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991.
Atactic vinyl homopolymers, synthesized from non-dissymmetric monomers, are produced racemic or enantiomerically pure depending on the length of the polymer chain. The optical inactivity of high molecular weight samples of these polymers arises by a mechanism unknown in small molecules: intermolecular compensation of diastereomers. 相似文献
992.
The potential functions for simple amides, several peptides and a small protein have been worked out for the MM3 force field. Structures and energies were fit as previously with MM2, but additionally, we fit the vibrational spectra of the simple amides (average rms error over four compounds, 34 cm?1), and examined more carefully electrostatic interactions, including charge-charge and charge-dipole interactions. The parameters were obtained and tested by examining four simple amides, five electrostatic model complexes, two dipeptides, six crystalline cyclic peptides, and the protein Crambin. The average root-mean-square deviation from the X-ray structures for the six cyclic peptide crystals was only 0.10 Å for the nonhydrogen atomic positions, and 0.011 Å, 1.0°, and 4.9° for bond lengths, bond angles, and torsional angles, respectively. The parameter set was then further tested by minimizing the high resolution crystal structure of the hydrophobic protein Crambin. The resultant root-mean-square deviations for the non-hydrogen atomic data, in the presence of the crystal lattice, are 0.22 Å, 0.023 Å, 2.0°, and 6.4° for coordinates, bond lengths, bond angles, and torsional angles, respectively. 相似文献
993.
E. Ben-Hur M. Green A. Prager R. Kol I. Rosenthal 《Photochemistry and photobiology》1987,46(5):651-656
Abstract— The concentrations of Na+ , K+ , Ca2+ , Mg2+ and CI− ions in the cytoplasm of octopus photoreceptor cells were determined before and after illumination by electron probe X-ray microanalysis. The concentrations of these elements in the dark-adapted photoreceptor cells were: Na+ , 68.4; K+ , 111.4; Ca2+ , 4.0; Mg2+ , 16.4; and CI− , 102.9 m M /kg of cytoplasm. Illumination raised the concentration of Na+ by 58 m M and that of Cl− by 23 m M and reduced the K+ concentration by 47 m M /kg of cytoplasm. A trace increase of intracellular Ca2+ and a trace decrease of Mg2+ were observed. These results confirm the hypothesis that sodium ions flow in on illumination, and suggest the influx of chloride ions and the outflux of potassium ions during illumination. The intracellular concentrations of Na+ , K+ and Cl+ can give the basis for calculating the ion permeability of ion channels in octopus photoreceptor cell membranes, using values of the membrane potentials obtained by electrophysiological studies 相似文献
994.
Joseph T. Sprague Julia C. Tai Young Yuh Norman L. Allinger 《Journal of computational chemistry》1987,8(5):581-603
The molecular mechanics (MMP2) program and procedures for the treatment of conjugated hydrocarbons, and some of the results which they can achieve are described. The program is an updated version of the similar MMP1 program, but contains some differences. It is based on an SCF π system calculation, rather than on the VESCF method used earlier. All parameters are compatible with those in the MM2 program. Hence it is possible to calculate heats of formation, resonance energies, and structures for conjugated hydrocarbons in a way that is consistent with the calculations on non-conjugated molecules. The overall results as far as structure and energy are somewhat better than they were with the MMP1 program. 相似文献
995.
996.
P. P. Povinec M. K. Pham J. A. Sanchez-Cabeza G. Barci-Funel R. Bojanowski T. Boshkova W. C. Burnett F. Carvalho B. Chapeyron I. L. Cunha H. Dahlgaard N. Galabov L. K. Fifield J. Gastaud J. -J. Geering I. F. Gomez N. Green T. Hamilton F. L. Ibanez M. Ibn Majah M. John G. Kanisch T. C. Kenna M. Kloster M. Korun L. Liong Wee Kwong J. La Rosa S. -H. Lee I. Levy-Palomo M. Malatova Y. Maruo P. Mitchell I. V. Murciano R. Nelson A. Nouredine J. -S. Oh B. Oregioni G. Le Petit H. B. L. Pettersson A. Reineking P. A. Smedley A. Suckow T. D. B. van der Struijs P. I. Voors K. Yoshimizu E. Wyse 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(2):383-393
A reference material designed for the determination of anthropogenic and natural radionuclides in sediment, IAEA-384 (Fangataufa
Lagoon sediment), is described and the results of certification are presented. The material has been certified for 8 radionuclides
(40K, 60Co, 155Eu, 230Th, 238U, 238Pu, 239+240Pu and 241Am). Information values are given for 12 radionuclides (90Sr, 137Cs, 210Pb (210Po), 226Ra, 228Ra, 232Th, 234U, 235U, 239Pu, 240Pu and 241Pu). Less reported radionuclides include 228Th, 236U, 239Np and 242Pu. The reference material may be used for quality management of radioanalytical laboratories engaged in the analysis of radionuclides
in the environment, as well as for the development and validation of analytical methods and for training purposes. The material
is available from IAEA in 100 g units.
Retired from IAEA in 2003. 相似文献
997.
