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51.
Solute-induced quenching of sonoluminescence (SL) is reported for aqueous solutions of two homologous series of methyl esters and ketones using low (20 kHz) and high (515 kHz) ultrasound frequencies. SL data at 20 kHz from aqueous solutions containing alcohols and carboxylic acids are also presented to compare with previously published results at 515 kHz. In addition to supporting the previous findings on the existence of stable and transient bubbles at 515 and 20 kHz, respectively, the results suggest that the hydrogen-bonding characteristics of the solutes also play a major role in the extent of SL quenching. An increase in the SL intensity at low concentrations for most of the solutes suggests that these solutes increase the number of "active" bubbles by hindering the coalescence of bubbles. It is concluded that the effect of the solutes on the SL signal from aqueous solutions at both frequencies is primarily due to the balance of two factors, namely, the incorporation of solute within the bubble, leading to SL quenching, and the prevention of coalescence of the bubbles, leading to SL enhancement. At the higher frequency, SL quenching by the solutes is the main influence on the emission yield. However, at the lower frequency, hindrance to coalescence by the solutes dominates at lower concentrations and leads to SL enhancement. The implications of these results for optimizing conditions for aqueous sonochemical reactions are discussed. 相似文献
52.
Summary We compare two methods (Mulliken charges and a distributed multipole analysis, DMA) of representing an ab initio charge distribution for calculating the electrostatic field and potential outside the molecule, using pyrimidine and the RNA base uracil as examples. This is done using a 3-D graphical display of the electrostatic fields, which, when used with real-time rotation, zooming and clipping, has many advantages for qualitatively assessing the electrostatic interactions of a molecule. The errors involved in using Mulliken point charges may be of similar magnitude to the total electrostatic field in regions which are important in recognition processes. The DMA representation automatically includes the anisotropic electrostatic effects of non-spherical features in the charge distribution of each atom, and yet the displayed electrostatic fields around the atoms which have lone-pair density do not show marked anisotropy. 相似文献
53.
54.
Summary There are a number of reagents available for fluorescent labelling of primary amines. These include dansyl chloride, o-phthalaldehyde,
fluorescamine, and a new reagent, 9-fluorenylmethylchloroformate (FMOC), reported recently. This paper describes a reversed-phase
HPLC procedure for the separation and fluorescence detection of polyamines following pre-column derivatization with FMOC.
The polyamines studied by this method include putrescine, cadaverine, spermidine, and spermine. Experiments were carried out
to determine maximum fluorescence excitation and emission wavelengths, optimum reaction pH, linear ranges, and minimum detection
limits for each of the polyamines. The HPLC method includes a gradient program which provides complete separation from serum
hydrolysate components and specificity for the four polyamines with detection limits ranging from 2 to 9 pg. This procedure
was applied to hydrolyzed serum samples. 相似文献
55.
Slaughter BD Unruh JR Price ES Huynh JL Bieber Urbauer RJ Johnson CK 《Journal of the American Chemical Society》2005,127(34):12107-12114
We used single-pair fluorescence resonance energy transfer (spFRET) measurements to characterize denatured and partially denatured states of the multidomain calcium signaling protein calmodulin (CaM) in both its apo and Ca(2+)-bound forms. The results demonstrate the existence of an unfolding intermediate. A CaM mutant (CaM-T34C-T110C) was doubly labeled with fluorescent probes AlexaFlour 488 and Texas Red at opposing globular domains. Single-molecule distributions of the distance between fluorophores were obtained by spFRET at varying levels of the denaturant urea. Multiple conformational states of CaM were observed, and the amplitude of each conformation was dependent on urea concentration, with the amplitude of an extended conformation increasing upon denaturation. The distributions at intermediate urea concentrations could not be adequately described as a combination of native and denatured conformations, showing that CaM does not denature via a two-state process and demonstrating that at least one intermediate is present. The intermediate conformations formed upon addition of urea were different for Ca(2+)-CaM and apoCaM. An increase in the amplitude of a compact conformation in CaM was observed for apoCaM but not for Ca(2+)-CAM upon the addition of urea. The changes in the single-molecule distributions of CaM upon denaturation can be described by either a range of intermediate structures or by the presence of a single unfolding intermediate that grows in amplitude upon denaturation. A model for stepwise unfolding of CaM is suggested in which the domains of CaM unfold sequentially. 相似文献
56.
