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71.
The following experiment reinforces students working knowledge of statistics by utilizing the t test to compare the results of two independent methods for the determination of dissolved oxygen (DO). In this experiment students utilize a dissolved oxygen probe to determine the levels of DO in natural waters at two sampling locations while obtaining samples of water from the laboratory for analysis using the classic Winkler titration. The importance of using proper sampling methods and techniques to obtain representative samkples is a large focus of the prelaboratory discussion and is continually stressed during fieldwork. After analyzing the water samples by the DO mete and the Winkler titration, students pool the class data and are asked to determine if the two methods for dissolved oxygen agree at each sampling location. The students are then asked to determine if the DO levels at the different sampling locations are statistically different or not. The students are asked to consider why their results agree or differ from the theoretical value they calculate using Henrys law. 相似文献
72.
π-Electron delocalization in neutral and protonated “doubly-extended-guanidine,” (H2N)2C?N? CH?N—CH?NH, has been studied by ab initio methods at the self-consistent field (SCF) STO -3G and 3-21G levels for a large number of tautomeric, rotameric, pseudocyclic, and monocyclic (disubstituted triazine) forms. These π systems have been characterized in terms of a number of structural and energetic parameters: degree of single/bond character from bond lengths and π bond orders, electron distributions, and tautomer, rotamer, and protonation energies. The acyclic neutral forms exhibit largely alternant single–double bond patterns as predicted by classical bonding structures but with, however, significant deviations due to conjugation. The acyclic protonated forms exhibit bond patterns consistent with resonance delocalized structures extending over the whole molecule (“doubly-extended guanidinium”) or part of the molecule (“extended-guanidinium”) or guanidinium . All systems showed alternant charge distributions with electron-deficient carbons. The energy results have been analyzed in terms of possible contributions from steric interactions, lone-pair repulsions, purportive electrostatic interactions in pseudocyclic forms, overall π-system conformation (extended, kinked, or folded), and specific through-space π-overlap interactions in some pseudocyclic forms. It was found that these other interactions usually dominate the specifically π effects so that the general concept of preferential π delocalization in straight lines does not hold for the acyclic systems. Some interesting examples of pseudocyclic forms exhibiting strongly stabilizing intramolecular interactions attributed to π through-space coupling are identified. These systems with incipient-ring characteristics present intermediate bonding models between the acyclic and closed-ring π systems. The extent of stabilization of the guanidinium-type cations by resonance delocalization in cyclic systems depended on whether it reinforced or interfered with the overall ring delocalization. 相似文献
73.
Jill Merry 《Fresenius' Journal of Analytical Chemistry》1995,352(1-2):148-151
VKI, the Danish national reference laboratory for environmental chemistry, has prepared a range of reference materials for quality control of nutrient analyses in environmental samples. The relevant concentration levels and potential matrix interference effects vary with the sample type. In sea water, nutrient concentrations are in the g/L range and salt concentrations can cause matrix interferences. Two certified reference materials for nutrient analyses were prepared representing the salinities in the Kattegat (20–25 parts per thousand) and the Baltic sea (below 10 parts per thousand). The preparation and certification were an integrated part of a larger project conducted by VKI on the optimisation of nutrient analyses in sea water. Part of the project has been participation in a European interlaboratory trial exercise QUASIMEME [1], which has enabled the Danish reference material data to be traced back to other European data. Denmark has now a certified reference material that will help to ensure the quality of marine analyses that are part of the ongoing national water monitoring plan. 相似文献
74.
Starting from 1-methylimidazole, a concise, scalable, three-step synthesis of the title compound is described. The required 2-chloroimidazole was prepared in very good yield by halogen-metal exchange between the 2-lithio derivative and hexachloroethane. 相似文献
75.
PaulA. Stocks PatrickG. Bray VictoriaE. Barton Mohammed Al‐Helal Michael Jones NunaC. Araujo Peter Gibbons StephenA. Ward RuthH. Hughes GiancarloA. Biagini Jill Davies Richard Amewu AmyE. Mercer Gemma Ellis PaulM. O'Neill 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2007,119(33):6394-6399
76.
Sally M. McIntyre Jill Wisnewski Ferguson Travis M. Witte R.S. Houk 《Spectrochimica Acta Part B: Atomic Spectroscopy》2011,66(3-4):248-254
The general method of comparing measured ion ratios to calculated ion ratios to determine a gas kinetic temperature (Tgas) is reviewed. Various mathematical refinements to the calculated partition functions are examined for their effect on the determined Tgas. It is found that (a) excited electronic states should be included for ArO+, neutral NO, and O2; (b) a 10% error in solvent load, sample gas flow rate, vibrational constant (ω), rotational constant (B) or measured ion ratio produces only a 1 to 3% error in Tgas; (c) a 10% error in dissociation energy (D0) creates nearly a 10% error in Tgas; and (d) high temperature corrections to the partition functions produce minimal change and can generally be neglected. 相似文献
77.
Crystal M. Schneck Adam J. Poncheri John T. Jennings Deanna L. Snyder Jill L. Worlinsky Swarna Basu 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2010,75(2):624-628
The interaction between 9-fluorenone, various indoles and solvents has been studied using steady-state fluorescence spectroscopy and quantum chemical calculations. It was determined that polar protic solvents such as methanol and ethanol significantly quenched the fluorescence of 9-fluorenone but various indoles reversed the solvent quenching. The effect of various solvents on the 9-fluorenone carbonyl vibration was investigated using infrared spectroscopy. Ab initio calculations using Gaussian03 were also carried out in order to determine the minimum energy conformations of these systems along with binding energies. 相似文献
78.
Wei Wei Dr. Shuzhou Li Dr. Jill E. Millstone Matthew J. Banholzer Xiaodong Chen Dr. Xiaoyang Xu George C. Schatz Prof. Chad A. Mirkin Prof. 《Angewandte Chemie (International ed. in English)》2009,48(23):4210-4212
Very long range surface‐enhanced Raman scattering is observed from a nickel nanowire that is separated by 120 nm from a pair of gold nanodisks. The excitation of the surface‐plasmon resonance (SPR) from the gold nanodisk pair generates an enhanced electromagnetic field near the nickel segment (SEM, left), leading to Raman intensity greater than the nickel alone (right).
79.
Kouvatsos N Meldrum JK Searle MS Thomas NR 《Chemical communications (Cambridge, England)》2006,(44):4623-4625
We have engineered a variant of the beta-clam shell protein ILBP which lacks the alpha-helical motif that caps the central binding cavity; the mutant protein is sufficiently destabilised that it is unfolded under physiological conditions, however, it unexpectedly binds its natural bile acid substrates with high affinity forming a native-like beta-sheet rich structure and demonstrating strong thermodynamic coupling between ligand binding and protein folding. 相似文献
80.
Nath KG Ivasenko O Miwa JA Dang H Wuest JD Nanci A Perepichka DF Rosei F 《Journal of the American Chemical Society》2006,128(13):4212-4213
We demonstrate a surprising cooperative adsorption process at the liquid-solid interface, involving self-assembly in which a three-fold hydrogen-bonding unit (trimesic acid, TMA) is forced into a linear pattern by noncovalent interaction with an alcohol. Our work shows that the unexpected linear pattern formed by coadsorption of TMA and alcohols can be modulated in size by choosing alcohols with different chain lengths. 相似文献