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991.
3,5-Bis(2-hydroxyphenyl)-1H-1,2,4-triazole (H2La) and 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid (H3Lb) have been prepared, and crystal structure of the intermediate 2-(2-hydroxyphenyl)-4H-1,3-benzoxazin-4-one has been determined. Temperature dependent 1H NMR spectroscopic measurements of H2La indicated dynamic behavior with the equilibrium between the two asymmetric tautomers. For H3Lb, pD-dependent 1H NMR spectroscopic measurements showed small but characteristic shifts in the range of 0 ≤ pD ≤ 1, indicative of a triazole
nitrogen atom protonation; the corresponding pK
a of 0.98 ± 0.04 was determined by spectrophotometric titrations. (H2O, 26°C, 1 M KCl/HCl). Formation of [FeIII(La)]+ (pH 2.5) and [FeIII(La)2]− (pH > 6) was verified by UV-Vis spectroscopy. Complex formation of H3Lb with Al3+ and VO2+ was investigated by 1H NMR spectroscopic titration and cyclic voltammetry, respectively. Single crystals of the phenoxo bridged [VVO(HLb)(EtO)]2·2EtOH were characterized by X-ray structural analysis.
Dedicated to Professor Milan Melník on the occasion of his 70th birthday 相似文献
992.
Methodologies for the quantification of measurement uncertainties associated with the determination of 210Pb- and 210Po-specific activities by liquid scintillation counting (LSC) and alpha-particle spectrometry are presented, and are demonstrated
using the soil reference material IAEA-326. Major contributors to the combined uncertainty associated with the measurement
result of 210Pb were the uncertainties of net count rates in the 210Pb energy region of the sample spectrum and in the 210Bi energy region of the blank spectrum. The predominant sources of uncertainty in the measurement of 210Po were the uncertainties of net count rates in the regions of interest of 209Po and 210Po. The relative standard uncertainty of 210Po exponentially increases with the time interval between the sampling date and the separation date of Po, and this effect
is strongly dependent on the 210Po/210Pb activity ratio. When the specific activity of 210Pb is much higher than that of 210Po in the sample, the relative standard uncertainty of the 210Po determination increases significantly within a short time interval between the sampling date (or reference date) and the
separation date of Po in samples. 相似文献
993.
In this paper, we introduce and study a model of a Monod–Haldene type food chain chemostat with pulsed substrate. We investigate
the subsystem with substrate and prey and study the stability of the periodic solutions, which are the boundary periodic solutions
of the system. The stability analysis of the boundary periodic solution yields an invasion threshold. By use of standard techniques
of bifurcation theory, we prove that above this threshold there are periodic oscillations in substrate, prey, and predator.
Simple cycles may give way to chaos in a cascade of period-doubling bifurcations. Furthermore, by comparing bifurcation diagrams
with different bifurcation parameters, we can see that the impulsive system shows two kinds of bifurcations, whose are period-doubling
and period-halfing. 相似文献
994.
Sephadex LH-20 column chromatography was used to separate flavonoid components in a heartsease methanol extract. One of the
main components was identified by NMR as violanthin (6-C-glucosyl-8-C-rhamnosylapigenin). As a first approximation, the other main flavonoid component was considered to be rutin (3-O-rhamnoglucosylquercetin), based on comprehensive comparison of retention times and UV spectra of reference molecules, as
well as molecular mass and fragmentation patterns obtained by mass spectrometry. The minor flavonoids were separated by polyamide
column and analyzed by LC-MS. The antioxidant capacity of different flavonoid fractions was determined using both Trolox equivalent
antioxidant capacity (TEAC) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) in vitro antioxidant assays. The highest electron-donor
capacity was found for the major flavonoid component (rutin), whereas one minor component-rich flavonoid fraction exhibited
the highest hydrogen-donor activity. 相似文献
995.
