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41.
Limberg C 《Angewandte Chemie (International ed. in English)》2003,42(48):5932-5954
The oxo-functionalization of organic substrates with the aid of metal oxo moieties is of fundamental importance not only in nature but also in academic and industrial research. Nevertheless the corresponding reaction mechanisms remain among the most enigmatic in chemistry and few of them are understood in detail. Recent research efforts have resulted in significantly improved information: in the cases of many oxygenation reactions evidence has been provided for the occurrence radical intermediates, even though the high selectivity observed suggests to a different mechanism. Examples stem from various areas of chemistry and include processes involving molecular metal oxo complexes, gas-phase and matrix-isolated species, metalloenzymes, and solid-state oxide surfaces. This review treats this seemingly wide variety of systems with the aim of providing an overview of common reactivity patterns and principles, as well as open problems. 相似文献
42.
V. M. Tapilin 《Journal of Structural Chemistry》2005,46(1):9-15
Electronic structure calculations have been performed for flat graphite sheets and periodically distorted sheets with different values of deformation angle and hydrogen adsorption. These characteristics do not all behave monotonically with increasing deformation angle. At small deformation angles, the electrons pass from the atoms of terraces to those of edges, while at larger angles the reverse pattern is observed. The position of the Fermi level also varies nonmonotonically. The binding energy of hydrogen depends significantly on the adsorption site, and the difference can be doubled. Based on the ata obtained, it was concluded that a priori statements derived from the “general concepts” on the difference in the physicochemical properties of, for example, terrace and edge atoms on high-index facets are unreliable. 相似文献
43.
The conformational change of the 39-43 residues of the amyloid beta-peptide (Abeta) toward a beta-sheet enriched state promotes self-aggregation of the peptide molecules and constitutes the major peptide component of the amyloid plaques in Alzheimer patients. The crucial question behind the self-aggregation of Abeta is related to the different pathways the peptide may take after cleavage from the amyloid precursor proteins at cellular membranes. This work is aiming at determining the conformation of the Abeta (1-40) adsorbed on hydrophobic Teflon and hydrophilic silica particles, as model sorbent surfaces mimicking the apolar transmembrane environment and the polar, charged membrane surface, respectively. The mechanism by which the Abeta interacts with solid surfaces strongly depends on the hydrophobic/hydrophilic character of the particles. Hydrophobic and electrostatic interactions contribute differently in each case, causing a completely different conformational change of the adsorbed molecules on the two surfaces. When hydrophobic interactions between the peptide and the sorbent prevail, the adsorbed Abeta (1-40) mainly adopts an alpha-helix conformation due to H-bonding in the apolar part of the peptide that is oriented towards the surface. On the other hand, when the peptide adsorbs by electrostatic interactions beta-sheet formation is promoted due to intermolecular association between the apolar parts of the adsorbed peptide. Irrespective of the characteristics of the solid sorbent, crowding the surface results in intermolecular association between adsorbed molecules leading to a strong aggregation tendency of the Abeta (1-40). [Diagram: see text] CD spectra of Abeta (1-40) at pH 7: A) in solution ([Abeta]=0.2 mg.ml(-1)) freshly prepared (line) and after overnight incubation (symbols);B) on Teflon (Gamma=0.5 mg.m(-2)). 相似文献
44.
Two methods are proposed, one for the location of saddle points and one for the calculation of steepest-descent paths on multidimensional surfaces. Both methods are based on a constrained simplex optimization technique that avoids the evaluation of gradients or second derivative matrices. Three chemical reactions of increasing structural complexity are studied within the PRDDO SCF approximation. Predicted properties of reaction hypersurfaces are in good overall agreement with those determined by gradient minimization and gradient following algorithms in connection with various ab initio SCF methods. Computational efforts required by the new procedures are discussed. 相似文献
45.
