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71.
The semiempirical PM5 method has been used to calculate fully optimized structures of magnesium-bacteriochlorin, magnesium-chlorin, magnesium-porphin, mesochlorophyll a, chlorophylls a, b, c(1), c(2), c(3), and d, and bacteriochlorophylls a, b, c, d, e, f, g, and h with all homologous structures. Hartree-Fock/6-31G* ab initio and density functional B3LYP/6-31G* methods were used to optimize structures of methyl chlorophyllide a, chlorophyll c(1), and methyl bacteriochlorophyllides a and c for comparison. Spectroscopic transition energies of the chromophores and their 1:1 or 1:2 solvent complexes were calculated with the Zindo/S CIS method. The self-consistent reaction field model was used to estimate solvent shifts. The PM5 calculations predict planar structure of the porphyrin ring and central position of the four coordinated magnesium atoms in all pigments studied, in accord with the experimental, ab initio, and density functional results, a significant improvement as compared to the older semiempirical PM3 approach. Only small differences in PM5 and B3LYP/6-31G* or Hartree-Fock/6-31G* minimum energy geometries of the reference molecules were observed. Calculations show that in 1:1 solvent complexes, where the magnesium atom is five coordinated, the magnesium atom is shifted out of the plane of the porphyrin ring towards the solvent molecule, while the hexa coordinated 1:2 complexes are again planar. The PM5 method gives atomic charges that are comparable with those obtained from the Hartree-Fock/6-31G* and B3LYP/6-31G* calculations. The single point ZINDO/S CIS calculations with PM5 minimum energy structure gave excellent correlations between calculated and experimental transition energies of the chlorophylls and bacteriochlorophylls studied. Such correlations may be used for prediction of transition energies of the chromophores in protein binding sites. Calculations also predict existence of dark electronic states below the main Soret absorption band in all chromophores studied. The results suggest that the semiempirical PM5 method is a fairly reliable and computationally efficient method in predicting molecular parameters of porphyrin-like molecules. 相似文献
72.
《Journal of separation science》2003,26(11):1028-1034
Reversed phase liquid chromatography with diode array detection (DAD) and electrospray mass spectrometric (ESI MSD) methods were developed for the identification of anthraquinone color components of cochineal, lac dye, and madder – red natural dyestuffs. Electrospray mass spectrometry was found to be more suitable than diode array detection for such analysis because of its higher sensitivity (detection limits in the range 30–90 ng mL–1) and selectivity. The developed method permitted unequivocal identification of carminic acid and laccaic acid A as coloring matters in examined preparations of cochineal and lac dye, respectively. In madder more chemical color species were found: alizarin, purpurin, lucidin, ruberythric acid, and also aluminum and calcium alizarin lake. Among the methods recommended so far, the present one allows fast, direct, and unequivocal identification of components of very complicated natural products used in art. 相似文献
73.
This paper demonstrates the application of composite multi-walled carbon nanotube (MWNT) polyvinylchloride (MWNT-PVC) based on 1,5-diphenylcarbazide as chromium ionophore in potentiometric measurement. The sensor shows a good Nernstian slope of 19.52 ± 0.40 mV/decade in a wide linear range concentration of 6.3 × 10−8 to 1.0 × 10−2 M for Cr(NO3)3. The detection limit of this electrode was found to be 3.2 × 10−8 M of Cr(NO3)3 and is applicable in a pH range of 3.0-6.8. It has a short response time of about 10 s. This chromium electrode has a good selectivity over 16 various metal ions. The practical analytical utility of this electrode was demonstrated by measurement of Cr(III) in drinking water and mineral water samples without any serious preliminary pre-treatment and chromium in multivitamin. 相似文献
74.
