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51.
The reaction of ring opening of epoxides under the action of a halogenic acid has been investigated using as model the system C2H4O + HF → CH2OHCH2F. The physical conditions chosen for the model more directly correspond to the gas phase reaction and under these conditions the preferred mechanism leads to the formation of a halohydrine having the same configuration at the C atom as the reagent. Parallel investigations have been performed on other mechanisms which postulate the preliminary formation of the conjugate acid of the oxirane (C2H4OH+) and proceed via the well known mechanismsA 1 orA 2. In this case the best mechanism corresponds to the so-called “borderlineA 2” mechanism. This last type of mechanism probably is the dominant one in protic solutions, but by coupling the present calculations with experimental conductometric measurements in anhydrous aprotic media one could consider the first concerted mechanism as a possible candidate also for the reaction in “inert” media. A qualitative analysis of the transition state indicates, in addition, that the propension for the retention path, is probably emphasized by the use of HF as reactant, and that with other acids, like HC1, or even by assuming the presence of dimers like HF·HF, the inversion path could be preferred. The investigations have been done by determining the geometry of the transition state and the reaction coordinate withab initio SCF STO-3G calculations on the whole nuclear configuration space (21 dimensions). These calculations have been supplemented by a few CI calculations on the same basis set and by a few SCF calculations with a larger basis set. 相似文献
52.
A new implementation of analytical gradients for the polarizable continuum model is presented, which allows Hartree-Fock and density functional calculations taking into account both electrostatic and nonelectrostatic contributions to energies and gradients for closed and open shell systems. Simplified procedures neglecting the derivatives of the cavity surface and/or using single spheres for XHn groups have also been implemented and tested. The solvent-induced geometry relaxation has been studied for a number of representative systems in order to test the efficiency of the procedure and to investigate the role of different contributions. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 404–417, 1998 相似文献
53.
54.
Acta Mechanica Sinica - Nature and technology often adopt structures that can be described as tubular helical assemblies. However, the role and mechanisms of these structures remain elusive. In... 相似文献
55.
We review in these notes the theory of equivariant embeddings of spherical homogeneous spaces. Given a spherical homogeneous space G/H, the normal equivariant embeddings of G/H are classified by combinatorial objects called colored fans, which generalize the fans appearing in the classification of toric varieties and which encode several geometric properties of the corresponding variety. 相似文献
56.
An improved iterative solution to solve the electrostatic problem in the polarizable continuum model
Christian Silvio Pomelli Jacopo Tomasi Vincenzo Barone 《Theoretical chemistry accounts》2001,105(6):446-451
We present a discrete iterative interpolation scheme (DIIS) to improve the convergence rate of electrostatic calculations
in the polarizable continuum model (PCM) to describe solvent effects on molecular solutes. The electrostatic calculations
may easily become the bottleneck of the calculation when the solute size is large. For large molecules iterative procedures
turn out to be computationally more convenient than matrix inversion or closure methods. The DIIS scheme is compared here
to another iterative procedure (DAMP) and to the biconjugate gradient (BCG) method. The comparisons show that DIIS leads to
a sizeable saving of computational time for the C-PCM and IEF-PCM methods (average 40%) compared to DAMP, and more than 50%
with respect to the BCG method.
Received: 5 October 2000 / Accepted: 13 November 2000 / Published online: 19 January 2001 相似文献
57.
Rosanna Bonaccorsi Roberto Cammi Jacopo Tomasi 《International journal of quantum chemistry》1986,29(3):373-378
Counterpoise corrections for the basis set superposition error to the components of the bimolecular interaction energy are defined for three methods of decomposition. The results for the case of the NH3 + BH3 interaction are presented and discussed. 相似文献
58.
We report on the observation of nonlocalized modes or necklace states of light waves in disordered systems in the Anderson localized regime. The samples consist of positional-disordered binary multilayer systems. Anderson localized modes manifest themselves as narrow high-transmission peaks in the transmission spectrum, whereas the average of the logarithm of the transmission coefficient decreases linearly with thickness. Optical necklace states are observed as modes with a characteristic multiresonance time response and relatively fast decay time. 相似文献
59.
Sgrignani Jacopo Novati Beatrice Colombo Giorgio Grazioso Giovanni 《Journal of computer-aided molecular design》2015,29(5):441-450
Journal of Computer-Aided Molecular Design - AmpC β-lactamase is a hydrolytic enzyme conferring resistance to β-lactam antibiotics in multiple Gram-negative bacteria. Therefore,... 相似文献
60.