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61.
Systems incorporating catalyst–substrate non-covalent interactions are emerging as a versatile approach to address site-selectivity challenges in remote functionalization reactions. Given the achievements that have been made in this regard using metals such as iridium, manganese and rhodium, it is surprising that non-covalent catalyst direction has not been utilized in reactions incorporating palladium-catalyzed C–H activation steps, despite palladium being arguably the most versatile metal for C–H activation. Herein, we demonstrate that electrostatically directed, site-selective C–Cl oxidative addition is compatible with a subsequent C–H activation step, proceeding via a concerted metalation deprotonation-type mechanism. This results in site-selective cross-coupling of dichloroarenes with fluoroarenes and fluoroheteroarenes, with selectivity controlled by catalyst structure. This study demonstrates that Pd-catalyzed C–H activation can be used productively in combination with a non-covalently-directed mode of catalysis, with important implications in both fields.Electrostatically-directed oxidative addition is compatible with a subsequent C–H activation step, enabling site-selective coupling of remote chlorides with fluoroarenes and fluoroheteroarenes. 相似文献
62.
An Anglo-German-Japanese bio-organic meeting was held in Fukuoka, Japan, from 10th to 14th January, 1999, to repair the lack of sufficient scientific contact between Europe and Japan in the field of bio-organic chemistry. Participants judge the meeting as “highly successful, both from a scientific and a social standpoint” and intend to deepen the collaboration. 相似文献
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Yasuji Yamada D. Miljkovic P. Wehrli B. Golding P. Lliger R. Keese K. Müller A. Eschenmoser 《Angewandte Chemie (International ed. in English)》1969,8(5):343-348
Work towards a synthesis of vitamin B12 has inspired a new type of corrin synthesis. The key step is a light-induced 1,16-hydrogen transfer leading to an antarafacial (π→σ)-cycloisomerization of a seco-corrinoid metal complex. The construction of the seco-corrinoid ligand system is achieved by coupling monocyclic ring precursors in their enamide or enamine form through the methods of sulfide-contraction via oxidative coupling and of iminoester-enamine condensation. 相似文献
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A decrease in the group velocity of 0.5 GHz phonons at 0.023 K has been observed in fused SiO2. This is attributed to pulse reshaping associated with the saturation of two-level tunneling states in the glass. 相似文献
69.
Dykman MI Golding B McCann LI Smelyanskiy VN Luchinsky DG Mannella R McClintock PV 《Chaos (Woodbury, N.Y.)》2001,11(3):587-594
We discuss activated escape from a metastable state of a system driven by a time-periodic force. We show that the escape probabilities can be changed very strongly even by a comparatively weak force. In a broad parameter range, the activation energy of escape depends linearly on the force amplitude. This dependence is described by the logarithmic susceptibility, which is analyzed theoretically and through analog and digital simulations. A closed-form explicit expression for the escape rate of an overdamped Brownian particle is presented and shown to be in quantitative agreement with the simulations. We also describe experiments on a Brownian particle optically trapped in a double-well potential. A suitable periodic modulation of the optical intensity breaks the spatio-temporal symmetry of an otherwise spatially symmetric system. This has allowed us to localize a particle in one of the symmetric wells. (c) 2001 American Institute of Physics. 相似文献
70.
Ab initio molecular orbital theory is used to investigate the coenzyme B(12)-dependent reactions catalyzed by diol dehydratase. The key step in such reactions is believed to be a 1,2-hydroxyl migration, which occurs within free-radical intermediates. The barrier for this migration, if unassisted, is calculated to be too high to be consistent with the observed reaction rate. However, we find that "pushing" the migrating hydroxyl, through interaction with a suitable acid, is able to provide significant catalysis. This is denoted retro-push catalysis, the retro prefix signifying that the motion of the migrating group is in the direction opposite to the electron motion. Similarly, the "pulling" of the migrating group, through interaction of the spectator hydroxyl with an appropriate base, is found to substantially reduce the rearrangement barrier. Importantly, the combination of these two effects results in a barrier reduction that is notably greater than additive. This synergistic interplay of the push and the pull provides an attractive means of catalysis. Our proposed retro-push--pull mechanism leads to results that are consistent with isotope-labeling experiments, with experimental rate data, and with the crystal structure of the enzyme. 相似文献