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991.
Alain Sevin Daniel Masure Claude Giessner-Prettre Michel Pfau 《Helvetica chimica acta》1990,73(3):552-573
A theoretical study of the enantioselective Michael-type addition of chiral secondary enamines to enones has been achieved. In a first step, the structures of various free enamines have been investigated at the ab initio and MNDO levels. The results clearly show that upon substitution of the prototype vinylamine, the N-center is pyramidalized. The study of enamines with chiral N-substituents such as (S)-Ph(Me)CH or (S)-cyclohexyl(Me)CH reveals a very complex pattern, where up to 8 local energy minimums are characterized whose examination shows that no prediction can be. done regarding the final enantioselectivity of their reaction with enones. These sets of conformers can be regarded as nearly energetically degenerate, at least for the three or four ones of lowest energy. The study of the compact complexes formed between the latter optimal conformers and acrylaldehyde shows that: (i)syn complexation with respect to the N lone pair is the only one which remains possible for steric reason, (ii) small geometrical rearrangements take place as the complexation proceeds, (iii) no clear-cut correspondence exists between the relative sequence of the low-energy conformers of the free enamines and the sequence of the low-energy complexes, (iv) the examination of the relative gradients of the complexation energies provides an index for predicting the relative facilities of the enantiometric pathways, in good agreement with the experimental facts. Our study emphasizes the great complexity of systems of realistic size and brings about critical conclusions regarding classical ad hoc models. 相似文献
992.
Infrared spectra of some compounds containing weak and medium-strong NH..S hydrogen bonds have been investigated. Spectroscopic and structural data have been correlated and compared with those obtained for NH..O and NH..N systems. At the same strength, the N..S distance is 0.51 and 0.36 Å longer than the corresponding N..O and N..N distances, respectively. 相似文献
993.
We prove a diffusion law for a disordered Lorentz gas obtained by modification of a model of Gates, Gerst, Kac in Ref. 1, even though the motion is not a Markovian one in the technical sense of the word. 相似文献
994.
Maryvonne Lorient Régnault Von der Mühll Alain Tressaud Jean Ravez 《Solid State Communications》1981,40(9):847-852
Dielectric and thermocurrent measurements have been carried out on (NH4) 3AlF6 and (NH4) 3FeF6 ceramic samples. A maximum of permittivity is observed close to the transition temperature (TT(NH4) 3AlF6 = 217K; TT(NH43FeF6 = 264K. In the low-temperature phase a polarization current of about 10-9A is obtained and can be reversed when the sign of the polarization field is changed, a property which could correspond to a ferroelectric behavior. However, no pyroelectric current is detected when the temperature decreases from TT. Another hypothesis, based on a field-induced polarization, has been considered : the depolarization current could be due to charge displacements from potential minima favored by rising temperature. In any way, the low-temperature phase is characterized by a remanent polarization. 相似文献
995.
996.
Alain Battais Bernard Boutevin Louis Cot William Granier Yves Pietrasanta 《Journal of fluorine chemistry》1979,13(6):531-550
Action of potassium fluoride on chlorofluorinated telomers of general formula Cl-(CFCl-CF2)n-CCl3 leads to a mixture of perfluorinated cis isomers of internal olefins with the more substitued isomer being preponderant. The influence of temperature and solvant has also been studied. Only reactions carried out in tetramethylene sulfone or preferably in dimethylsulfoxide lead to perfluorinated alkenes. We suggest a mechanism with elimination followed by fluorination. 相似文献
997.
Rosalinda Contreras Douraid Houalla Alain Klaébé Robert Wolf 《Tetrahedron letters》1981,22(40):3953-3954
By reaction of Borane Dimethyl Sulfide on the bicyclophosphorane the adduct is obtained with 56 % yield; by-products and are characterized . 相似文献
998.
Alain Soum Michel Fontanille Pierre Sigwalt 《Journal of polymer science. Part A, Polymer chemistry》1977,15(3):659-673
Anionic polymerization of 2-vinylpyridine has been studied, initiated either by dialkyl and diaryl magnesium or by living oligomers in tetrahydrofuran solution at 20°C. Spectrophotometric observations indicate the existence of stable active species with different structures. Kinetic data for propagation and viscosimetric measurements on two-ended polymers agree with a cyclic structure of living polymers and lead us to the conclusion of the existence of reactive negative “tripleions.” 相似文献
999.
Jacques Dagaut Pierre Dizabo Alain Pompon Joelle Fillaux 《Journal of mass spectrometry : JMS》1975,10(10):905-912
Three fragmentations (loss of phenyl, hydrogen and a RCO radical) of benzocyclobutene derivatives of differently substituted 1,4-benzodioxin were studied. In some cases these molecules, deuteriated in a specific position, revealed the existence of several isomers of the molecular ion. 相似文献
1000.