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61.
Liu CT Maxwell CI Edwards DR Neverov AA Mosey NJ Brown RS 《Journal of the American Chemical Society》2010,132(46):16599-16609
The methanolytic cleavage of a series of O,O-dimethyl O-aryl phosphorothioates (1a?g) catalyzed by a C,N-palladacycle, (2-[N,N-dimethylamino(methyl)phenyl]-C1,N)(pyridine) palladium(II) triflate (3), at 25 °C and sspH 11.7 in methanol is reported, along with data for the methanolytic cleavage of 1a?g. The methoxide reaction gives a linear log k2?OMe vs sspKa (phenol leaving group) Br?nsted plot having a gradient of βlg = ?0.47 ± 0.03, suggesting about 34% cleavage of the P?OAr bond in the transition state. On the other hand, the 3-catalyzed cleavage of 1 gives a Br?nsted plot with a downward break at sspKa (phenol) 13, signifying a change in the rate-limiting step in the catalyzed reaction, with the two wings having βlg values of 0.0 ± 0.03 and ?1.93 ± 0.06. The rate-limiting step for good substrates with low leaving group sspKa values is proposed to be substrate/pyridine exchange on the palladacycle, while for substrates with poor leaving groups, the rate-limiting step is a chemical one with extensive cleavage of the P?OAr bond. DFT calculations support this process and also identify two intermediates, namely, one where substrate/pyridine interchange has occurred to give the palladacycle coordinated to substrate through the S═P linkage and to methoxide (6) and another where intramolecular methoxide attack has occurred on the P═S unit to give a five-coordinate phosphorane (7) doubly coordinated to Pd via the S? and through a bridging methoxide linked to P and Pd. Attempts to identify the existence of the phosphorane by 31P NMR in a d4-methanol solution containing 10 mM each of 3, trimethyl phosphorothioate (a very slow cleaving substrate), and methoxide proved unsuccessful, instead showing that the phosphorothioate was slowly converted to trimethyl phosphate, with the palladacycle decomposing to Pd0 and free pyridine. These results provide the first reported example where a palladacycle-promoted solvolysis reaction exhibits a break in the Br?nsted plot signifying at least one intermediate, while the DFT calculations provide further insight into a more complex mechanism involving two intermediates. 相似文献
62.
David R. Edwards Christopher I. Maxwell Robert W. Harkness Alexei A. Neverov Nicholas J. Mosey R. Stan Brown 《Journal of Physical Organic Chemistry》2012,25(3):258-266
An experimental determination of the βEq value for equilibrium transfer of the O,O‐dimethyl phosphorothioyl group between oxyanion nucleophiles in water and methanol at 25 °C is presented. The respective βEq values in the two solvents are experimentally the same at ?1.45 ± 0.08 and ?1.39 ± 0.12. Based on the observation that the Brønsted correlation for the nucleophilic reaction of phenoxides in water with substrate 1d (dimethyl 4‐nitrophenyl phosphorothioate, pKaHOAr of 7.14) is linear over the entire range of phenoxides employed (5.53 ≤ pKaNu ≤ 12.38), the reaction for phenoxide nucleophiles displacing phenoxide leaving groups is probably concerted. The obtained data allow one to calculate, for a symmetrical transition state involving 2,4,5‐trichlorophenoxide as a nucleophile and leaving group, an approximately 60% P–OAr cleavage and about 40% P–Nuc bond formation. A computational method is presented for the rapid prediction of the βEq values for such processes in water and methanol, and the results are compared with known values from the literature. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
63.
Titanium diselenide TiSe2 is doped for the first time with an acceptor impurity through substitution of Cr3+ ions for Ti4+ ions in the host lattice. The structure and electrical resistivity of the TiSe2-based solid solutions are investigated in the vicinity of the compensation point, and the influence of the compensation point on the stability of the charge-density-wave state is analyzed. It is demonstrated that disorder in the guest sublattice does not lead to Anderson localization of charge carriers. 相似文献
64.
It is shown how one can transform scalar first-order ordinarydifferential equations which admit non-local symmetries of theexponential type to integrable equations admitting canonicalexponential non-local symmetries. As examples we invoke theAbel equation of the second kind, the Riccati equation and naturalgeneralizations of these. Moreover, our method describes howa double reduction of order for a second-order ordinary differentialequation which admits a two-dimensional Lie algebra of generatorsof point symmetries can be affected if the second-order equationis first reduced in order once by a symmetry which does notspan an ideal of the two-dimensional Lie algebra. 相似文献
65.
66.
67.
Liu CT Maxwell CI Pipe SG Neverov AA Mosey NJ Brown RS 《Journal of the American Chemical Society》2011,133(50):20068-20071
Palladacycle 1 catalyzes the methanolytic cleavage of N-methyl-N-(4-nitrophenyl)thiobenzamide (4) via a mechanism involving formation of a Pd-bound tetrahedral intermediate (TI). The rate constant for decomposition of the complex formed between 1, methoxide, and 4 is 9.3 s(-1) at 25 °C; this reaction produces methyl thiobenzoate and N-methyl-4-nitroaniline. The ratio of the second-order rate constant for the catalyzed reaction, given as k(cat)/K(d), relative to that of the methoxide-promoted reaction is 3 × 10(8), representing a very large catalysis of thioamide bond cleavage by a synthetic metal complex. 相似文献
68.
S. V. Neverov V. L. Sigacheva V. A. Petrosyan 《Russian Journal of Electrochemistry》2011,47(10):1134-1138
The potentiostatic (galvanostatic) electrolysis of methyl esters of trichloroacetic acid on mercury (steel) cathodes in the
cathodic compartment of a diaphragm cell in the presence of acrylonitrile affords a mixture of two stereoisomers of methyl
ester of 1-chloro-2-cyanocyclopropanecarboxylic acid. The process involves a stage of dehalogenation of the original methyl
ester followed by the reaction of the formed anion with acrylonitrile and proceeds more efficiently on a mercury cathode in
the dimethylformamide medium with the current efficiency of 94% and the substance yield of 25%. In contrast to methyl trichloroacetate,
the electrolysis of methyl dichloroacetate proceeds by an indirect mechanism involving its deprotonation on the steel cathode
and by the cleavage of the C-Cl bond on the mercury cathode. In both cases, the acrylonitrile addition leads to formation
of the corresponding cyclopropane structures. Moreover, the process is the most efficient on the steel cathode in the MeCN
medium. In the latter case, the current efficiency of methyl ester of 1-chloro-2-cyanocyclopropanecarboxylic acid is 31% and
substance yield is 41%. Peculiarities of these processes are discussed. 相似文献
69.
70.
Piezoelectricity is usually expressed as an interaction between mechanical and electrical variables. The physics involved is hence governed by a coupling between Maxwell's equations of electromagnetism and the equations of elasticity. Such a coupling takes us through the piezoelectric constitutive relations. In this work, the second order anisotropic constitutive equations are treated, and in particular, the number of independent material constants is computed for all the 32 crystallographic classes.Paper presented, in reduced version, at the 12th Italian National Congress of Theoretical and Applied Mechanics (AIMETA '95), October 1995, Naples, Italy. 相似文献