Spectrophotometric determination of nitrite based on its catalytic effect on the oxidation of carminic acid by bromate 总被引:4,自引:0,他引:4
A highly sensitive and selective method is described for the determination of trace amounts of nitrite based on its effect on the oxidation of carminic acid with bromate. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of carminic acid at 490 nm after 3 min of mixing the reagents. The optimum reaction conditions were 1.8×10−1 mol l−1 H2SO4, 3.8×10−3 mol l−1 KBrO3, and 1.2×10−4 mol l−1 carminic acid at 30°C. By using the recommended procedure, the calibration graph was linear from 0.2 to 14 ng ml−1 of nitrite; the detection limit was 0.04 ng ml−1; the R.S.D. for six replicate determinations of 6 ng ml−1 was 1.7%. The method is mostly free from interference, especially from large amounts of nitrate and ammonium ions. The proposed method was applied to the determination of nitrite in rain and river water. 相似文献
75.
Six pentacyclic triterpene acids, ursolic acid, oleanolic acid, betulinic acid, 23-hydroxybetulinic acid, glycyrrhetinic acid, and senegenin, were metabolized by the microbe Nocardia sp. NRRL 5646 to selectively furnish their corresponding 28-methyl esters. Notably, ursolic acid (1) was converted to oleanolic acid methyl ester (4) via two intermediates, oleanolic acid (2), and ursolic acid methyl ester (3), which are formed by participation of ‘retro-biosynthetic’ methyl migration from C-19 to C-20. Senegenin (11) was selectively converted to a nortriterpene methyl ester, senegenic acid methyl ester (12), via an unprecedented C-C bond cleavage. The stereochemical assignments of compounds 11 and 12 were made unambiguously for the first time using 2D NMR spectroscopy. 相似文献
76.
Pekka Juusola Pentti Minkkinen Lasse Leino Jarmo K. Laihia 《Monatshefte für Chemie / Chemical Monthly》2007,138(10):951-965
Summary. (E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation
constants (pK
a1 and pK
a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation
(pK
a1 = 3.43 ± 0.12 and pK
a2 = 5.80 ± 0.04) and (pK
a1 = 2.7 ± 0.3 and pK
a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride
solutions up to the isotonic salt concentration (0.154 mol dm−3), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at
temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK
a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric
dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived.
The obtained pK
a1 and pK
a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature.
An erratum to this article is available at . 相似文献
77.
This paper presents the influence of a concentrated mass location on the natural frequencies of a cracked double-beam. The double-beam consists of two different beams connected by an elastic medium. The concentrated mass is located on the main beam. The relationship between the natural frequency and the location of concentrated mass is established and called “Frequency–Mass Location” (FML). The numerical simulations show that when there is a crack, the frequency of the double-beam changes irregularly when the concentrated mass is attacked at the crack position. This irregular change can be amplified by the wavelet transform and this is useful for crack detection: the crack location can be detected by the location of peaks in the wavelet transform of the FML. Finite element model for the cracked double-beam carrying a concentrated mass is presented and numerical simulations are also provided. 相似文献
78.
An analysis is performed to study transient free convective boundary layer flow of a couple stress fluid over a vertical cylinder, in the absence of body couples. The solution of the time-dependent non-linear and coupled governing equations is carried out with the aid of an unconditionally stable Crank–Nicolson type of numerical scheme. Numerical results for the steady-state velocity, temperature as well as the time histories of the skin-friction coefcient and Nusselt number are presented graphically and discussed. It is seen that for all flow variables as the couple stress control parameter, Co, is amplified, the time required for reaching the temporal maximum increases but the steady-state decreases. 相似文献
79.
Tryptanthrin is a natural product with numerous important pharmacological properties. Tryptanthrin and its analogs are commonly prepared by condensation of isatoic anhydride and isatin. In this Letter we investigate the formation of tryptanthrin derivatives upon Oxone-induced oxidative dimerization of indole-3-carbaldehydes. 相似文献
80.
(+)-Decytospolides A and B, natural products containing the tetrahydropyran skeleton, were synthesized via Prins cyclization as the key step. 相似文献