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11.
The non-stoichiometry and chemical diffusion coefficient of SrFe1−xCoxO3-δ have been measured by steady state and transient thermogravimetry in the temperature range 750–1200 °C at different oxygen
partial pressures. At high oxygen partial pressures, the chemical diffusion coefficient was in the range 1·10−4 – 7·10−4 cm2/s. This, combined with high concentration of disordered vacancies make these materials perhaps the fastest solid oxygen ion
diffusers known at high temperatures and high oxygen partial pressures. However, due to the high concentration of defects
in SrFe1−xCoxO3-δ the compound transforms from a cubic (disordered) perovskite to a brownmillerite type of structure under reduced oxygen partial
pressures below approx. 900 °C. Due to this phase transition, the mobility of oxygen vacancies in SrFe1−xCoxO3-δ decreases up to about an order of magnitude at 850 °C. We also observe an ordering effect at 1000 °C, although smaller in
size, and this is suggested to be due to short range ordering of four-coordinated polyhedra of Fe. For possible use as oxygen
separation membranes, phase stability against sulphur and carbon containing atmospheres is also discussed with respect to
the formation of carbonates and sulphates.
Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999. 相似文献
12.
Dr. Mick Hornum Julie Stendevad Dr. Pawan K. Sharma Dr. Pawan Kumar Rasmus B. Nielsen Dr. Michael Petersen Dr. Poul Nielsen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(30):7387-7395
Nucleotides that contain two nucleobases (double-headed nucleotides) have the potential to condense the information of two separate nucleotides into one. This presupposes that both bases must successfully pair with a cognate strand. Here, double-headed nucleotides that feature cytosine, guanine, thymine, adenine, hypoxanthine, and diaminopurine linked to the C2′-position of an arabinose scaffold were developed and examined in full detail. These monomeric units were efficiently prepared by convergent synthesis and incorporated into DNA oligonucleotides by means of the automated phosphoramidite method. Their pairing efficiency was assessed by UV-based melting-temperature analysis in several contexts and extensive molecular dynamics studies. Altogether, the results show that these double-headed nucleotides have a well-defined structure and invariably behave as functional dinucleotide mimics in DNA duplexes. 相似文献
13.
Poul Erik Hansen Bahjat A. Saeed Rita S. Rutu Teobald Kupka 《Magnetic resonance in chemistry : MRC》2020,58(8):750-762
1J(15N,H) coupling constants for enaminones and NH-forms of intramolecularly hydrogen-bonded Schiff bases as model compounds for sp2-hybridized nitrogen atoms are evaluated using density functional theory (DFT) to find the optimal functionals and basis sets. Ammonia is used as a test molecule and its one-bond coupling constant is compared with experiment. A methylamine Schiff base of a truncated molecule of gossypol is used for checking the performance of selected B3LYP, O3LYP, PBE, BHandH, and APFD density functionals and standard, modified, and dedicated basis sets for coupling constants. Both in vacuum and in chloroform, modeled by the simple continuum model of solvent, the modified basis sets predict significantly better the 1J(15N,H) value in ammonia and in the methylamine Schiff base of a truncated molecule of gossypol than the standard basis sets. This procure is then used on a broad set of intramolecularly hydrogen-bonded molecules, and a good correlation between calculated and experimental one-bond NH coupling constants is obtained. The 1J(15N,H) couplings are slightly overestimated. The calculated data show for hydrogen-bonded NH interatomic distances that the calculated values depend on the NH bond lengths. The shorter the bond lengths, the larger the 1J(15N,H). A useful correlation between 1J(15N,H) and NH bond length is derived that enables realistic predictions of one-bond NH coupling constants. The calculations reproduce experimentally observed trends for the studied molecules. 相似文献
14.
Temperature-induced structure and microstructure changes in hexacelsians (BaAl2Si2O8) that have been synthesised from the Ba-exchanged LTA and FAU zeolites (hexacelsianLTA and hexacelsianFAU) show that the phase transition near 580?K exists only in hexacelsianLTA. The X-ray powder diffraction method has been used to follow the evolution of the structure during the phase transition, as described here. The excess thermodynamic quantities Gibbs free energy (G), entropy (S) and enthalpy (H) are obtained through the Landau theory of phase transition. The constants of proportionality between the G and ordering parameter (Q) are: h?=??170345?J?mol?1, a?=??66.6?J?mol?1?K?1 and b?=??410534?J?mol?1. The abrupt change in the trigonal distortion of the single six-member tetrahedral [SiO4]4? and [AlO4]5? ring near 580?K is responsible for the phase transition. The phase transition is non-convergent, ferroelastic, pure and proper. 相似文献
15.
