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101.
Unrestricted density functional theory (UB3LYP), CASSCF, and CASPT2 calculations have been employed to compute the relative energies of the C(s) and C(2v) geometries of several 1,5-disubstituted semibullvalenes. Substitution at these positions with R = F, -CH(2)-, or -O- affords semibullvalenes that are predicted to have C(2v) equilibrium geometries. Calculated singlet-triplet energy splittings and the energies of isodesmic reactions are used to assess the amount of bishomoaromatic character at these geometries. The results of these calculations show that employing strain to destabilize the C(s) geometries of semibullvalenes can lead to a significant decrease in the amount of bishomoaromatic stabilization of the C(2v) geometries, due to reduced through-space interaction between the two allyl groups. However, the C(2v) equilibrium geometries of the 1,5-disubstituted semibullvalenes with R = F and -RR- = -O- do benefit from stabilizing through-bond interactions between the two allyl groups. These interactions involve mixing of the bisallyl HOMO with the low-lying C-F or C-O sigma orbital combinations of the same symmetry. In contrast, for -RR- = -CH(2)-, through-bond interactions destabilize the bisallyl HOMO and are predicted to make the ground state of this semibullvalene a triplet. 相似文献
102.
An amplitude modulation of the electron spin echo envelope has been observed for radiation-produced trapped electrons in 10 M NaOD/D2O at 77°K but not in 10 M N2OH/H2O. The modulation has been simulated theoretically by generalizing the single crystal model of Rowan et al. to disordered systems. The modulation has been interpreted as due to dipolar interactions with deuterons in molecules of the second solvation shell around the trapped electrons. 相似文献
103.
Tandem liquid chromatography-mass spectrometry coupled to online radioactive material detection (LC/RAM/MS/MS) is a technique that is used routinely for in vivo and in vitro drug metabolism studies and allows for a simultaneous correlation between radiochemical peaks and mass spectral data. The compound diethylene glycol monobutyl ether (DGBE), a component of a commercially available scintillation cocktail for RAM analysis, was identified as a source of overwhelming chemical noise in a mass spectrometer which was used in an LC/RAM/MS/MS configuration. In this report, we describe the identification of DGBE as the source of the chemical noise and the methods that were used to minimize the exposure of the mass spectrometer to volatile components of the scintillation cocktail. 相似文献
104.
105.
106.
Thitivat Suebsaeng Charles A. Wilkie Vern T. Burger Janet Carter Charles Eric Brown 《Journal of polymer science. Part A, Polymer chemistry》1984,22(4):945-957
The solid reaction products from pyrolysis of polyethylene terephthalate in the presence and absence of red phosphorus were characterized by CP/MAS 13C-NMR, FR-IR, and MAS 31P-NMR spectroscopy. Over the temperature range of 300–400°C, polyethylene terephthalate was converted in a sealed vial to a highly crosslinked polymer of terephthalic acid. Pyrolysis in the presence of red phosphorus, which functions as a flame retardant by increasing the amount of char, yielded an intractible polyaromatic phosphate ester. After thermal cleavage of polyethylene terephthalate with formation of free carboxyl and vinyl ester groups, there are two competing reaction pathways. The smaller molecular weight fragments may enter the vapor phase where they undergo further degradation primarily to CO2, CO, and acetaldehyde, as described by others. However, if volatilization of the oligomeric fragments is inhibited, an alternate reaction pathway gives rise to the formation of highly crosslinked char. Red phosphorus decreases the volatility of the oligomeric fragments by converting them to phosphates and thereby enhances char formation. 相似文献
107.
The indole alkaloid sitsirikine has been synthesised by a biomimetic conversion of strictosidine and its C-16 configuration established as R by cyclisation to a 16,17-dihydroheteroyohimbine. 相似文献
108.
