenic double bond which has been enunciated by quantum chemical calculations. Quantum chemical calculation also forbids the involvement of hula-twist because of high activation barrier of twisting. HPLC profiles and proton-NMR measurements of the irradiated analogs confirm the presence of Z and E isomers, whose possibility of formation can be accomplished only by the rotation along the exomethylenic double bond. The present observations can be extended to p-HBDI in order to understand the role of protein scaffold in reducing the nonradiative pathways, leading to highly luminescent nature of GFP. 相似文献
163.
Stela M. C. Fernandes Olandir V. Correa Lalgudi V. Ramanathan 《Journal of Thermal Analysis and Calorimetry》2011,106(2):541-543
This article presents the influence of surface additions of nanocrystalline rare earth (RE) oxides CeO2, La2O3, and CeO2 + La2O3 on the isothermal oxidation behavior of Fe20Cr and Fe20Cr5Al at 1000 °C. Thermogravimetric studies revealed parabolic kinetics
in all cases and the scale thickness on specimen surfaces varied with the nature of RE oxide. The oxidation resistance of
specimens coated with two RE oxides was significantly higher than those coated with either one of the two oxides. The marked
increase in the oxidation resistance of the alloys coated with two RE oxides is due to optimization of RE ion radius and RE
oxide grain size/shape. 相似文献
164.
Jeon B Sankaranarayanan SK van Duin AC Ramanathan S 《The Journal of chemical physics》2011,134(23):234706
Corrosion is a fundamental problem in electrochemistry and represents a mode of failure of technologically important materials. Understanding the basic mechanism of aqueous corrosion of metals such as Cu in presence of halide ions is hence essential. Using molecular dynamics simulations incorporating reactive force-field (ReaxFF), the interaction of copper substrates and chlorine under aqueous conditions has been investigated. These simulations incorporate effects of proton transfer in the aqueous media and are suitable for modeling the bond formation and bond breakage phenomenon that is associated with complex aqueous corrosion phenomena. Systematic investigation of the corrosion process has been carried out by simulating different chlorine concentration and solution states. The structural and morphological differences associated with metal dissolution in the presence of chloride ions are evaluated using dynamical correlation functions. The simulated atomic trajectories are used to analyze the charged states, molecular structure and ion density distribution which are utilized to understand the atomic scale mechanism of corrosion of copper substrates under aqueous conditions. Increased concentration of chlorine and higher ambient temperature were found to expedite the corrosion of copper. In order to study the effect of solution states on the corrosion resistance of Cu, partial fractions of proton or hydroxide in water were configured, and higher corrosion rate at partial fraction hydroxide environment was observed. When the Cl(-) concentration is low, oxygen or hydroxide ion adsorption onto Cu surface has been confirmed in partial fraction hydroxide environment. Our study provides new atomic scale insights into the early stages of aqueous corrosion of metals such as copper. 相似文献
165.
Byoungseon Jeon Subramanian K.R.S. Sankaranarayanan Adri C.T. van Duin Shriram Ramanathan 《哲学杂志》2013,93(32):4073-4088
We investigate oxidation and oxide growth on single-crystal copper surfaces using reactive molecular dynamics simulation. The kinetics of surface oxide growth are strongly correlated with the microstructure of the metal substrates. Simulating oxide layer growth along the (100), (110), and (111) orientations of crystalline copper, oxidation characteristics are investigated at temperatures of 300?K and 600?K. The oxidation kinetics are found to strongly depend on the surface orientation, ambient temperature, and surface defects. The effect of surface morphology on oxidation characteristics is analyzed by comparing oxygen adsorption on various sites and the structure factor. The surface oxide formed on (100) retains the initial crystal structure in the 300–600?K range. The (100) surface shows the highest oxidation rate at both temperature conditions but saturates, facilitating oxygen adsorption on hollow sites. The oxidation kinetics of the (100) orientation are found to be not significantly affected by surface defects. (110) shows modest oxidation at 300?K but the highest oxidation is observed at 600?K. By surface disorder and reconstruction, the oxide layer is produced continuously. The (111) surface is sensitive to ambient temperature and surface defects, showing that surface reconstruction is a key element for further oxidation. The charge distribution of oxidized Cu atoms indicates multiple groups of stoichiometric oxides, while the fraction of CuO-like characteristics increases significantly on the (110) and (111) orientations at higher temperature (600?K). The energetics and mechanisms of oxidation on Cu metal substrates at the nanoscale are discussed in detail, and comparisons with available experimental and other theoretical studies are presented wherever possible. 相似文献
166.
