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排序方式: 共有152条查询结果,搜索用时 15 毫秒
1.
Vishal Sipani Ann Kirsch Alec B. Scranton 《Journal of polymer science. Part A, Polymer chemistry》2004,42(17):4409-4416
The effective propagation rate constant (kp; averaged over all the propagating active centers) was characterized for solvent‐free cationic photopolymerizations of phenyl glycidyl ether over the entire range of conversions, including the high conversion regime in which mass transfer limitations become important. The profile for the kp as a function of conversion was found to exhibit a constant plateau value at low to intermediate conversions, followed by a monotonic increase above a threshold value of conversion. To explain this trend, it is proposed that at high conversion the diffusional mobility of the photoinitiator counterion is reduced whereas the mobility of the cationic active center remains high because of reactive diffusion. Therefore, with increasing conversion, the average distance between the active centers and counterions may increase, resulting in an increase in the propagation rate constant. The profiles for the kp values were investigated as a function of the temperature, photoinitiator anion, and photoinitiator concentration. As the photoinitiator concentration was increased, the plateau value of the effective propagation rate constant decreased whereas the threshold conversion increased. All of the experimental trends are consistent with the proposed increase in ion separation at high conversions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4409–4416, 2004 相似文献
2.
Alec B. Scranton John Klier Nikolaos A. Peppas 《Journal of Polymer Science.Polymer Physics》1991,29(2):211-224
The dilute solution complexation equilibrium between linear macromolecules and smaller complementary oligomers is considered when: (1) the oligomers are free in solution; and (2) the oligomers are covalently attached at one end to the polymer. A general statistical mechanical framework is developed and is illustrated using a simple random walk model for polymer conformation. The statistical mechanical partition functions are formulated using a generating function technique, allowing thermodynamic averages in the complexed state to be calculated. Loops, trains, and tails of all possible length are allowed in the conformation of a complexed oligomer. Simulation results for the free oligomer case are compared with those obtained for oligomers covalently attached to the polymeric molecular. The model provides a theoretical explanation for the experimentally observed enhancement of complexation of oligomers grafted to the complementary polymers. 相似文献
3.
Edmond Amouyal Bertrand Zidler Patrick Keller Alec Moradpour 《Chemical physics letters》1980,74(2):314-317
Rate constants kq for the quenching of the excited state of Ru(bipy)32+ by a series of viologen salts having different redox potential E have been determined in deaerated aqueous solutions at pH = 5 by laser flash photolysis. The kq values are found to decrease with increasing —E and to correlate with the reaction free-energy change ΔG. Such a correlation is shown to be consistent with the Rehm—Weller model for electron-transfer reactions. 相似文献
4.
Alec Norton 《Journal of Geometric Analysis》1994,4(3):403-424
The classical Morse-Sard Theorem says that the set of critical values off:R n+k →R n has Lebesgue measure zero iff ∈C k+1. We show theC k+1 smoothness requirement can be weakened toC k+Zygmund. This is corollary to the following theorem: For integersn >m >r > 0, lets = (n ?r)/(m ?r); iff:R n →R m belongs to the Lipschitz class Λ s andE is a set of rankr forf, thenf(E) has measure zero. 相似文献
5.
Alec B. Scranton Nikolaos A. Peppas 《Journal of polymer science. Part A, Polymer chemistry》1990,28(1):39-57
A statistical model for network formation by the free-radical copolymerization and crosslinking reaction with small crosslinker content is used to obtain expressions for structural averages as a function of reaction parameters. The analysis accounts for reaction features that are characteristic of free-radical mechanisms and can be adapted to include cyclization, various modes of termination, and the gel effect. Profiles for structural averages such as the weight average molecular weight, the sol weight fraction, the molecular weight between crosslinks, and the number of elastically active network chains are obtained as functions of conversion. 相似文献
6.
Complex dynamics in equilibrium asset pricing models with boundedly rational,heterogeneous agents 下载免费PDF全文
We study a simple model based upon the Lucas framework where heterogeneous agents behave rationally in a fully intertemporal setting but do not know other investors' personal preferences, wealth or investment portfolios. As a consequence, agents initially do not know the equilibrium asset pricing function and must make guesses, which they update via adaptive learning with constant gain. We demonstrate that even in this simple environment the economy can, depending on parameters, exhibit either stable convergence to equilibrium, or chaotic dynamical behavior of asset prices and trading volume without converging to the rational expectations equilibrium of the Lucas model. This contradicts the assertion that the Lucas model is stable in the face of modest deviations from the strong assumptions required to compute the equilibrium. © 2013 Wiley Periodicals, Inc. Complexity 19: 38–55, 2014 相似文献
7.
Guillaume Pisella Alec Gagnebin Prof. Dr. Jérôme Waser 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(45):10199-10204
Multicomponent reactions provide efficient means to access molecular complexity. Herein, we report a copper-catalyzed three-component reaction of diazo compounds, alcohols and ethynyl benziodoxole (EBX) reagents for the synthesis of propargyl ethers. Extensive variations of the three partners of the reaction is possible, leading to highly functionalized and structurally diverse products under mild conditions. Alkynylation of a copper ylide intermediate is postulated as key step for this transformation. 相似文献
8.
T. Parker Maloney Alexander F. Dohoda Alec C. Zhu Forrest E. Michael 《Chemical science》2022,13(7):2121
Herein we report an intermolecular propargylic C–H amination of alkynes. This reaction is operationally convenient and requires no transition metal catalysts or additives. Terminal, silyl, and internal alkynes bearing a wide range of functional groups can be aminated in high yields. The regioselectivity of amination for unsymmetrical internal alkynes is strongly influenced by substitution pattern (tertiary > secondary > primary) and by relatively remote heteroatomic substituents. We demonstrate that amination of alkynes bearing α-stereocenters occurs with retention of configuration at the newly-formed C–N bond. Competition experiments between alkynes, kinetic isotope effects, and DFT calculations are performed to confirm the mechanistic hypothesis that initial ene reaction of a selenium bis(imide) species is the rate- and product-determining step. This ene reaction has a transition state that results in substantial partial positive charge development at the carbon atom closer to the amination position. Inductive and/or hyperconjugative stabilization or destabilization of this positive charge explains the observed regioselectivities.Selenium catalysis enables a general intermolecular propargylic C–H amination of alkynes. The concerted mechanism gives rise to high regioselectivity for the more electron-rich end of the alkyne and retention of the C–H propargylic stereocenter. 相似文献
9.
Florence Geneste Alec Moradpour Georges Dive Daniel Peeters Jacques Malthête Jean-Fran?ois Sadoc 《The Journal of organic chemistry》2002,67(2):605-607
The total number of possible retrosynthetic bisections of C(60) leads to nine different isometric C(30) fragments. These molecules include five chiral units, four of which derive from partitions corresponding to four distinct "Coupes du Roi". The energies, curvatures, and homodesmotic stabilization energies of the C(30) fragments are evaluated at the ab initio 6-31G level. 相似文献
10.