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11.
A method for semi-automatic background estimation in energy-dispersive x-ray fluorescence spectra is outlined. Two cubic splines were investigated and the spline called the Butland interpolant was chosen for further investigation. Prior to the calculation of the spline, peak regions are set up, and suitable knots are defined outside the peak regions. To set up the peak regions, an automatic peak-search routine and a calibration equation are used. For a given peak, the latter relates the full width at half maximum (FWHM) to the peak centre. In turn, the size of the peak regions are defined by the FWHM multiplied by a constant given by the user. The method was tested on several types of spectra. It was found that the optimal size of the peak region decreased with increasing peak density. Reproducibility tests showed that the standard deviation of the summation of counts within a peak region and after background subtraction was less than would be expected from the counting statistics. 相似文献
12.
The reaction of [Cr(NCCH(3))(6)](2+) with dioxygen in acetonitrile (MeCN) solution acidified with HBF(4) gave red crystals of the binuclear complex [(CH(3)CN)(5)Cr(OH)Cr(NCCH(3))(5)](BF(4))(5) (1). From the X-ray crystal structure of 1, the Cr-O-Cr angle was found to be 147.5(2) degrees. Magnetic susceptibility measurements of 1 showed an antiferromagnetic coupling between the two chromium(III) centers with a triplet energy J = 35.9(1) cm(-1). On redissolution of 1 in MeCN, the hydroxo bridge was deprotonated, and a green solution of the complex [(CH(3)CN)(5)CrOCr(NCCH(3))(5)](4+) formed. The electronic absorption spectrum of this solution is very similar to the spectrum of the classical complex [(H(3)N)(5)CrOCr(NH(3))(5)](4+) with intense bands in the UV and near-UV region. From the temperature dependence of the absorption spectrum near 12900 cm(-1), the triplet energy J was found to be 1067(19) cm(-1). The acidity of the hydroxo bridge in 1 is very high with an acid dissociation constant K(a) > 1 M. 相似文献
13.
Rossmeisl J Kristensen I Gregersen M Jacobsen KW Nørskov JK 《Journal of the American Chemical Society》2003,125(52):16383-16386
The natural amino acids have different preferences of occurring in specific types of secondary protein structure. Simulations are performed on periodic model beta-sheets of 14 different amino acids, at the level of density functional theory, employing the generalized gradient approximation. We find that the statistically observed beta-sheet propensities correlate very well with the calculated binding energies. Analysis of the calculations shows that the beta-sheet propensities are determined by the local flexibility of the individual polypeptide strands. 相似文献
14.
Human beta2-glycoprotein I (beta2gpI) is a phospholipid and heparin binding plasma glycoprotein involved in autoimmune diseases characterized by blood clotting disturbances (thrombosis) together with the occurrence of autoantibodies against beta2gpI. With the final goal of assessing autoantibody influence on binding interactions of beta2gpI we have studied the development of capillary electrophoresis (CE)-based assays for interactions of negatively charged ligands with beta2gpI. In the development of suitable conditions for analysis at neutral pH of this basic protein (pI about 8) we found the pH hysteresis behavior of fused silica surfaces useful since the protonated surface after an acid pre-wash counteracted protein adsorption efficiently in contrast to more laborious procedures including acrylamide/dimethylacrylamide coatings that did not permit analysis of this particular protein. This simple approach made estimates of heparin-beta2gpI interactions possible and the principle was shown also to work for detection of betagpI binding to anionic phospholipids. Utilizing the pH hysteresis effect may be a simple solution to the adsorption problems often encountered in analyses of proteins by CE. 相似文献
15.
Intercalation of trioxatriangulenium ion in DNA: binding,electron transfer,x-ray crystallography,and electronic structure 总被引:1,自引:0,他引:1
Reynisson J Schuster GB Howerton SB Williams LD Barnett RN Cleveland CL Landman U Harrit N Chaires JB 《Journal of the American Chemical Society》2003,125(8):2072-2083
Trioxatriangulenium ion (TOTA(+)) is a flat, somewhat hydrophobic compound that has a low-energy unoccupied molecular orbital. It binds to duplex DNA by intercalation with a preference for G-C base pairs. Irradiation of intercalated TOTA(+) causes charge (radical cation) injection that results in strand cleavage (after piperidine treatment) primarily at GG steps. The X-ray crystal structure of TOTA(+) intercalated in the hexameric duplex d[CGATCG](2) described here reveals that intercalation of TOTA(+) results in an unusually large extension of the helical rise of the DNA and that the orientation of TOTA(+) is sensitive to hydrogen-bonding interactions with backbone atoms of the DNA. Electronic structure calculations reveal no meaningful charge transfer from DNA to TOTA(+) because the lowest unoccupied molecular orbital of TOTA(+), (LUMO)(T), falls in the gap between the highest occupied molecular orbital, (HOMO)(D), and the (LUMO)(D) of the DNA bases. These calculations reveal the importance of backbone, water, and counterion interactions, which shift the energy levels of the bases and the intercalated TOTA(+) orbitals significantly. The calculations also show that the inserted TOTA(+) strongly polarizes the intercalation cavity where a sheet of excess electron density surrounds the TOTA(+). 相似文献
16.
Zamfir AD Lion N Vukelic Z Bindila L Rossier J Girault HH Peter-Katalinic J 《Lab on a chip》2005,5(3):298-307
A thin chip polymer-based microsprayer has been coupled to a hybrid quadrupole time-of-flight mass spectrometer (QTOF MS) and introduced in carbohydrate research. The feasibility of the approach is demonstrated for mapping, sequencing and structural elucidation of glycoconjugates originating from human body fluids and tissues such as a glycopeptide mixture from normal human urine and an isolated and purified GT1 ganglioside fraction from normal adult human brain. The optimization procedure required by each glycoconjugate category is described and the advantages of the system in terms of flexibility and adaptability to QTOF MS, stability of the ESI MS signal, carbohydrate ionization and sequencing, sensitivity, speed of analysis and sample consumption are discussed. 相似文献
17.
Rasmussen T Jensen JF Ostergaard N Tanner D Ziegler T Norrby PO 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(1):177-184
The selectivity-determining step in enantioselective copper-catalyzed cyclopropanation with diazo compounds has been studied by experimental and computational methods. The addition of the very reactive metallacarbene intermediate in an early transition state to the substrate alkene is concerted but strongly asynchronous, with substantial cationic character on one alkene carbon in the neighborhood of the transition state. Evidence from isotope effects and Hammett studies supports the nature of the transition state. Formation of a metallacyclobutane intermediate by a [2+2] addition is kinetically disfavored. Ligand-substrate interactions influencing the enantio- and diastereoselectivity have been identified, and the preferred orientation of the alkene substrate during the addition is suggested. 相似文献
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Niels ten Brummelhuis Marcus Weck 《Journal of polymer science. Part A, Polymer chemistry》2014,52(11):1555-1559
Reversible addition‐fragmentation chain‐transfer (RAFT) polymerization was used to control the alternating copolymerization of styrene and 2,3,4,5,6‐pentaflurostyrene. The RAFT polymerization yields a high degree of control over the molecular weight of the polymers and does not significantly influence the reactivity ratios of the monomers. The controlled free‐radical polymerization could be initiated using AIBN at elevated temperatures or using a redox couple (benzoyl peroxide/N,N‐dimethylaniline) at room temperature, while maintaining control over molecular weight and dispersity. The influence of temperature and solvent on the molecular weight distribution and reactivity ratios were investigated. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1555–1559 相似文献