Stamatelatos IE Kasviki K Green S Gainey M Kalef-Ezra J Beddoe A 《Analytical and bioanalytical chemistry》2004,379(2):192-197
The design, calibration, dosimetry and performance evaluation of a prompt-gamma neutron activation analysis facility for in vivo body composition studies in small animals (i.e. rats or rabbits) is discussed. The system design was guided by Monte Carlo transport calculations using MCNP-4C code. A system was built and performance evaluation was made using a 185-GBq Pu-Be neutron source. Prompt-gamma rays produced by neutron capture reactions were detected by a combination of a NaI(Tl) scintillation and a HPGe semiconductor detectors. Nitrogen and chlorine were quantified by analysis of the 10.83-MeV and 6.11-MeV peaks, respectively. Appropriate corrections for the animal body size were determined. The facility described allows the in vivo determination of protein and extracellular space in sets of experimental animals. 相似文献
998.
A better understanding of the interactions between poly(ethylene oxide) (PEO)-based nonionic surfactants and smectite is important
to fully comprehend the transport and the fate of nonionic surfactants in the environment and to design novel organo-clay
composites. We studied the bonding between the surfactants and smectite and the molecular conformations of the surfactants
in the interlayer of smectite. A reference polymer PEG and three nonionic surfactants—Brij 56, Brij 700, and PE-PEG—were intercalated
into a smectite. The polymers and the composites were characterized with X-ray diffraction (XRD) and Fourier transform infrared
(FT-IR) spectroscopy. The XRD and FT-IR results indicate that the bulk surfactants existed as crystalline materials at room
temperature, and surfactant molecules had both helical/extended diblock and planar zigzag conformations. The surfactants intercalated
smectite and expanded the d(001) spacing of smectite to nearly 1.8 nm. The shapes and positions of the IR bands of interlayer
surfactants were similar to those of the melted (amorphous) bulk polymers: the wagging vibrations of the CH2 merged to a single band at 1,350 cm−1, the twisting bands of CH2 had 9 cm−1 or more blue shifts. These changes imply that the PEO segments of the surfactants existed with a distorted and extended conformation
in the interlayer of smectite, and this extended conformation was an intermediate form of the helical and planar zigzag conformations.
The molecular conformation of the interlayer surfactant was not affected by the seven types of exchangeable cations (Na+, K+, Ca2+, Mg2+, Cu2+, Ni2+, and H+) tested. There were 20 cm−1 or more red shifts from the C–O–C stretching bands when the surfactants were adsorbed. The red shifts suggest that surfactants
were bonded to smectite mainly through (1) H-bonding between oxygen atoms of the PEO segments and water molecules in hydration
shells of the exchangeable cations, and (2) direct coordination or ion–dipole interaction between the oxygen atoms of the
PEO segments and the exchangeable cations. With the extended conformation, the oxygen atoms of the PEO segments have maximum
exposure to the bonding water molecules and exchangeable cations. 相似文献
999.
Photoelectron spectra, with photon energies varying from 18 to 120 eV, have been measured for Ni(eta(5)-C(5)H(5))(NO). Relative partial photoelectron cross sections and branching ratios have been evaluated for the first three valence ionization bands. He I and He II photoelectron spectra have been remeasured for Ni(eta(5)-C(5)H(5))(NO) and Ni(eta(5)-C(5)H(4)CH(3))(NO). In the latter case, the fine structure on the first band differs from that in the previously published spectrum. Density functional calculations have been carried out to determine the ionization potentials of the lowest lying states of Ni(eta(5)-C(5)H(5))(NO) as well as the corresponding photoionization cross sections and the resulting branching ratios using the LCGTO-DF and LDKL-DF methods, respectively. Both experimental and theoretical investigations lead to an ion state ordering (2)E(1) < (2)E(2) approximately (2)A(1)< (2)E(1) and an assignment of (2)E(1) states to the first and third bands with the (2)A(1) and (2)E(2) states comprising the second band. This differs from the original assignment in the literature, where the (2)A(1) ionization was assigned to a high-energy shoulder on the first band. The separation of this shoulder from the main band maximum of 0.23 eV (1850 +/- 81 cm(-)(1)) suggests that it may be caused by excitation of the NO stretching vibration in the ion. The neutral molecule has a NO stretch of 1832 cm(-)(1); the calculated energies for the neutral molecule and the cation are 1845 and 1911 cm(-)(1), respectively. Agreement between calculated and experimental ionization energies and good matching of the theoretical and measured branching ratios support the new assignment of the photoelectron spectrum. 相似文献
1000.
Francesca P. A. Fabbiani Charlotte K. Leech Kenneth Shankland Andrea Johnston Philippe Fernandes Alastair J. Florence Norman Shankland 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(11):o659-o663
Bendroflumethiazide, or 3‐benzyl‐6‐(trifluoromethyl)‐3,4‐dihydro‐2H‐1,2,4‐benzothiadiazine‐7‐sulfonamide 1,1‐dioxide, is reported to crystallize as 1:1 solvates with acetone, C15H14F3N3O4S2·C3H6O, and N,N‐dimethylformamide, C15H14F3N3O4S2·C3H7NO. A detailed investigation of the crystal packing and intermolecular interactions is presented by means of Hirshfeld surface analysis. This analysis confirms the atomic positions of methyl H atoms of the solvent molecules that were inferred from the X‐ray data and provides a useful tool for structure validation. 相似文献