The formation region of the various types of layered titanium hydrogen phosphate hydrates was investigated. The materials were prepared by hydrothermal methods, treating amorphous titanium phosphate with phosphoric acid (8 to 16M) in the temperature range 175 to 250°C. The materials obtained were:α-Ti(HPO4)2·H2O,γ-Ti(PO4)(H2PO4)·2H2O, and its anhydrous formβ-Ti(PO4)(H2PO4). The structure ofβ-Ti(PO4)(H2PO4) has been determined by Rietveld powder refinement of high resolution neutron diffraction data. The structure is refined in the monoclinic space groupP21/n(No. 14). The unit cell parameters are:a=18.9503(4) Å,b=6.3127(1) Å,c=5.1391(1) Å,β=105.366(2)°;Z=4. The final agreement factors were:Rp=2.9% andRwp=3.8%. The structure ofβ-Ti(PO4)(H2PO4) is built from TiO6octahedra linked together by tertiary phosphate (PO4) and dihydrogen phosphate ((OH)2PO2) tetrahedra. The layers are held together by hydrogen bonds. 相似文献
16.
A cyclic dinucleotide with a butylene linker between the upper 2'-C position and the 3'-O-phosphate linkage was synthesised from simple nucleoside building blocks via a tandem ring-closing metathesis and hydrogenation procedure. The major of two phosphorus epimers was incorporated into an oligodeoxynucleotide, as well as into an LNA-DNA mixmer oligonucleotide. These were evaluated as parts in three different secondary structures, a duplex, a bulged duplex and a three-way junction, with both DNA and RNA complements. In the DNA:RNA hybrid molecule, the oligodeoxynucleotide containing this single 2'-C to 3'-O-phosphate butylene linkage was found to stabilise a three-way junction. 相似文献
17.
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19.
1H, 2H and 13C NMR studies of cyclobutene and a series of isotopically enriched species have led to a determination of the 1H? 1H, 13C? 1H, 13C? 2H and 13C? 13C coupling constants in these compounds. In agreement with general observations, 1J(CH) is found to depend on the hybridization of the carbon atoms. Likewise, 2J(HH), 2J(CC), 3J(HH) and 3J(CH), but not 2J(CH), depend on the angles between the bonds connecting the coupled nuclei. When comparing cyclobutene with thiete 1,1-dioxide (thiete sulphone) an increase of almost 20 Hz is observed for 1J(C-2, H-2) in the latter compound. All but one of the observed deuterium isotope effects on chemical shifts are negative. In the case of isotope effects upon the one-bond coupling constants, the obtained values support the results of the theoretical calculations of Sergeev and Solkan. 相似文献
20.
Dr. Evanildo G. Lacerda Jr. Dr. Fadhil S. Kamounah Prof. Dr. Kaline Coutinho Prof. Dr. Stephan P. A. Sauer Prof. Dr. Poul Erik Hansen Prof. Dr. Ole Hammerich 《Chemphyschem》2019,20(1):78-91
Prediction of chemical shifts in organic cations is known to be a challenge. In this article we meet this challenge for α-protonated alkylpyrroles, a class of compounds not yet studied in this context, and present a combined experimental and theoretical study of the 13C and 1H chemical shifts in three selected pyrroles. We have investigated the importance of the solvation model, basis set, and quantum chemical method with the goal of developing a simple computational protocol, which allows prediction of 13C and 1H chemical shifts with sufficient accuracy for identifying such compounds in mixtures. We find that density functional theory with the B3LYP functional is not sufficient for reproducing all 13C chemical shifts, whereas already the simplest correlated wave function model, Møller–Plesset perturbation theory (MP2), leads to almost perfect agreement with the experimental data. Treatment of solvent effects generally improves the agreement with experiment to some extent and can in most cases be accomplished by a simple polarizable continuum model. The only exception is the NH proton, which requires inclusion of explicit solvent molecules in the calculation. 相似文献