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861.
L. Ambrosone V. Vitagliano C. Della Volpe R. Sartorio 《Journal of solution chemistry》1991,20(3):271-291
The evolution of free diffusion boundaries with various initial perturbations is described and compared with a set of experimental
data for the system KCl-Water. The necessity of extrapolating to infinite time of not only the experimental diffusion coefficient
(as a function of 1/t'), but also the experimental Q quantily (as a function of 1/t'2) is stressed. 相似文献
862.
Treatment of 1-(2-phenylselenoethyl)-1,2,5,6-tetrahydropyridine (15) with tributyltin hydride affords only the product of reduction, demonstrating the reluctance of the 5-hexenyl radical 9 to undergo ring closure. When the nature of the radical is modified, either by introduction of an ester group at C4 or via its quaternary ammonium salt, cyclization occurs readily; while the radical 52 gives an excellent yield of 1-methyl-1-azoniabicyclo[3.2.1.]octyl bromide (55) uncontaminated with the product of reduction, the bicyclic product from 21 is accompanied by some reduced material. Production of the unwanted alkene can be eliminated in the latter by recourse to the quaternary ammonium ester 1-(2-bromoethyl)-4-carbethoxy-1-methyl-1,2,5,6-tetrahydropyridinium bromide (35) which, when exposed to tributyltin hydride, affords a 1:1 endo/exo mixture of 4-carbethoxy-1-methyl-1-azoniabicyclo[3.2.1]octyl bromide (37) exclusively. These results support the demonstration of the powerful polar effect of an ester function when attached to the double bond of a 5-hexenyl system, a property which can be exploited in the case of the radical 58. Treatment of the precursor, 1-(2-bromoethyl)-3-carbethoxy-1-methyl-3-pyrrolinium bromide (60), with tributyltin hydride generates 58 which is found to cyclize with high regioselectivity, affording a convenient high-yielding synthesis of the endo/exo isomers of 3-carbethoxy-1-methyl-1-azoniabicyclo[2.2.1]heptyl bromide 57. The isomeric bicyclo[2.2.1]heptyl ester 63 was not detected. These observations are in accordance with predictions based upon frontier molecular orbital considerations. 相似文献
863.
Samples (1500 μl) are inserted between zones of water and ascorbic acid solution with subsequent addition of 1,10-phenanthroline at pH 5.0. The signal provides a plateau region corresponding to Fe(Il) followed by a peak corresponding to total iron. The proposed system allows up to 90 injections per hour. Linear working ranges are 0.1–9 and 0.3–12 mg l?1 Fe(II) and total iron, with relative standard deviations of < 0.6 and 1.2%, respectively. Results obtained for various ground waters agree well with those obtained by a standard method. 相似文献
864.
Application of a potentiometric electronic tongue as a classification tool in food analysis 总被引:3,自引:0,他引:3
This paper reports on the application of a potentiometric sensor array to the food analysis field, in order to distinguish simple tastes and to classify food samples. This array is formed by a set of non-specific all-solid-state potentiometric sensors and has been used in combination with principal component analysis (PCA) for the classification of food samples in batch and in flow injection mode. First attempt was to classify synthetic samples prepared with controlled variability. Once this ability is proven, satisfactory classification results are presented for commercial waters, orange-based drinks and tea samples. An interesting correlation is achieved between the natural juice content and its first calculated component, which allows for a very simple tool for screening purposes. 相似文献
865.
Summary In this second paper we propose an algorithm for the construction of the -tables and of the Padé-Hermite tables [3]. Then we present some numerical applications. 相似文献
866.
Wing-Chi Cheng Tsan-Sang Yau Wai-Nang Cheng Kok-Choi Cheng Della Wai-Mei Sin 《Accreditation and quality assurance》2007,12(9):483-490
An accurate and precise method for the determination of total 19-norandrosterone (19-NA), a major metabolite of nandrolone,
in human urine was developed based on isotope dilution gas chromatography-high resolution mass spectrometry (ID GC-HRMS).
The 19-NA glucuronide, together with deuterated 19-NA (d4-19-NA) (as the internal standard, IS), was subjected to enzymatic hydrolysis using β-glucuronidase, followed by solid phase
extraction (XtractT, mixed-mode column) and liquid–liquid extraction cleanup. The native and deuterated analogues of 19-NA
were then derivatised to the corresponding bis(trimethylsilyl) derivatives using N-methyl-N-trimethylsilyltrifluoroacteamide (MSTFA):NH4I:Dithiothretitol (1,000:2:3 w/w). Identification was achieved under selected ion monitoring of the respective trimethylsilyl
derivatives at ion masses m/z 405.26450 and 420.28800 for 19-NA and m/z 409.28920 and 424.31270 for d4-19-NA within the specific time windows (±1% of the relative retention time to the calibration standard). A linear calibration
curve (r
2 > 0.9995) was obtained based on seven calibration points (five replicates at each level) in the range 0.05–10 ng/g. The detection
limit for 19-NA was found to be 4 pg/g. The method has been applied for the determination of 19-NA by fortifying 19-NA glucuronide
at three concentrations (0.2, 2.1 and 7.2 ng/g) in blank urine samples with excellent accuracy and reproducibility. To circumvent
the iterative process of exacting matching, a single-point calibration procedure was adopted, where the acceptance criteria
for the isotopic ratio in the sample (RF
s) and calibration blends (RF
c) was set close to unity (0.95–1.05). This method was successfully applied in a pilot inter-comparison study, with results
in good agreement with the fortified value and other participants’ results (relative standard deviation, RSD < 2.0%) with
an expanded relative uncertainty (coverage factor of 2 at 95% confidence level) of 4.7%. It was found in our determination
that the main contributors to the uncertainty budget originated from the measurements of the purity of the reference material
and the response factor of the calibration standard. 相似文献
867.
