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71.
Patricia H. Risso Domingo Mariano Borraccetti César Araujo María Eugenia Hidalgo Carlos A. Gatti 《Colloid and polymer science》2008,286(12):1369-1378
κ-Casein (κ-CN) aggregation by heating has been studied at pH 7.2 and 5.2 using UV-visible spectrophotometry, sodium dodecyl
sulfate polyacrylamide gel electrophoresis, spectrofluorometric study of the 1–8 aniline naphtalene sulfonate (ANS)–κ-CN binding
and circular dichroism (CD) spectroscopy. The aggregation process to form aggregates like micelles or submicelles and the
structural characteristics of these aggregates were pH dependent. Far-UV CD showed that the aggregates obtained by heating
presented changes in the κ-CN secondary structure. Near-UV CD spectra showed a certain degree of tertiary organization in
the Tyr environment for the protein heated or unheated, only at pH 5.2. ANS binding at both pH was quite different and depends
on the self-association process. Heating produced exposition of hydrophobic binding sites only at pH 7.2, including those
in the neighborhood of the κ-CN Trp residue. 相似文献
72.
Gutiérrez-Tauste D Domènech X Domingo C Ayllón JA 《Chemical communications (Cambridge, England)》2007,(44):4659-4661
The straightforward modification of commercial hexafluorotitanic acid with organic derivatives containing a tetraalkylammonium cation produced stable precursor solutions with only a small fraction of water, which were then used as a reaction medium for the synthesis of nanocrystalline TiO(2). 相似文献
73.
The pseudo-intramolecular Diels-Alder (DA) reaction between a 2-substituted furan (1) and a N-maleimide derivative (2) has been analyzed using DFT methods. Formation of two hydrogen bonds between the appendages on furan and maleimide derivatives favors thermodynamically the formation of a molecular complex (MC1) through an efficient molecular recognition process. The large enthalpy stabilization associated with the molecular recognition overcomes the unfavorable activation entropy associated with the bimolecular process. As a consequence, the subsequent DA reaction is clearly accelerated through a pseudo-intramolecular process. 相似文献
74.
Highly enantioselective rearrangement of beta-amino alcohols was realized by using a catalytic amount of trifluoroacetic anhydride. 相似文献
75.
Pentacyclic pyrrolo- and pyrido[1,2-a]xanthene[1,9-de]azepines were synthesized in various oxidation states by assembling the azepine ring following two strategies: 7-endo-trig cyclization of the aryl radical derived from a gamma-methylene lactam and cyclodehydration of aldehydes. Other strategies examined (Heck reaction and intramolecular acylation) did not afford azepines, but six-membered nitrogenated rings. 相似文献
76.
Jorge Soto‐Delgado Luis R. Domingo Ramiro Araya‐Maturana Renato Contreras 《Journal of Physical Organic Chemistry》2009,22(6):578-584
The polar Diels–Alder (DA) reactions of 2‐acetyl‐1,4‐benzoquinone (acBQ) with methyl substituted 1,3‐butadienes have been studied using DFT methods at the B3LYP/6‐31G(d) level of theory. These reactions are characterized by a nucleophilic attack of the unsubstituted ends of the 1,3‐dienes to the β conjugated position of the acBQ followed by ring‐closure. The reactions present a total regioselectivity and large endo selectivity. The analysis based on the global electrophilicity of the reagents at the ground state, and the natural bond orbital (NBO) population analysis at the transition states correctly explain the polar nature of these cycloadditions. The large electrophilic character of acBQ is responsible for the acceleration observed in these polar DA reactions. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
77.
Gonzalez-Béjar M Stiriba SE Domingo LR Pérez-Prieto J Miranda MA 《The Journal of organic chemistry》2006,71(18):6932-6941
Diels-Alder reactions between indoles (InHs) and 1,3-cyclohexadienes (CHDs) were achieved by using aromatic ketones as photosensitizers. For instance, irradiation of deaerated dichloromethane solutions containing benzoylthiophene (BT, 1 mM), indole (20 mM), and phellandrene (40 mM), in the presence of an acylating agent, led to the N-acetylated Diels-Alder cycloadduct in 46% yield (endo:exo ratio of 1.8:1). Energy transfer from the BT triplet to the CHD gave rise to diene dimers as byproducts. Several combinations of CHDs, InHs, and aromatic ketones were tested; the Diels-Alder reaction was found to be a general process, except when the indole nucleus was substituted at position 2 or 7 and when aromatization of the CHD was favored. Theoretical calculations support a stepwise mechanism involving a triplet ternary complex TC(T1), arising from a nearly barrierless reaction between CHD and the 3(BT...HIn) exciplex. All subsequent steps proceed downhill in the triplet excited state, leading to a triplet cycloadduct-sensitizer CA-BT(T1) radical pair. Attempts to detect this species, which is basically an aminyl/BT ketyl radical pair, were performed by laser excitation of a solution containing BT, phellandrene, and indole. The observed transient absorption spectra could be compatible with the added spectra of the expected components of the radical pair. 相似文献
78.
A new synthetic entry to enantiopure cis-decahydroquinolines is reported. Endo and exo derivatives of cis-1-benzyl-2-(hydroxymethyl)octahydroindol-6-one ethylene acetal undergo ring enlargement upon treatment with TFAA and then Et3N (thermodynamic conditions) to give enantiopure 1-benzyl-3-hydroxydecahydroquinolin-7-one derivatives in 77 and 82% yield, respectively. For 2-(1-hydroxyethyl) analogues, the best synthetic result is obtained from the (2S,1'R) endo isomer, which under kinetic reaction conditions (MsCl, THF, -20 degrees C, then AgOAc at rt ) gives the expanded product in 54% yield. 相似文献
79.
[reaction: see text] Theoretical calculations at the B3LYP/6-31+G(d), MP2/6-31+G(d), and G3(MP2) levels have been carried out to understand the alternative reaction pathways (the cyclopropyl ring cleavage (RC) and the retrocycloaddition reaction (rCA)) of a constrained tricyanocyclopropyl anionic derivative. The more energetically favorable path is found to be the RC process, a formally "forbidden" rearrangement (Leiviers, M.; Tam, I.; Groves, K.; Leung, D.; Xie, Y.; Breslow, R. Org. Lett. 2003, 5, 19, 3407) yielding an allylic anion system via a concerted transition structure, in agreement with experimental outcomes. rCA is more energetically favorable along a two-stage mechanism, via an intermediate, than a synchronous concerted process. By using isodesmic reactions, we have found that B3LYP presents limitations when it calculates carbon-carbon bond-breaking processes along the present rCA reaction. A detailed analysis of the nature of the topology of the reactive potential energy surface for the RC process points out the presence of a valley-ridge inflection point in the uphill part. An explanation for the low-energy barrier associated with RC is furnished on the analysis of the evolution of the twisting (dis-/conrotatory) motions of cyano substituents in the cyclopropyl ring as well as on the number and type of electron pairs provided by the electron localization function (ELF). 相似文献
80.