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21.
22.
Alves GA Amato S Anjos JC Appel JA Bracker SB Cremaldi LM Darling CL Dixon RL Errede D Fenker HC Gay C Green DR Jedicke R Kaplan D Karchin PE Kwan S Leedom I Lueking LH Luste GJ Mantsch PM de Mello Neto JR Metheny J Milburn RH de Miranda JM da Motta Filho H Napier A Rafatian A dos Reis AC Reucroft S Ross WR Santoro AF Sheaff M Souza MH Spalding WJ Stoughton C Streetman ME Summers DJ Takach SF Wu Z 《Physical review letters》1993,70(6):722-725
23.
Peter C. Burgers J. L. Holmes F. P. Lossing Frank R. Povel Johan K. Terlouw 《Journal of mass spectrometry : JMS》1983,18(8):335-339
The structures of the m/z 87, [C4H7O2]+, ions generated by dissociative ionization of CH3CGXCOOCH3 and XCH2CH2COOCH3 (X = CH3, Cl, Br, and I) have been investigated via their unimolecular and collisionally activated fragmentations and by apperance energy measurements. For both precursors loss of X = CH3 produced, via H atom transfer, ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_2 = CH}\mathop {\rm C}\limits^{\rm + } \left({{\rm OH}} \right){\rm OCH}_{\rm 3} $\end{document} (a), ΔHf = 386 kj mol?1. In marked contrast, loss of I˙ from ionized CH3CHICOOCH3 and ICH2CH2COOCH3 proceeded without rearrangement to yield respectively ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_3}\mathop {\rm C}\limits^{\rm + } {{\rm HCOOCH_3}} $\end{document} (b), ΔHf = 480 kJ mol?1 and (c), ΔHf = 450 kJ mol?1. These different fragmentation behaviours are explained via photoelectron spectra which show that the formal charge site in the precursor ion is at the carbonyl oxygen when X = CH3 but at the halogen atom when X = I. The precursor molecules X = Cl and Br display both of the above characteristics, CH3CHXCOOCH3 yielding mixtures of a and b and XCH2CH2COOCH3 producing a and c ions. 相似文献
24.
The collision-induced dissociation characteristics of the benzyl, tropyl and tolyl cations which serve to identify these [C7H7]+ isomers (the m/z 74–77 relative abundances) were not preserved in their neutralization-reionization (NR) mass spectra. However, analyses of mixtures of the [C7H7]+ isomers made by comparison of NR mass spectra gave similar results to those obtained from collisional activation (CA) mass spectra. The radicals produced by electron transfer from Xe to tolyl cations were not observed to rearrange to benzyl radicals on the time-scale of the NR experiment, a result in conflict with other gas-phase studies. 相似文献
25.
THE FUNCTION OF PHYTOCHROME IN THE NATURAL ENVIRONMENT—III. MEASUREMENT AND CALCULATION OF PHYTOCHROME PHOTOEQUILIBRIA 总被引:1,自引:0,他引:1
Abstract— Phytochrome photoequilibria have been measured in dark-grown Phaseolus uulgaris L . and Cucurbita pepo I . hypocotyl hooks which had been exposed to various natural and artificial radiation sources. Mean phytochrome photoequilibria ( φ ) varied from 0.20 within a wheat canopy to 0.54 above, although lower values were occasionally observed in densely shaded areas. Greater variation in phyto chrome photoequilibria and lower levels of Pfr were recorded within a sugar beet canopy. The range of photoequilibria was φ= 0.04 in dense shade to φ= 0.54 above the canopy. Photoequilibrium was achieved within 5 s in mid-day sunlight and approximately 30 s in dense canopy shade.
A close correlation was found between φ and the ratio of the quantum flux in the red and far-red wavelength bands (ζ) in broad spectrum (400–800 nm) radiation. This relationship allows direct prediction of φ from a knowledge of ζ. Phytochrome showed greatest sensitivity to spectral changes in the range ζ= zero to ζ= 1.0, which is the range found in the natural environment.
The observations provide support for the hypothesis that phytochrome is involved in the detection of shading by plants. 相似文献
A close correlation was found between φ and the ratio of the quantum flux in the red and far-red wavelength bands (ζ) in broad spectrum (400–800 nm) radiation. This relationship allows direct prediction of φ from a knowledge of ζ. Phytochrome showed greatest sensitivity to spectral changes in the range ζ= zero to ζ= 1.0, which is the range found in the natural environment.