Akihito Soeda Paweł Kurzyński Ravishankar Ramanathan Andrzej Grudka Jayne Thompson Dagomir Kaszlikowski 《Physics letters. A》2013
We show that sharp measurements of total magnetization cannot be used to reveal contextuality in macroscopic many-body systems of spins of arbitrary dimension. We decompose each such measurement into set of projectors corresponding to well-defined value of total magnetization. We then show that such sets of projectors are too restricted to construct Kochen–Specker sets. 相似文献
167.
168.
The dependence of the viscosity of fluids on pressure has been well established by experiments and it needs to be taken into consideration in problems where there is a large variation of pressure in the flow domain. In this paper we consider the flow of a fluid in the annulus between two cylinders whose viscosity depends on the pressure. First we consider the steady flow in the annulus due to the rotation of one cylinder with respect to the other. Then we study the problem of flow in the annular region due to torsional and longitudinal oscillations of one cylinder with respect to the other. In both the problems considered the flow is found to be markedly different from that for the incompressible Navier–Stokes fluid with constant viscosity. 相似文献
169.
Rahul Maity Himan Dev Singh Ankit Kumar Yadav Debanjan Chakraborty Ramanathan Vaidhyanathan 《化学:亚洲杂志》2019,14(20):3736-3741
Here, we report two novel water‐stable amine‐functionalized MOFs, namely IISERP‐MOF26 ([NH2(CH3)2][Cu2O(Ad)(BDC)]?(H2O)2(DMA), 1 ) and IISERP‐MOF27 ([NH2(CH3)2]1/2[Zn4O(Ad)3(BDC)2]?(H2O)2(DMF)1/2, 2 ), which show selective CO2 capture capabilities. They are made by combining inexpensive and readily available terephthalic acid and N‐rich adenine with Cu and Zn, respectively. They possess 1D channels decorated by the free amine group from the adenine and the polarizing oxygen atoms from the terephthalate units. Even more, there are dimethyl ammonium (DMA+) cations in the pore rendering an electrostatic environment within the channels. The activated Cu‐ and Zn‐MOFs physisorb about 2.7 and 2.2 mmol g?1 of CO2, respectively, with high CO2/N2 and moderate CO2/CH4 selectivity. The calculated heat of adsorption (HOA=21–23 kJ mol?1) for the CO2 in both MOFs suggest optimal physical interactions which corroborate well with their facile on‐off cycling of CO2. Notably, both MOFs retain their crystallinity and porosity even after soaking in water for 24 hours as well as upon exposure to steam over 24 hours. The exceptional thermal and chemical stability, favorable CO2 uptakes and selectivity and low HOA make these MOFs promising sorbents for selective CO2 capture applications. However, the MOF′s low heat of adsorption despite having a highly CO2‐loving groups lined walls is quite intriguing. 相似文献
170.
Mahesh B. Muluk Pramod S. Phatak Shriram B. Pawar Sambhaji T. Dhumal Naziya N. M. A. Rehman Prashant P. Dixit Prafulla B. Choudhari Kishan P. Haval 《中国化学会会志》2019,66(11):1507-1517
A series of novel substituted (E)‐N′‐benzylidene‐4‐methyl‐2‐(2‐propylpyridin‐4‐yl)thiazole‐5‐carbohydrazide derivatives ( 6a‐l ) have been synthesized by following the multistep synthetic route starting from prothionamide. The resulting compounds were characterized via 1H, 13C NMR, and HRMS spectral data. The synthesized carbohydrazides were evaluated for their in vitro antimicrobial and antioxidant activities. Tested molecules have displayed moderate to good growth inhibition activity. Among the screened compounds, 6b , 6e , 6j, and 6k are found to be the more promising antimicrobial agents. A 2,2‐diphenyl‐1‐picrylhydrazyl assay was used to test the antioxidant activity of the carbohydrazides. The carbohydrazide derivatives 6b and 6i have shown better free‐radical scavenging ability than the other investigated compounds. 相似文献