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511.
Iodine/silver(I) oxide (I(2)/Ag(2)O) reacts highly stereoselectively in the single, double, and triple anti epoxidation of the spherical 1,4,7-triene, 10-methyl-2,3:5,6:8,9-tris(cyclohexano)triquinacene 1. All of the three epoxides 3, 4, and 5 obtained with this reagent contain the epoxy groups at the convex side of the triquinacene framework. The stereochemical course of the epoxidation with I(2)/Ag(2)O is clearly distinct from that observed with m-chloroperbenzoic acid (MCPBA), which gives the same monoepoxide (3) but exclusively anti,syn di- and triepoxides (6-8) bearing at least one epoxy group at the concave side of the triquinacene framework. Epoxidation of the related three-fold 1,4-cyclohexadiene, tris(cyclohexeno)triquinacene 2, with MCPBA occurs similarly to the conversion of 1, whereas I(2)/Ag(2)O reacts with high regioselectivity at the less electron-rich peripheral double bonds of 2 giving triepoxides 12 and 13. The molecular structure of triepoxide 8 has been elucidated in detail by X-ray crystal structure analysis. 相似文献
512.
Cürten B Kullmann PH Bier ME Kandler K Schmidt BF 《Photochemistry and photobiology》2005,81(3):641-648
The photorelease of a caged neurotransmitter can be used to investigate the function of neuronal circuits in tissues. We have designed and synthesized a stable, caged gamma-aminobutyric acid (GABA) derivative, 4-[[(2H-1-benzopyran-2-one-7-amino-4-methoxy)carbonyl]amino] butanoic acid (BC204), that releases the neurotransmitter in physiological medium when irradiated with UV light at 300-400 nm in PBS at pH 7.4. The release of GABA occurs with the formation of the major photoproduct, 7-amino-4-(hydroxymethyl)-2H-1-benzopyran-2-one, via a solvolytic photodegradation mechanism of the coumarin moiety and was confirmed by electrospray mass spectrometry/mass spectrometry (ESI MS/MS). BC204 is chemically stable and shows no intrinsic activity after many hours under physiological dark conditions. These properties suggest that BC204 is an excellent form of caged GABA that is well suited for long-term biological studies. 相似文献
513.
W. Semmler P. Raghavan M. Senba R. S. Raghavan 《Zeitschrift für Physik B Condensed Matter》1981,45(1):29-37
The quadrupole interaction constants of69GeT1 and113SnSn were measured by means of the perturbed angular distribution technique as a function of temperature in the range of 80KT508 K and 80KT480 K, respectively. Isomeric states in69Ge and113Sn were populated by the heavy ion reactions56Fe(16O, 2p n) and100Mo(16O, 3n) and recoil implanted into polycrystalline Tl- and single crystalline Sn-backings. In the case of113SnSn, where the quadrupole coupling is weak, a special single crystal geometry was employed to enhance the sensitivity of the measurement. Within the limits of the errors the temperature dependence for both systems follows the empirical T1.5-dependence. While a strong temperature dependence comparable to InIn is observed for69GeT1, that for113SnSn is weaker than expected. The strength of the temperature dependence for113SnSn does not agree with the predictions of a lattice vibration model proposed recently for the temperature dependence of the quadrupole interaction.Supported in part by NSFAssociate of the Graduate Faculty of Rutgers University 相似文献
514.
The separation of oligosaccharides derivatized with various esters of aminobenzoic acid by means of reversed-phase nano-HPLC (nHPLC) with on-line ESI mass spectrometry and off-line MALDI-TOF mass spectrometry as well as MEKC is described. For this purpose methyl, ethyl and butyl aminobenzoates and heptyloxyaniline were used as derivatization agents for homologous maltodextrins and oligosaccharides from human milk. Four different C(18) stationary phases were tested for this purpose because the type of stationary phase was shown to have a dramatic effect on the performance of the separation. Optimal results were obtained using n-butyl aminobenzoate as label and an encapsulated ODS stationary phase. The on-line coupling of nHPLC to ESI MS allowed to separate and identify various oligosaccharides from human milk. This technique enabled the exact attribution of the molecular structure to a signal in the chromatogram. In a second approach oligosaccharides were separated by nHPLC and subsequently fractionated. The fractions were analyzed by MALDI-TOF mass spectrometry. The results obtained by this approach confirmed the ESI MS data. An analogous separation profile was obtained by using sodium dodecyl sulfate in MEKC, which proves that the retention mechanisms of both techniques are identical. 相似文献
515.
H. Metzner R. Sielemann R. Butt S. Klaumünzer W. Semmler 《Hyperfine Interactions》1983,15(1-4):413-415
A recoil implantation technique using heavy ion nuclear reactions has been used to implant the PAC-activity111In deeply (several m) into high-purity tantalum for a study of radiation induced lattice defects. In the region of annealing stage III (around 260K) trapping of defects is observed at the111In/111Cd probes in three well defined configurations. Striking similarities are found compared with the results of our former experiments on defect annealing in niobium. 相似文献
516.
Karoline Fendler Beate Hager Heinz Falk 《Monatshefte für Chemie / Chemical Monthly》2007,138(9):859-862
Summary. Two model compounds for the tryptophane variant of the green fluorescent protein chromophore containing a 3-indolyl and 2-pyrrolyl
moiety were prepared. For the first one the (Z)-diastereomer was found to be more stable than the (E)-diastereomer by 5.7 kJ mol−1. It could be photo-diastereomerized and its thermal equilibration was studied, whereas the second one underwent photo-destruction.
From an Arrhenius plot an activation barrier for the (E) to (Z) diastereomerization of 85.6 kJ mol−1 could be determined. Thus, it could be demonstrated that in contrast to the corresponding phenyl derivative studied recently
the tyrosine- and tryptophane-derived chromophores of the green fluorescent protein are amenable to fast thermal diastereomerization,
which is of fundamental importance for the fluorescence and photoswitching processes in the corresponding proteins. 相似文献
517.
Beate Massmann 《Journal of Geometric Analysis》1992,2(6):555-574
A compact complex manifoldX is an equivariant compactification of a homogeneous manifoldG/H (G a connected complex Lie group,H a closed complex subgroup ofG), if there exists a holomorphic action ofG onX such that theG-orbit of some pointx inX is open and H is the isotropy group ofx. GivenG andH, for some groups (e.g.,G nilpotent) there are necessary and sufficient conditions for the existence of an equivariant Kähler compactification which are proven in this paper. 相似文献
518.
519.
520.
Mathilde Giffard Michel Bardet Beate Bersch Jacques Covs Sabine Hediger 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2009,200(1):153-160
The effect of selective pulses on the apparent carbon longitudinal relaxation is investigated in three fully 13C-labeled systems, histidine as a model system and two proteins MerP and YajG. It is shown that the longitudinal relaxation of a selectively excited carbon spin is greatly enhanced, mainly because of fast spin-diffusion. This relaxation enhancement allows reducing the time necessary for polarization recovery between two experiments. This effect can be exploited either to improve the sensitivity of NMR experiments or to reduce the experimental time. Using selective carbon excitation combined with fast pulsing on fully 13C-labeled proteins, a sensitivity improvement of 20–45% over standard cross-polarization methods is predicted from the measured relaxation times. 相似文献