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951.
The synthesis of polyolefin graft copolymers made with coordination polymerization was studied by dynamic Monte Carlo simulation. Narrow molecular weight distribution macromonomers, containing terminal vinyl groups made with atom-transfer radical polymerization (ATRP), were incorporated randomly into the polyolefin backbone. In addition to average molecular weights and polydispersity index, the model predicts the complete molecular weight distribution (MWD) and branching density of the graft copolymer. The effect of the concentration of macromonomers on the grafting efficiency was also studied. 相似文献
952.
Lithiated 2,4,4-trimethyl-2-oxazoline 2a and 2-chloromethyl-4,4-dimethyl-2-oxazoline 2b react smoothly with a number of nitrones 3 to produce α,β-unsaturated oxazolines 6 and 7 highly stereoselectively. 相似文献
953.
Karl Gademann Bernhard Jaun Dieter Seebach Remo Perozzo Leonardo Scapozza Gerd Folkers 《Helvetica chimica acta》1999,82(1):1-11
Temperature-dependent NMR and CD spectra of methanol solutions of a β-hexapeptide and of a β-heptapeptide at temperatures between 298 and 393 K are reported. They establish the fact that the 314-helical secondary structures of the two β-peptides, 1 and 2 , do not `melt' in the temperature range investigated. This is in sharp contrast to the behavior of the helices of α-peptides and proteins which undergo cooperative unfolding (`denaturing') upon heating. A non-cooperative mechanism is proposed, with a stepwise, rather than an `un-zipping' opening of H-bonded rings (cf. Fig. 6). The experimental results are regarded as evidence that, of the three effects which have been identified as contributing to the stability of β-peptide helices, i.e., H-bonding, hydrophobic interactions, and ethane staggering, the latter one is predominant. 相似文献
954.
Takumi Oishi Leonardo I. Lugo-Fuentes Yichuan Jing J. Oscar C. Jimenez-Halla Joaquín Barroso-Flores Masaaki Nakamoto Yohsuke Yamamoto Nao Tsunoji Rong Shang 《Chemical science》2021,12(47):15603
Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H–OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH−) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.A (bisborane)triarylphosphine was developed to spontaneously generate H2 from water under ambient conditions, revealing an unprecedented multi-centered electron relay mechanism for a metal-free umpolung of proton to hydride. 相似文献
955.
956.
Leonardo Rodriguez Wasdin Muoz Gonzalo Martin Edgar Ocando-Mavarez 《Heteroatom Chemistry》1997,8(1):85-89
The transition-state geometries of retroene elimination reactions of propene from allylamines have been calculated by using the semiempirical AM1 method. The most favored geometry resembles a half chair or a flattened boat. It is also found that the transition states are of polar character and that the negative charge on the N atom decreases in the transition state; thus, the reaction is favored by electron donor substituents on the nitrogen atom, as observed experimentally. © 1997 John Wiley & Sons, Inc. 相似文献
957.
The explanation of the anomeric effect in terms of underlying quantum properties is still controversial almost 70 years after its introduction. Here, we use a method called Relative Energy Gradient (REG), which is able to compute chemical insight with a view to explaining the anomeric effect. REG operates on atomic energy contributions generated by the quantum topological energy decomposition Interacting Quantum Atoms (IQA). Based on the case studies of dimethoxymethane and 2-fluorotetrahydropyran, we show that the anomeric effect is electrostatic in nature rather than governed by hyperconjugation. 相似文献
958.
Numerical Algorithms - A quasi-Toeplitz (QT) matrix is a semi-infinite matrix of the kind $A=T(a)+E$ where $T(a)=(a_{j-i})_{i,j\in \mathbb Z^{+}}$ , $E=(e_{i,j})_{i,j\in \mathbb Z^{+}}$ is compact... 相似文献
959.
Arnone Massimo Bianchi Michele Leonardo Quaranta Anna Grazia Tassinari Gian Luca 《Computational Management Science》2021,18(2):213-237
Computational Management Science - In this paper, after a review of the most common financial strategies and products that insurance companies use to hedge catastrophic risks, we study an option... 相似文献
960.
Salim Leonardo Alfredo Bonotto Daniel Marcos 《Journal of Radioanalytical and Nuclear Chemistry》2019,320(3):587-595
Journal of Radioanalytical and Nuclear Chemistry - Radionuclides in mineral coal and related rocks are of great concern to the human health, among them the radon gas, as it has been considered the... 相似文献