Borjana V. Donkova Katja I. Milenova Dimitar R. Mehandjiev 《Central European Journal of Chemistry》2008,6(1):115-124
The precursors with a low manganese content ≤ 0.07% Mn were synthesized by spontaneous crystallization from Zn2+, Mn2+ and C2O4
2−-containing solutions. The initial ratio Zn2+:C2O4
2− = 1:1 and 1:2 influences the morphology and prevailing orientations of the crystallites in the oxalate samples. The presence
of such small Mn content in the samples does not change the morphology or size of the crystals. The ZnO and Mn/ZnO oxides
with manganese content from 0.51×10−2 to 15.1×10−2 Wt % are obtained after thermal decomposition of the oxalates. The oxides preserved the morphology of the precursors. The
catalytic tests show that the pure ZnO has a poor activity for CO oxidation reaction. Its doping with Mn promotes the catalytic
activity (up from twice to five times) in spite of the very low contents of the dopants. The observed increase of the activity
depends on both dopant concentration and Zn2+:C2O4
2− ratio, probably due to the different mechanism of the manganese inclusion and different morphology of the oxides. The catalysts
of the 1:2 series are more active in CO oxidation reaction.
相似文献
996.
Sevim Hamamci Veysel T. Yilmaz Sedat Gumus Orhan Büyükgüngör 《Structural chemistry》2008,19(1):123-129
A silver(I) complex of saccharinate (sac) with pyrazine (pyz), [Ag(sac)(pyz)] n , has been synthesized and characterized by elemental analysis, IR, thermal analysis, and single-crystal X-ray diffractometry. The complex crystallizes orthorhombic space group Pnma with unit cell parameters of a = 13.0073(9) Å, b = 6.4907(6) Å, c = 13.4007(9) Å, V = 1131.37(15) Å3, and Z = 4. [Ag(sac)(pyz)] n is a one-dimensional coordination polymer, in which the sac ligand acts as a monodentate ligand through the N atom and the trigonal silver centers are linked by the bridging pyz ligands. The individual chains are connected into two-dimensional supramolemular network by aromatic π(sac)···π(pyz) stacking interactions. The FTIR spectrum of [Ag(sac)(pyz)] n has been recorded in the region and 4,000–400 cm?1. The optimized geometry, frequency, and intensity of the vibrational bands of [Ag(sac)(pyz)] n were obtained by density functional theory (DFT) at the B3LYP level. The vibrational frequencies were calculated and the scaled values have been compared with the experimental FTIR data. The observed and calculated frequencies are found to be in good agreement. 相似文献
997.
Amanda L. Santos Gilberto O. Chierice Kenneth S. Alexander Alan Riga Ellen Matthews 《Journal of Thermal Analysis and Calorimetry》2009,96(3):821-825
Eugenol is the main volatile compound extracted oil from clove bud, Syzygium aromaticum L., and used in traditional medicine, as a bactericide, fungicide, anesthetic, and others. Its extraction was performed using
hydrodistillation which is the most common extraction technique. Its components and thermal behavior were evaluated using
gas chromatography (GC) and differential scanning calorimetry (DSC), which provide a better characterization of these natural
compounds. This extracted product was compared to the standard eugenol results. The GC results suggested ~90% eugenol was
found in the total extracted oil, and some of its boiling characteristics were 270.1 °C for peak temperature and 244.1 J g−1 for the enthalpy variation.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
998.
Mass analysis in islands of stability with linear quadrupoles with added octopole fields 总被引:1,自引:0,他引:1
Konenkov N Zhao X Xiao Z Douglas DJ 《Journal of the American Society for Mass Spectrometry》2007,18(5):826-834
Mass analysis with linear quadrupole mass filters is possible by forming "islands" in the stability diagram with auxiliary quadrupole excitation. In this work, computer simulations are used to calculate stability boundaries, island positions, and peak shapes and ion transmission for mass analysis with linear quadrupole mass filters that have added octopole fields of about 2 to 4%. Rod sets with exact geometries that have quadrupole and octopole fields only in the potential, and round rod sets, with multipoles up to N = 10 (the twenty pole term) included in the calculations, show the same stability boundaries, island positions, and peak shapes. With the DC voltage applied to the rods so that the Mathieu parameter a < 0, conventional mass analysis is possible without the use of an island. With the DC polarity reversed so that a > 0, the resolution and transmission are poor preventing conventional mass analysis. In principle, mass analysis in an island is possible with operation at either of two tips. Provided the correct island tip is chosen for mass analysis, peak shapes comparable to those with a > 0 and no excitation are possible, both with a > 0 and with a < 0. In the latter case, the use of an island of stability allows mass analysis when the added octopole otherwise prevents conventional mass analysis. 相似文献
999.
