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991.
Marcos IS Conde A Moro RF Basabe P Díez D Mollinedo F Urones JG 《Molecules (Basel, Switzerland)》2008,13(5):1120-1134
An efficient synthesis of ent-halimanolide 2 (15,16-epoxy-12-oxo-ent-halima- 5(10),13(16),14-trien-18,2beta-olide), from ent-halimic acid has been achieved, corroborating the structure of the natural compound and establishing its absolute configuration. 相似文献
992.
Michaux C Wouters J Perpète EA Jacquemin D 《The journal of physical chemistry. B》2008,112(8):2430-2438
The incremental hydration of the glycine cation is investigated using an ab initio approach fully correcting for basis set superposition errors and explicitly incorporating electron-correlation effects. Structures with zero to four surrounding water molecules have been determined. It is demonstrated that the successive aggregates follow a Darwinian family tree, the most stable complexes systematically belonging to the same branch of the tree. In strong contrast with neutral glycine, the direct hydrogen bonding to the glycine cation is favored over bridging water structures. The agreement between experimental and theoretical hydration enthalpies and Gibbs free energies is impressive, as ab initio estimates almost systematically fit the experimental error bars. For GlyH(+)-(H2O) and GlyH(+)-(H2O)3, we show that two structures are generated by the experimental setup. The present approach also resolves most of the previous theory/experiment discrepancies and provides patterns for the evolution of the vibrational spectra: a decrease of the hydrogen-bond stretching frequency indicating second-shell water molecules. Additionally, the impact of bulk solvent solvation is investigated, as four discrete water molecules still do not fully hydrate the protonated glycine. 相似文献
993.
A novel domino reaction--stereoselective Ireland-Claisen rearrangement and asymmetric Michael addition--is described. A protocol starting from Baylis-Hillman adducts 3a-f using chiral lithium amide affords optically active gamma-substituted delta-amino acids 4a-f with high diastereoselectivities and enantioselectivities. The acid can be isolated easily from large-scale reactions and transformed to 2,3-disubstituted piperidines 11 or 2-substituted nipecotic acid derivates 12. 相似文献
994.
A CZE model is presented for peptide characterization on the basis of well-established physicochemical equations. The effective mobility is used as basic data in the model to estimate relevant peptide properties such as, for instance, hydration, net and total electrical charge numbers, hydrodynamic size and shape, particle average orientation, and pH-microenvironment from the charge regulation phenomenon. Therefore 102 experimental effective mobilities of different peptides are studied and discussed in relation to previous work. An equation for the estimation of peptide hydration as a function of ionizing, polar, and non-polar amino acid residues is included in the model. It is also shown that the shape-orientation factor of peptides may be either lower or higher than one, and its value depends on a complex interplay among total charge number, molar mass, hydration, and amino acid sequence. 相似文献
995.
Jan Janczak Genivaldo Júlio Perptuo 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(2):o91-o94
Crystals of the title compound, 2C3H7N6+·C10H6O6S22−·C3H6N6·5H2O, are built up of neutral 2,4,6‐triamino‐1,3,5‐triazine (melamine), singly protonated melaminium cations, naphthalene‐1,5‐disulfonate dianions and water molecules. Two independent anions lie across centres of inversion in the space group P. The melamine molecules are connected by N—H...N hydrogen bonds into two different one‐dimensional polymers almost parallel to the (010) plane, forming a stacking structure along the b axis. The centrosymmetric naphthalene‐1,5‐disulfonate anions interact with water molecules via O—H...O hydrogen bonds, forming layers parallel to the (001) plane. The cations and anions are connected by N—H...O and O—H...N hydrogen bonds to form a three‐dimensional supramolecular framework. 相似文献
996.
Djairo G. de Figueiredo Ireneo Peral Julio D. Rossi 《Annali di Matematica Pura ed Applicata》2008,187(3):531-545
In this paper we look for existence results for nontrivial solutions to the system,
with Dirichlet boundary conditions, u = v = 0 on and α, β < N. We find the existence of a critical hyperbola in the (p,q) plane (depending on α, β and N) below which there exists nontrivial solutions. For the proof we use a variational argument (a linking theorem).
I. Peral was partially supported by project MTM2004-02223 of MEC, Spain. J. D. Rossi partially supported by Universidad de
Buenos Aires (grant TX066), ANPCyT and Fundacion Antorchas. 相似文献
997.
Mayte Pé rez-Llanos Julio D. Rossi 《Proceedings of the American Mathematical Society》2008,136(2):593-596
We provide examples of solutions to parabolic problems with nontrivial blow-up sets of dimension strictly smaller than the space dimension. To this end we just consider different diffusion operators in different variables, for example, or . For both equations, we prove that there exists a solution that blows up in the segment .
998.
The Neumann problem for nonlocal nonlinear diffusion equations 总被引:1,自引:0,他引:1
Fuensanta Andreu José M. Mazón Julio D. Rossi Julián Toledo 《Journal of Evolution Equations》2008,8(1):189-215
We study nonlocal diffusion models of the form
Here Ω is a bounded smooth domain andγ is a maximal monotone graph in . This is a nonlocal diffusion problem analogous with the usual Laplacian with Neumann boundary conditions. We prove existence
and uniqueness of solutions with initial conditions in L
1 (Ω). Moreover, when γ is a continuous function we find the asymptotic behaviour of the solutions, they converge as t → ∞ to the mean value of the initial condition.
Dedicated to I. Peral on the Occasion of His 60th Birthday 相似文献
999.
We study the asymptotic behavior for solutions to nonlocal diffusion models of the form u
t
= J * u – u in the whole with an initial condition u(x, 0) = u
0(x). Under suitable hypotheses on J (involving its Fourier transform) and u
0, it is proved an expansion of the form
, where K
t
is the regular part of the fundamental solution and the exponent A depends on J, q, k and the dimension d.
Moreover, we can obtain bounds for the difference between the terms in this expansion and the corresponding ones for the expansion
of the evolution given by fractional powers of the Laplacian, .
相似文献
1000.
Carballido-Gamio J Xu D Newitt D Han ET Vigneron DB Majumdar S 《Magnetic resonance imaging》2007,25(5):665-670
Diffusion tensor imaging (DTI) of the lumbar spine could improve diagnostic specificity. The purpose of this work was to determine the feasibility of and to validate DTI with single-shot fast spin-echo (SSFSE) for lumbar intervertebral discs at 1.5 and 3 T. Six normal volunteers were scanned with DTI-SSFSE using an eight- and a three-b-value protocol at 1.5 and 3 T, respectively. Apparent diffusion coefficient (ADC) values were computed and validated based on those obtained at 1.5 T from corresponding diffusion tensor scans using line scan diffusion imaging (LSDI), a technique that has been previously validated for use in the spine. Pearson correlation coefficients for LSDI and DTI-SSFSE ADC values were .88 and .89 for 1.5 and 3 T, respectively, with good quantitative agreement according to the Bland-Altman method. Results indicate that DTI-SSFSE is a candidate as a clinical sequence for obtaining diffusion tensor images of the lumbar intervertebral discs with scan times shorter than 4 min. 相似文献