Vinzenz Bachler Gottfried Olbrich Oskar E. Polansky Yuh-Kang Pan 《Theoretical chemistry accounts》1979,50(4):327-342
A CNDO/2 SCF perturbation theory is presented for interpreting the form of CNDO/2 potential energy surfaces of unimolecular reactions. The analysis is performed by calculating the energy change E arising from a distortion of the molecular geometry along the reaction coordinate. E is decomposed into different perturbational contributions which are appropriate for an interpretation of the perturbation energy E. Moreover, E is resolved into energy parts arising from a single occupied orbital and contributions due to pairwise orbital interactions. In this way one evaluates numerically how the form of the occupied and unoccupied orbitals determines the magnitude of E. If the distortion occurs along a definite symmetry coordinate, group-theoretical arguments can be applied to discuss the magnitude of characteristic components of the perturbation energy. The SCF perturbation theory is used to analyze the isomerization of ethylene, cis-2-butene and cis-2-butenenitrile.This work was partially supported by Nato-Grant No. 1072 相似文献
46.
Gun Wirtanen Timo Alanko Tiina Mattila-Sandholm 《Colloids and surfaces. B, Biointerfaces》1996,5(6):319-326
Biofilm growth of Bacillus subtilis, Pseudomonas fragi, Pediococcus inopinatus and Listeria monocytogenes was studied on stainless steel surfaces at room and low temperatures to evaluate the results of traditional hygiene measures. The results were compared with those of image analysis of stainless steel surfaces in an epifluorescence microscope. Statistical analyses were carried out to determine the variations between the conventional cultivation swab method, the glycocalyx amount obtained using swabbing, and the values of the areas of the biofilm, slime and cells. As a general rule, old biofilms showed total counts at approximately the same levels as the young biofilm. The results showed that temperature affected the results for all strains except B. subtilis. The strains of Pe. inopinatus and Ps. fragi showed increased attachment at 6°C and L. monocytogenes at 25°C. The biofilm slime was more easily detached than the cells. The results indicated that the traditional swab method is not reliable for the measurement of biofilm formation on surfaces. 相似文献
47.
Paul G. Mezey 《Theoretical chemistry accounts》1985,67(1):43-61
The family of all possible reaction mechanisms on a potential surface has an algebraic structure with potential applications in quantum chemical molecular design and synthesis planning.Transformation properties and equivalence relations of reaction paths on potential energy hypersurfaces lead to a topological definition of reaction mechanisms. The family of all fundamental reaction mechanisms on the hypersurface has a group structure,the fundamental group of an appropriately defined topological space. Isomorphism and homomorphism relations between fundamental groups of reaction mechanisms are used to characterize the chemically important topological properties of various subsets of a hypersurface, or those of different excited state hypersurfaces. 相似文献
48.
49.
The photochemical reaction channels of cyclobutanone have been studied at the CASSCF level with a 6‐31G* basis set. Starting from the n‐π* excited‐state (S1) cyclobutanone, the three reactions can take place: decarbonylation (produce CO and cyclopropane or propylene), cycloelimination (produce ketene and ethylene), and ring expansion (produce oxacarbene). Our computation indicates that decarbonylation products CO and triplet trimethylene are formed on the triplet n‐π* excited state (T1) in a stepwise way via a biradical intermediate after intersystem crossing (ISC) to T1 from S1. And, then, the triplet trimethylene undergoes a second ISC to the ground state (S0) to produce the singlet trimethylene from which cyclopropane can be produced rapidly only overcoming a 1 to 2‐kcal/mol barrier while propylene can be formed as a secondary product. The cycloelimination products ketene and ethylene are formed on the S0 in a concerted mechanism after internal conversion (IC) to S0 from S1 via a biradical conical intersection. The reaction channels corresponding to ring expansion on the S0, T1, and S1 states have also been discussed, and the likeliest reaction path is that oxacarbene is formed on the ground state following S1/S0 internal conversion. The surface topology of cyclobutanone on the S1 surface is characterized by a transition state separating the minimum from the S1/S0 conical intersection, which is consistent with the previous computations and can explain the wavelength dependence of the fluorescence emission yield. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 相似文献
50.
Fractal dimension of a carious tooth surface was determined using an electrochemical method. The method was based on time-dependent diffusion towards electrode surfaces, which is one of the most useful and reliable methods for the determination of fractal dimension of electrode surfaces. For this purpose, the tooth was covered with a gold layer, which acted as an electrode in electrochemical experiments. It is suggested that the fractal dimension can be used as a quantitative measure of the state of dental surfaces. The method presented demonstrates the power of electrochemical techniques for the determination of fractal dimension of surface of non-conducting objects. 相似文献