Solid-state synthesis of a conducting polythiophene via an unprecedented heterocyclic coupling reaction 总被引:2,自引:0,他引:2
Meng H Perepichka DF Bendikov M Wudl F Pan GZ Yu W Dong W Brown S 《Journal of the American Chemical Society》2003,125(49):15151-15162
Prolonged storage ( approximately 2 years) or gentle heating (50-80 degrees C) of crystalline 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) affords a highly conducting, bromine-doped poly(3,4-ethylenedioxythiophene) (PEDOT), as confirmed by solid-state NMR, FTIR, CV, and vis-NIR spectroscopies. The novel solid-state polymerization (SSP) does not occur for 2,5-dichloro-3,4-ethylenedioxythiophene (DCEDOT), and requires a much higher temperature (>130 degrees C) for 2,5-diiodo-3,4-ethylenedioxythiophene (DIEDOT). X-ray structural analysis of the above dihalothiophenes reveals short Hal.Hal distances between adjacent molecules in DBEDOT and DIEDOT, but not in DCEDOT. The polymerization may also occur in the melt but is significantly slower and leads to poorly conductive material. Detailed studies of the reaction were performed using ESR, DSC, microscopy, and gravimetric analyses. SSP starts on crystal defect sites; it is exothermic by 14 kcal/mol and requires activation energy of approximately 26 kcal/mol (for DBEDOT). The temperature dependence of the conductivity of SSP-PEDOT (sigma(rt) = 20-80 S/cm) reveals a slight thermal activation. It can be further increased by a factor of 2 by doping with iodine. Using this approach, thin films of PEDOT with conductivity as high as 20 S/cm were fabricated on insulating flexible plastic surfaces. 相似文献
109.
The halocyclization reaction of 4-penten-1-ol mediated by various bis(2-substituted pyridine) and (2,6- disubstituted pyridine)bromonium triflates (P(2)Br(+)OTf(-)) was investigated to determine the influence of the substituents on the mechanism of reaction. In all cases, the reaction proceeds via a two-step process where the starting P(2)Br(+) reversibly dissociates to a reactive monosubstituted PBr(+), which then is captured by 4-penten-1-ol to form halocyclized product (2-bromomethyltetrahydrofuran). The dissociation rate constant of P(2)Br(+) (k(d)) is sensitive to the steric bulk at the 2- and 6-positions, and in the case of the 2, 6-dicyclohexylpyridine or 2,6-dicyclopentylpyridine, the P(2)Br(+) species are too unstable to isolate. The partitioning ratio of the reactive intermediate (PBr(+)) between reversal and product formation (k(-)(d)/k(2)) is not particularly sensitive to the nature of the pyridine, the limiting values being 3-7 except in the case of bis(2(-)-menthylpyridine)bromonium triflate where the k(-)(d)/k(2) ratio is approximately 80. The reaction of 4-penten-1-ol and its OD isotopomer with bis(lutidine)bromonium triflate was investigated to determine the deuterium kinetic isotope effect (dkie) on the bromocyclization reaction. The (k(-)(d)/k(2))(H/D) ratio is 1.0, indicating that the rate-limiting step for the bromocyclization is probably formation of a PBr(+)-4-penten-1-ol complex which does not involve substantial changes in the bonding of the OH. The cyclization of 4-penten-1-ol and 4-pentenoic acid mediated by bis(2(-)-menthylpyridine)bromonium triflate produces an enantiomeric excess in the cyclized products of only 2.4% and 4.8% respectively. 相似文献
110.
Martyn A. Brown Bruce R. McGarvey Andrzej Ozarowski Dennis G. Tuck 《Journal of organometallic chemistry》1998,550(1-2)
The primary process in the reaction of hexaphenylditin with various substituted orthoquinones (Q) is shown to involve attack by the quinone at a phenyl ligand. The intermediate thus formed decomposes to yield Ph3Sn(SQ·), where S(Q·−) is the corresponding semiquinonate. Rearrangement of these species in solution gives rise to biradicals, while intramolecular electron transfer may lead to the formation and precipitation of Ph2Sn(CAT), where CAT2− is the corresponding substituted catecholate. The identification of these processes depends in part on electron paramagnetic resonance spectroscopy. The reaction of Ph3SnCl or Ph2SnCl2 with Na(TBSQ·) (TBSQ·−=3,5-di-tert-butyl-orthobenzosemiquinonate) results in the formation of Ph2Sn(TBSQ·), which can undergo redistribution and intramolecular electron transfer, so that the solution chemistry of these latter systems is similar to that of the products of the Sn2Ph6+Q reaction. 相似文献