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601.
Reif B Hohwy M Jaroniec CP Rienstra CM Griffin RG 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,145(1):132-141
We present a novel solid-state magic angle-spinning NMR method for measuring the NH(i)-NH(i+1) projection angle θ(i,i+1) in peptides. The experiment is applicable to uniformly (15)N-labeled peptides and is demonstrated on the chemotactic tripeptide N-formyl-l-Met-l-Leu-l-Phe. The projection angle θ(i,i+1) is directly related to the peptide backbone torsion angles φ(i) and psi(i). The method utilizes the T-MREV recoupling scheme to restore (15)N-(1)H interactions, and proton-mediated spin diffusion to establish (15)N-(15)N correlations. T-MREV has recently been shown to increase the dynamic range of the (15)N-(1)H recoupling by gamma-encoding, and permits an accurate determination of the recoupled NH dipolar interaction. The results are interpreted in a quasi-analytical fashion that permits efficient extraction of the structural parameters. 相似文献
602.
Crepin C Broquier M Dubost H Galaup JP Le Gouet JL Ortega JM 《Physical review letters》2000,85(5):964-967
Infrared picosecond accumulated photon echo experiments have been performed for the first time, using the Orsay Free Electron Laser, on the v = 0-->v = 1 transition of CO in solid nitrogen. The vibrational dephasing time is found to be exceptionally long ( T2>/=120 ns) at low temperature. The analysis of the observed spectral diffusion leads one to assume different energy transfer mechanisms depending on the CO concentration. 相似文献
603.
H. M. Araújo R. J. Walker S. A. Rinehart M. J. Griffin P. A. R. Ade 《International Journal of Infrared and Millimeter Waves》2001,22(7):965-982
Submillimetre astronomy with ground-based instruments is currently restricted to spectral windows below 1 THz where the atmosphere is not very opaque. Recently, it was shown that supra-terahertz windows also unfold under very good atmospheric conditions, and could be explored for this effect. In particular, the 200 m window (1.5 THz) can transmit up to 30% on exceptionally dry weather at high-altitude sites. This study was conducted in parallel with the designs of THUMPER, a Two Hundred Micron Photometer for the James Clerk Maxwell Telescope, in Hawaii. Here we consider the weather conditions required and how often they arise, both by modelling the spectral transmittance of the atmosphere at 200 m and by analysing opacity data at 225 GHz. Implications for submillimetre astronomy are discussed. This study could benefit the ongoing analysis of other observation sites. 相似文献
604.
This paper discusses time-domain electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) experiments aimed at elucidating the enzymatic mechanism of ribonucleotide reductase (RNR), the enzyme responsible for the conversion of ribonucleotides to deoxyribonucleotides. The article begins with a discussion of the current state of the art of instrumentation for high-frequency EPR and ENDOR and some suggestions as to future developments. We then provide an introduction to the chemistry of RNR and a discussion of the high-field EPR and ENDOR spectra of the tyrosyl radical (Y?) in the R2 subunit of class I RNR. Finally, we describe two examples illustrating the use of high-frequency EPR and ENDOR to elucidate the enzymatic mechanism of RNR. EPR and ENDOR have played an important role for these studies since the mechanism involves several different radical intermediates. These intermediates are all present in low concentrations relative to the Y? concentration and they possess similarg-values. Spectral overlap, therefore, has been a problem with X-band EPR. At high frequencies the spectra are resolved to the point that individual powder lineshapes are discernible. In addition, we describe our approach, on the basis of differential relaxation, to suppress the spectrum of the dominant Y?. High-frequency EPR and ENDOR therefore has permitted us to determine the structure of several radical intermediates which in turn have contributed to the understanding of the enzymatic mechanism of RNR. 相似文献
605.
606.
de Jeu WH Obraztsov EP Ostrovskii BI Ren W McMullan PJ Griffin AC Sánchez-Ferrer A Finkelmann H 《The European physical journal. E, Soft matter》2007,24(4):399-409
The layer correlations in main-chain smectic liquid-crystal polymer and elastomer systems have been studied using high-resolution
X-ray scattering. In contrast to side-chain smectic polymers, in main-chain systems the polymer chains are oriented parallel
to the layer normal. As a result they couple directly to the lamellar structure and any polymer defect is translated into
layer distortions. For the homopolymers the resulting X-ray lineshapes are well described by Lorentzians. This is interpreted
as an average of algebraically decaying order in domains with dimensions of hundreds of nm and a wide dispersion of sizes.