Anni M Della Sala F Raganato MF Fabiano E Lattante S Cingolani R Gigli G Barbarella G Favaretto L Görling A 《The journal of physical chemistry. B》2005,109(12):6004-6011
The intramolecular radiative and nonradiative relaxation processes of three thiophene-S,S-dioxide derivatives with different molecular rigidity are investigated in different solutions and in inert matrix. We show that the fluorescence quantum efficiency and the relaxation dynamics are strongly dependent on the environment viscosity, whereas they are almost independent of the environment polarity. We demonstrate that this strong dependence is due to an environment dependent nonradiative decay rate, whereas no relevant variations of the radiative decay rate are observed. We demonstrate that the dipole coupling with the solvent does not provide an efficient nonradiative decay channel and that the S(n) - S(1) vibrational relaxation is very efficient in all of the molecules and for all of the investigated environments. Moreover first-principles time-dependent density-functional theory calculations in the correct, i.e., excited-state, molecular conformation, suggest that significant contributions of intersystem crossing to the triplet manifold can be excluded. We then conclude that the main nonradiative process determining the fluorescence quantum efficiency of this class of molecules is S(1) - S(0) internal conversion (IC). An explanation for the IC rate dependence in terms of the environment viscosity, molecular rigidity, S(1) - S(0) energy-gap, and molecular volume is presented. 相似文献
868.
Arylation of benzoxazolone 1 by 2,4-dinitrochlorobenzene 2 under solid-liquid phase transfer catalysis (PTC) gave 3-(2,4-dinitro)phenylbenzoxazolone 3 and 10-(2,4-dinitro)phenyl-3-nitrophenoxazine 4 . Neither PTC nor copper catalysis achieved arylation of 1 by less activated aryl halides. 相似文献
869.
Much work has been carried out in recent years on the beneficial effect of phenolic compounds as natural antioxidants which help to neutralize free radicals. In fact, researchers have focused their attention on the pathological role of free radicals in a variety of diseases, among which the most important are atherosclerosis and cancer. Thus, among the components of the so-called 'Mediterranean Diet', phenolic compounds have received increased attention as epidemiological studies have shown that consumption of foods and beverages rich in phenolics is correlated with reduced incidence of heart disease. In this study, four phenolic compounds: (1) 3,4,5-trihydroxybenzoic acid (gallic acid); (2) trans 3,4',5-trihydroxystilbene (trans-resveratrol); (3) 3,3',4',5,7-pentahydroxyflavone (quercetin) and its glycoside (4) 3,3',4',5,7-pentahydroxyflavone-3-rutinoside (Rutin) have been subjected to antioxidant study by a fluorimetric assay. In this method, the rate of peroxidation induced by 2,2'-azobis (2-methylpropionamidine) dihydrochloride was monitored through the loss of fluorescence of the protein B-phycoerythrin (B-PE). Under appropriate conditions, the loss of B-PE fluorescence in the presence of reactive species is an index of oxidative damage of the protein. The inhibition of the action of reactive species by phenolic compounds, reflected in the protection against the loss of B-PE fluorescence in the fluorimetric assay, is a measure of its antioxidant capacity against the reactive species. The antioxidant effects of phenolic compounds have been investigated at different concentrations to relate activity to structural effects. It has been observed that the fluorescence decay due to peroxyl radical attack on B-PE decreases exponentially with time. As a reference compound for antioxidant capacity we used 6-hydroxy-2,5,7,8-tetramethyl-chroman-2-carboxylicacid (trolox), a water soluble tocopherol analogue. This compound reacts rapidly with peroxyl radicals, and, until the trolox is consumed, no loss in phycoerythrin fluorescence is observed. A linear correlation of the net protection value with the concentration of trolox was demonstrated. The phenolic compounds studied react with peroxyl radicals in a similar way to trolox. Quercetin and rutin were shown to have strong antioxidant activities. The results obtained here are in agreement with previous studies confirming that quercetin is the most antioxidant of the four polyphenolics. 相似文献
870.
Bacchi A Costa M Della Cà N Gabriele B Salerno G Cassoni S 《The Journal of organic chemistry》2005,70(13):4971-4979
4-Yn-1-ones containing different substituents, prop-2-ynyl alpha-ketoesters, and prop-2-ynyl alpha-ketoamides have been caused to react catalytically under oxidative carbonylation conditions to give tetrahydrofuran, dioxolane and oxazoline, dihydropyridinone, and tetrahydropyridinedione derivatives in satisfactory yields. Reactions were carried out in MeOH or MeCN/MeOH mixtures at 65-100 degrees C in the presence of catalytic amounts of PdI(2) in conjunction with KI under 32 bar (at 25 degrees C) of a 3:1 mixture of CO and air. Anti and syn 5-exo-dig cyclization modes account for the formation of different products. It has been found that cyclopentenone, dihydropyridinone, and tetrahydropyridinedione derivatives, formed when the reaction is carried out at higher temperature and for a longer time, can also be selectively obtained through an acid treatment of tetrahydrofuran and oxazoline derivatives involving an unusual rearrangement. The structures of 6-methoxy-2,2-dimethyl-3-oxo-5-phenyl-2,3-dihydropyridine-4-carboxylic acid methyl ester and 2,2,5-trimethyl-3,6-dioxo-1,2,3,6-tetrahydropyridine-4-carboxylic acid methyl ester have been confirmed by X-ray diffraction analysis. 相似文献