Structural characterization of intact proteins is enhanced by prevalent fragmentation pathways rarely observed for peptides 总被引:2,自引:0,他引:2
Jennifer S. Cobb Michael L. Easterling Jeffrey N. Agar 《Journal of the American Society for Mass Spectrometry》2010,21(6):949-959
While collisionally activated dissociation (CAD) pathways for peptides are well characterized, those of intact proteins are
not. We systematically assigned CAD product ions of ubiquitin, myoglobin, and bovine serum albumin generated using high-yield,
in-source fragmentation. Assignment of >98% of hundreds of product ions implies that the fragmentation pathways described
are representative of the major pathways. Protein dissociation mechanisms were found to be modulated by both source declustering
potential and precursor ion charge state. Like peptides, higher charge states of proteins fragmented at lower energies next
to Pro, via mobile protons, while lower charge states fragmented at higher energies after Asp and Glu, via localized protons.
Unlike peptides, however, predominant fragmentation channels of proteins occurred at intermediate charge states via non-canonical
mechanisms and produced extensive internal fragmentation. The non-canonical mechanisms include prominent cleavages C-terminal
to Pro and Asn, and N-terminal to Ile, Leu, and Ser; these cleavages, along with internal fragments, led to a 45% increase
in sequence coverage, improving the specificity of top-down protein identification. Three applications take advantage of the
different mechanisms of protein fragmentation. First, modulation of declustering potential selectively fragments different
charge states, allowing the source region to be used as the first stage of a low-resolution tandem mass spectrometer, facilitating
pseudo-MS3 of product ions with known parent charge states. Second, development and integration of automated modulation of
ion funnel declustering potential allows users access to a particular fragmentation mechanism, yielding facile cleavage on
a liquid chromatography timescale. Third, augmentation of a top-down search engine improved protein characterization. 相似文献
1000.
Garrido Frenich A Romero-González R Martínez Vidal JL Martínez Ocaña R Baquero Feria P 《Analytical and bioanalytical chemistry》2011,399(6):2043-2059
This work compares two miniaturised sample preparation methods, solid phase microextraction (SPME) and hollow fiber liquid
phase microextraction (HF-LPME), in combination with gas chromatography coupled to tandem mass spectrometry with a triple
quadrupole analyzer for the determination of 77 pesticides in drinking water. In the case of SPME, extraction temperature
and time were optimized by experimental design, although other parameters, as desorption time, pH, and ionic strength, were
also evaluated. The extraction and desorption solvents [octanol/dihexyl ether (75:25, v/v) and cyclohexane, respectively],
as well as the extraction and desorption time, ionic strength, and pH, were studied for the HF-LPME procedure. Under the optimal
conditions, recoveries (70.2–113.5% for SPME and 70.0–119.5% for HF-LPME), intra-day precision (2.1–19.4% for SPME and 4.3–22.5%
for HF-LPME), inter-day precision (5.2–21.5% for SPME and 8.4–27.3% for HF-LPME), and limits of detection, between 0.1 and
28.8 ng/L for SPME and 0.2 and 47.1 ng/L for HF-LPME and overall uncertainty (9.6–25.2% for SPME and 13.3–27.5% for HF-LPME)
were established for both extraction procedures. Finally, the proposed methods were successfully applied to the analysis of
41 drinking water samples, and similar results were obtained with both extraction approaches. 相似文献