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391.
Abstract

The solution structure of antifreeze glycoproteins (AFGP's) of the polar fish Tetramatomus borchgrevinki has been investigated by 2D 1 H NMR spectroscopy as well as molecular modeling calculations (MM2). The simple glycotripeptide repeating structure in the shorter AFGP's (fractions 7 & 8) makes the structural analysis amenable. The resonance assignments of AFGP's 7 & 8 were determined by two-dimensional NMR techniques (COSY, Relayed-COSY, Phase Sensitive DQCOSY, NOESY). Information about the protein secondary structure was obtained by the coupling constants between the back-bone amide and α-carbon protons (obtained by phase sensitive COSY). Additional three dimensional constraints were obtained from NOESY through-space connectivities. The three dimensional solution structures of several AFGP's glycotripeptide fragments were based on MM2 calculations. The model structure was compared with the experimental data. Exchange rates of amide protons measured by dynamical spectroscopy show that the threonine and some of the alanine amide protons have two different and distinct exchange rates. GalNAc and the C-terminal Ala' amide protons appear to show relatively slow exchznge rates. The results suggets that the amide protons are not involved in any strong intramolecular hydrogen bonding.  相似文献   
392.
NMR properties of 1,8-diphosphanaphthalenes with prerequisites for strong through-space interaction were studied. The exceptional magnitude of 4J(PP) 246 Hz was found in Nap[P(NMe2)2][P(OMe)(NMe2)] (Nap = naphthalene-1,8-diyl). The PIII, PV mixed valence systems Nap[P(E)(OMe)2][P(OMe)2] (E = O, S, Se) show much lower magnitudes of through-space interaction than expected.  相似文献   
393.
Synthetic and bonding aspects of heavier Group 15 (P, As, Sb, Bi) and 16 (S, Se, Te) peri-substituted naphthalenes, are discussed in this review. An important and unifying feature of the chemistry of these systems is the lively discussion about the nature of the interaction between peri-atoms. Are atoms bonded when they are closer than the sum of their van der Waals radii? Is there any (weak) bonding, or just a strained repulsive interaction? Positioning atoms of Group 15 and 16 at the naphthalene 1,8-positions provides leading systems with which to study these bonding issues.  相似文献   
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396.
We discuss the design and performance of a very sensitive low-field magnetometer based on the detection of free spin precession of gaseous, nuclear polarized 3He or 129Xe samples with a SQUID as magnetic flux detector. The device will be employed to control fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron. Furthermore, with the detection of the free precession of co-located 3He/129Xe nuclear spins it can be used as ultra-sensitive probe for non-magnetic spin interactions, since the magnetic dipole interaction (Zeeman-term) drops out. Characteristic spin precession times T2 * of up to 60 h were measured. The achieved signal-to-noise ratio of more than 5000:1 leads to an expected sensitivity level (Cramer-Rao lower bound) of δB≈1 fT after an integration time of 220 s and of δB≈10-4 fT after one day. By means of a co-located 3He/129Xe magnetometer, noise sources inherent in the magnetometer could be investigated, showing that CRLB is fulfilled, at least down to δB≈10-2 fT. The reason for such a high sensitivity is that free precessing 3He (129Xe) nuclear spins are almost completely decoupled from the environment. Therefore, this type of magnetometer is particularly attractive for precision field measurements where long-term stability is required.  相似文献   
397.
The muscarinic acetylcholine receptor family is a highly sought-after target in drug and molecular imaging discovery efforts aimed at neurological disorders. Hampered by the structural similarity of the five subtypes’ orthosteric binding pockets, these efforts largely failed to deliver subtype-selective ligands. Building on our recent successes with arecaidine-derived ligands targeting M1, herein we report the synthesis of a related series of 11 hydroxylated arecaidine esters. Their physicochemical property profiles, expressed in terms of their computationally calculated CNS MPO scores and HPLC-logD values, point towards blood–brain barrier permeability. By means of a competitive radioligand binding assay, the binding affinity values towards each of the individual human mAChR subtypes hM1hM5 were determined. The most promising compound of this series 17b was shown to have a binding constant towards hM1 in the single-digit nanomolar region (5.5 nM). Similar to our previously reported arecaidine-derived esters, the entire series was shown to act as hM1R antagonists in a calcium flux assay. Overall, this study greatly expanded our understanding of this recurring scaffolds’ structure–activity relationship and will guide the development towards highly selective mAChRs ligands.  相似文献   
398.
The spherulite structure and its genesis in WAX PE130 are discussed on the basis of DSC, X-ray small-angle and raster-electron-microscopical measurements. Modifications of this superstructure in the presence of a low-molecular-weight solvent are shown. The role of thermodynamics and kinetics is reflected. Conclusions are drawn which imply general aspects that governs the growth of spherulites.  相似文献   
399.
400.
With Li7Si2NO6, a new member of the family of lithium oxonitridosilicates with a so far unseen structure type could be synthesized. Using a high-temperature solid-state reaction in open nickel crucibles under nitrogen flow, it was possible to obtain single crystals from the starting materials SiO2, Li3N, and Li2O at temperatures of 900 °C. Single crystal X-ray diffraction data yielded lattice parameters of a=5.0934(2), b=7.4128(2), c=8.5918(2) Å, α=75.16(1)°, β=87.36(1)°, γ=73.01(1)° and a cell volume of V=299.75(2) Å3. The compound, crystallizing in the triclinic space group P (no. 2), consists of a highly condensed anionic network built up by [SiNO3]-, [LiO4]-, and [LiN2O2]-tetrahedra as well as lithium in octahedral coordination as completing cation. With an activation barrier of 695 meV for lithium migration, Li7Si2NO6 is a potential lithium-ion conductor. The structure allows a classification not only as a sorosilicate but also as a tecto-lithosilicate and most precisely as a lithium oxonitridolithosilicate, when the different coordinations of the lithium ions are taken into account. Interestingly, the new compound is none of the several proposed representatives of the lithium oxonitridosilicates, thus expanding this substance class unexpectedly.  相似文献   
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