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141.
This review covers our extensive research activities in the area of fluorine containing phospha- and arsaalkenes as well as selenocarbonyls, which differ considerably in their properties and reactivities from their alkyl and aryl counterparts and thus contribute in a gratifying manner to the still growing field of unsaturated element-carbon compounds of 3rd and 4th row main group elements E. Of particular interest is the influence of the fluorine substituents and other small groups (OR, NR2) with either inductive and/or mesomeric effects on the polarity and reactivity of the EC bond. Addition reactions of proton acidic and hydridic polar HX reactants as well as [2+2], [3+2] and [4+2] cycloadditions have been thoroughly studied. The results obtained allow a classification of the EC systems within five different types, A to E, and prove a change from “normal” to “inverse” heteroalkenes in this sequence. The ligand properties of some derivatives have also been investigated in some detail. 相似文献
142.
The semiempirical AM I method was used for the calculation of conformational potential energy surfaces of hydrogen trioxide and tetraoxide. The most stable conformation of trioxide is itsanti-form, which is characterized by the torsion angle = 82.3°, and that of tetraoxide is theanti, anti-form with = 80.7° (H-O-O-O) and 74.1° (O-O-O-O). The structures of the stable conformers are determined by the interaction between lone clectron pairs of oxygen atoms. The dipole-dipole interaction of OH groups is weaker than that in hydrogen peroxide. The results were used for the estimation of the polyoxide increment of the Benson's additivity scheme, f
H°[O-(O)2] = 48±6 k1 mol–1.Translated from lzveshyn Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1356-1361, June, 1996. 相似文献
143.
Kovácik V Bekesová S Tvaroska I Hirsch J Chmelík J 《Journal of mass spectrometry : JMS》2004,39(12):1554-1561
Fragmentation mechanisms of electrospray ionization (ESI) mass spectrometry of aldobiuronic and pseudoaldobiuronic acid derivatives were elucidated by multistage mass spectrometric (MS(n), n = 2-5) measurements of selected ions. Characteristic under the conditions of ESI-MS analysis is the production of alkali metal (Na and K) cationized adducts. The probability the of locations of Na cations in per-O-methylated compounds was proved by quantum chemical calculations, using the Jaguar program. The most probably position of alkali metal attachment is the carboxy group of the methoxycarbonyl C-5 group of the uronic acid unit. Characteristic cleavages vary according the kind of O-derivatization. In most cases they take place on the acidic part of the dimer and at the interglycosidic oxygen atom. As a result, the criteria for the differentiation of aldobiouronic and pseudoaldobiouronic acids derivatives were elucidated. 相似文献
144.
Perczel A Jákli I McAllister MA Csizmadia IG 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(11):2551-2566
Folding properties of small globular proteins are determined by their amino acid sequence (primary structure). This holds both for local (secondary structure) and for global conformational features of linear polypeptides and proteins composed from natural amino acid derivatives. It thus provides the rational basis of structure prediction algorithms. The shortest secondary structure element, the beta-turn, most typically adopts either a type I or a type II form, depending on the amino acid composition. Herein we investigate the sequence-dependent folding stability of both major types of beta-turns using simple dipeptide models (-Xxx-Yyy-). Gas-phase ab initio properties of 16 carefully selected and suitably protected dipeptide models (for example Val-Ser, Ala-Gly, Ser-Ser) were studied. For each backbone fold most probable side-chain conformers were considered. Fully optimized 321G RHF molecular structures were employed in medium level [B3LYP/6-311++G(d,p)//RHF/3-21G] energy calculations to estimate relative populations of the different backbone conformers. Our results show that the preference for beta-turn forms as calculated by quantum mechanics and observed in Xray determined proteins correlates significantly. 相似文献
145.
V. P. Feshin M. Yu. Kon'shin A. V. Radushev E. V. Feshina V. Yu. Gusev A. E. Lesnov V. I. Karmanov 《Russian Chemical Bulletin》1996,45(11):2505-2508
The basicity of hydrazides of the highest aliphatic carboxylic acids RC(O)NHNH2 (R = CnH2n+1,n = 5-12) has been studied by potentiometric titration, and IR and1H NMR spectroscopy.Ab initio Hartree-Fork calculations using the 6–31G* basis set with full optimization of geometry were carried out on the simplest acy1hydrazines and their possible protonated forms. Based on these calculations, and the 1R and1H NMR spectra, the tautomerism of alkylhydrazides and the structures of their protonated forms are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2645–2649, November, 1996. 相似文献
146.