The observations provide support for the hypothesis that phytochrome is involved in the detection of shading by plants. 相似文献
26.
Given a reaction mechanism we show how a symbolic computation approach can be used to develop the kinetic equations by identifying the reaction scheme with an equivalent matrix. Our method is also applicable in cases where the stoichiometric matrix approach fails. The specific algorithm that is written applies to schemes where individual reactions are at most ternary, but the way to generalize the procedure is also discussed. By using symbolic computing it is possible to determine general properties of the system. We demonstrate this by showing how to use the matrix to determine the system's conservation laws, which in turn can be used to reduce the number of equations in the system. As another application it is shown how to determine some of the species which have a zero equilibrium state. To illustrate the procedure, example reaction schemes are investigated. 相似文献
27.
Gies P Roy C McLennan A Pailthorpe M Hilfiker R Osterwalder U Monard B Moseley H Sliney D Wengraitis S Wong J Human S Bilimis Z Holmes G 《Photochemistry and photobiology》2003,77(1):58-67
In recent years the need to standardize measurement protocols for quantifying the degree of ultraviolet radiation (UVR) protection provided by clothing has led to the introduction of a number of standards around the world. To date, these standards have specified spectral measurements of UVR transmission by clothing and fabrics. Development of a standard test method has become an important part of the testing process, and this article presents results from an intercomparison involving 10 independent testing laboratories and 11 different UVR transmission measurement instruments. In addition to comparing the measured ultraviolet protection factors (UPF), this intercomparison also incorporates detailed scan results from all 10 laboratories and highlights differences in performance of the various instruments in different wavelength regions. Careful examination of these differences can indicate where changes to the systems could be made to allow improvements both in equipment performance and in agreement of the final results. The variability in the measurements of UPF in this study suggest that the protection categories in standards may need to be broadened. 相似文献
28.
Coen M Lindon JC Gavaghan C Holmes E Humpfer E Wilson ID Stanley PD Nicholson JK 《The Analyst》2001,126(5):548-550
High resolution 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra have been obtained on typical C18 bonded silicas used in chromatographic solid-phase extraction separations. It has been shown for the first time that water molecules distributed in distinct physico-chemical environments within the chromatographic system can be detected directly using a simple 1H MAS NMR measurement. The resonances assigned to water protons in differing physico-chemical environments have distinct chemical shifts, line widths, relaxation times (T1 and T2) and also exhibit temperature dependent coalescence behaviour. This novel MAS approach may lead to a better understanding of the environments of other analytes in mixtures during such separations. 相似文献
29.
Titration microcalorimetry is used to study the influences of iodide, bromide, and chloride counterions on the aggregation of vesicle-forming 1-methyl-4-(2-pentylheptyl)pyridinium halide surfactants. Formation of vesicles by these surfactants was characterised using transmission electron microscopy. When the counterion is changed at 303 K through the series iodide, bromide, to chloride, the critical vesicular concentration (cvc) increases and the enthalpy of vesicle formation changes from exo- to endothermic. With increase in temperature to 333 K, vesicle formation becomes strongly exothermic. Increasing the temperature leads to a decrease in enthalpy and entropy of vesicle formation for all three surfactants. However the standard Gibbs energy for vesicle formation is, perhaps surprisingly, largely unaffected by an increase in temperature, as a consequence of a compensating change in both standard entropy and standard enthalpy of vesicle formation. Interestingly, standard isobaric heat capacities of vesicle formation are negative, large in magnitude but not strikingly dependent on the counterion. We conclude that the driving force for vesicle formation can be understood in terms of overlap of the thermally labile hydrophobic hydration shells of the alkyl chains. Copyright 2000 Academic Press. 相似文献
30.
The position of deuterium in the products of the addition of HCL and DCI to exo 5, 6-d2 norbornene has been determined mass spectrometrically by measuring metastable ion abundances. The results show a stereo-selective hydrogen atom abstraction when the molecular ion of exo-2-norbornyl chloride fragements by loss of a chloroethyl radical. 相似文献