The elastomers show much broader peaks than the correponding polymers. This is attributed to strong non-uniform strain within
the finite-size domains due to defects of the layer structure.
An erratum to this article is available at . 相似文献
607.
Dr. Sara I. R. Costa Dr. Yong-Seok Choi Dr. Alistair J. Fielding Dr. Andrew J. Naylor Dr. John M. Griffin Prof. Dr. Zdeněk Sofer Prof. David O. Scanlon Dr. Nuria Tapia-Ruiz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(11):3875-3886
Na2Ti3O7 (NTO) is considered a promising anode material for Na-ion batteries due to its layered structure with an open framework and low and safe average operating voltage of 0.3 V vs. Na+/Na. However, its poor electronic conductivity needs to be addressed to make this material attractive for practical applications among other anode choices. Here, we report a safe, controllable and affordable method using urea that significantly improves the rate performance of NTO by producing surface defects such as oxygen vacancies and hydroxyl groups, and the secondary phase Na2Ti6O13. The enhanced electrochemical performance agrees with the higher Na+ ion diffusion coefficient, higher charge carrier density and reduced bandgap observed in these samples, without the need of nanosizing and/or complex synthetic strategies. A comprehensive study using a combination of diffraction, microscopic, spectroscopic and electrochemical techniques supported by computational studies based on DFT calculations, was carried out to understand the effects of this treatment on the surface, chemistry and electronic and charge storage properties of NTO. This study underscores the benefits of using urea as a strategy for enhancing the charge storage properties of NTO and thus, unfolding the potential of this material in practical energy storage applications. 相似文献
608.
Yoon OK Zuleta IA Robbins MD Barbula GK Zare RN 《Journal of the American Society for Mass Spectrometry》2007,18(11):1901-1908
A Bradbury-Nielsen gate (BNG) consists of two interleaved and electrically isolated sets of wires and can transmit or deflect charged particles by applying a varying voltage difference across the two wire sets. We present a simple template-based method to fabricate BNGs with wire spacings as small as 50 microm with minimal use of a microscope. The small wire spacing allows modulation rates at tens of megahertz. Using this method, we have fabricated four BNGs with wire spacings of 500, 200, 100, and 50 microm using 10 microm gold-coated tungsten wires. The performance of the four BNGs is characterized using an imaging detector and compared with theoretical predictions. 相似文献
609.
610.
Marchetti F Sayle KL Bentley J Clegg W Curtin NJ Endicott JA Golding BT Griffin RJ Haggerty K Harrington RW Mesguiche V Newell DR Noble ME Parsons RJ Pratt DJ Wang LZ Hardcastle IR 《Organic & biomolecular chemistry》2007,5(10):1577-1585
An efficient synthesis of 2-substituted O(4)-cyclohexylmethyl-5-nitroso-6-aminopyrimidines from 6-amino-2-mercaptopyrimidin-4-ol has been developed and used to prepare a range of derivatives for evaluation as inhibitors of cyclin-dependent kinase 2 (CDK2). The structure-activity relationships (SARs) are similar to those observed for the corresponding O(6)-cyclohexylmethoxypurine series with the 2-arylsulfonamide and 2-arylcarboxamide derivatives showing excellent potency. Two compounds, 4-(6-amino-4-cyclohexylmethoxy-5-nitrosopyrimidin-2-ylamino)-N-(2-hydroxyethyl)benzenesulfonamide (7q) and 4-(6-amino-4-cyclohexylmethoxy-5-nitrosopyrimidin-2-ylamino)-N-(2,3-dihydroxypropyl)benzenesulfonamide (7s), were the most potent with IC50 values of 0.7 +/- 0.1 and 0.8 +/- 0.0 nM against CDK2, respectively. The SARs determined in this study are discussed with reference to the crystal structure of 4-(6-amino-4-cyclohexylmethoxy-5-nitrosopyrimidin-2-ylamino)-N-(2,3-dihydroxypropyl)benzenesulfonamide (7j) bound to phosphorylated CDK2/cyclin A. 相似文献