A comprehensive B3LYP/6-31+G∗ study of the influence of BF3 on the [4+2] cycloaddition of vinylketene with formaldimine was conducted. For this purpose, the complete pathway was determined and changes in different magnetic properties (magnetic susceptibility, χ, magnetic susceptibility anisotropy, χanis, and the nucleus-independent chemical shifts, NICS) were monitored along the reaction profile with a view to estimate the aromatization associated to the process. We have also applied the ACID (anisotropy of the current-induced density) method with the same intention. 相似文献
147.
The interaction energy of a [Au{C(NHMe)2}2]+ ... [Au{C(NHMe)2}2]+ dimer is investigated using the MP2 method and the LANL2DZ basis set when isolated or embedded in ionic an [Au{C(NHMe)2}2]2anion2 aggregate, a good model for the environment that these dimers feel in ionic crystals. A repulsive interaction energy is obtained when the dimer is isolated. However, it is possible to find short AuI ... AuI separations in [Au{C(NHMe)2}2]2anion2 aggregates, because in these aggregates the sum of the cation ... anion interactions overweight the sum of the cation ... cation plus anion...anion interactions. This explains why short AuI ... AuI separations are found in ionic crystals. The AuI ... AuI interaction found in [Au{C(NHMe)2}2]2 anion2 aggregates shows the same features observed in energetically stable dimers presenting AuI... AuI bonds. This makes appropriate to use the name counterion-mediated bonds for the AuI... AuI interactions found in [Au{C(NHMe)2}2]2 anion2 aggregates and ionic crystals. 相似文献
148.
Taking into account the thiophilic properties of iodine, a very simple methodology to achieve 2′,3′-didehydro-2′,3′-dideoxy-β-nucleosides in high yield was performed, using mild, and inexpensive conditions, by means of the treatment of 2′-deoxy-3′,5′-dibenzoyl-2′-iodo-β-nucleoside derivatives with NaHS. The process has shown to be highly dependent of the relative geometry between the iodine atom and the adjacent leaving group. In this way, different essays carried out with pyranose derivatives have concluded in no reaction when the vicinal groups to eliminate do not adopt a trans-diaxial disposition. In addition, the treatment of 2-iodomethyl-cyclopropane-1,1-dicarboxylic acid diethyl ester under the same conditions softly and readily leads to the obtention of a mixture of the expected 2-allyl-malonic acid diethyl ester (as the minor product) and the thiirane derivative 2-thiiranylmethyl-malonic acid diethyl ester (as the major product). In this case, the responsible of the reaction progress are the nucleophilic properties of the sulfur atom rather than the thiophilic character of the iodine atom. 相似文献
149.
A molecular mechanics software enhanced to perform empirical energy calculations on crystals (KESSHOU) was further developed to handle intermolecular electrostatic interactions as well. The packing of the molecules of 1,4-dicyanobenzene and 1,4-diisocyanobenzene in the crystal was studied. The role of the van der Waals and the electrostatic interactions in the balance of nonbonded atom-atom interactions is analyzed. The packing forces are dominated by van der Waals forces. The electrostatic interactions have higher stabilizing contribution for the dicyano isomer than for the diisocyano form. The dependence of the results on the size of the crystal, the molecular mechanics force field (MM2 vs MM3), and the dielectric constant are also assessed. Ab initio MP2/6–311G** geometries of the isolated molecules are in accordance with the observed benzene ring distortions determined by electron diffraction. 相似文献
150.
Tatiana Murakhtina Jasper Heuft Evert Jan Meijer Daniel Sebastiani 《Chemphyschem》2006,7(12):2578-2584
A combined experimental and ab initio study is presented of the 1H NMR chemical shift distribution of aqueous hydrogen chloride solution as a function of acid concentration, based on Car-Parrinello molecular dynamics simulations and fully periodic NMR chemical-shift calculations. The agreement of computed and experimental spectra is very good. From first-principles calculations, we can show that the individual contributions of Eigen and Zundel ions, regular water molecules, and the chlorine solvation shell to the NMR line are very distinct and almost independent of the acid concentration. From the computed instantaneous NMR distributions, it is further possible to characterize the average variation in hydrogen-bond strength of the different complexes. 相似文献