Deborah L. Buhrman Piper I. Price Patrick J. Rudewiczcor 《Journal of the American Society for Mass Spectrometry》1996,7(11):1099-1105
The effect of coeluting matrix compounds on the quantitation of SR 27417 in human plasma using electrospray liquid chromatography-tandem mass spectrometry has been examined. During the method development stage of this assay, plasma samples spiked with the analyte at 100 pg/mL were extracted using three different procedures: a hexane liquid-liquid extraction, an ethyl acetate back-extraction, and a solid phase extraction. Ion intensity of the analyte was found to be related inversely to the percent ionization of coeluting matrix components as evidenced by full scan spectra. The ethyl acetate back-extraction, which contained the fewest coeluting components, resulted in the highest ion intensity for the analyte. An assay comparison was done by using the liquid-liquid hexane and the ethyl acetate back-extractions for sample preparation. Replicate 1-mL samples (n=5) at 11 concentrations from 5 to 2000 pg/mL were extracted and analyzed. The results for the ethyl acetate back-extracted samples were acceptable from 2000 to 5 pg/mL with accuracy ranging from ?11.6 to 2.61% of the nominal concentrations. In contrast, the hexane liquid-liquid method had poor accuracy and precision below 20 pg/mL. The difference is explained by suppression of analyte ion intensity. These results are consistent with the current theory of electrospray ionization. 相似文献
57.
Design and Construction of a Pulsed Field-Gradient NMR Probe for a High-Field Superconducting Magnet
We describe the design and construction of a high-resolution pulsed field-gradient NMR probe for use in a high-field superconducting magnet. Overview and details of the design and construction of the probe case, rf circuitry, temperature control and shielded gradient coils for a heteronuclear probe suitable for a spectrometer (Bruker MSL-300) equipped with a wide bore (i.e., 89 mm) magnet (Oxford) are given. The interior of the probe is designed so mat it may be easily moved to variable height so as to accept easily various gradient coils. A design for a novel jig for the winding of saddle-geometry rf coils is given. 相似文献
58.
The dissociation kinetics of proton-bound dimers of betaine with molecules of comparable gas-phase basicity were investigated using blackbody infrared radiative dissociation (BIRD). Threshold dissociation energies were obtained from these data using master equation modeling. For bases that have comparable or higher gas-phase basicity, the binding energy of the protonated base.betaine complex is ~1.4 eV. For molecules that are ~2 kcal/mol or more less basic, the dissociation energy of the complexes is ~1.2 eV. The higher binding energy of the former is attributed to an ion-zwitterion structure which has a much larger ion-dipole interaction. The lower binding energy for molecules that are ~2 kcal/mol or more less basic indicates that an ion-molecule structure is more favored. Semiempirical calculations at both the AM1 and PM3 levels indicate the most stable ion-molecule structure is one in which the base interacts with the charged quaternary ammonium end of betaine. These results indicate that the measurement of binding energies of neutral molecules to biological ions could provide a useful probe for the presence of zwitterions and salt bridges in the gas phase. From the BIRD data, the gas-phase basicity of betaine obtained from the kinetic method is found to be 239.2 +/- 1.0 kcal/mol. This value is in excellent agreement with the value of 239.3 kcal/mol (298 K) from ab initio calculations at the MP2/6-31+g** level. The measured value is slightly higher than those reported previously. This difference is attributed to entropy effects. The lower ion internal energy and longer time frame of BIRD experiments should provide values closer to those at standard temperature. 相似文献
59.
Price PJ 《Physical review. B, Condensed matter》1992,45(16):9042-9045
60.