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61.
Donners JJ Heywood BR Meijer EW Nolte RJ Sommerdijk NA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(11):2561-2567
Poly(propylene imine) dendrimers that are modified with long alkyl chains self-assemble to form well-defined aggregates. The geometry and surface chemistry of the dendrimer assemblies can be varied through the addition of surfactants. These dendrimer/surfactant aggregates can be tuned to template the formation of the different phases of calcium carbonate. The use of octadecylamine results in the formation of polyhedral aggregates that become embedded within an amorphous calcium carbonate phase that persists in competition with the thermodynamic product, calcite. In combination with hexadecyltrimethylammonium bromide, small spherical aggregates are formed that induce the formation of vaterite. The use of the negatively charged surfactant SDS results in growth retardation by the Ca(2+)-induced agglomeration of dendrimer/surfactant aggregates into giant spherical particles. Eventually these particles become overgrown by rhombohedral calcite. 相似文献
62.
[Reaction: see text] We report a one-step, racemization-free method for the diversification of peptide thiazoles via direct lithiation of the thiazole ring. The method is compatible with N-Boc, N-trityl, carboxylic ester, and carboxamide protecting groups and has been used to directly functionalize the thiazole ring of cyclopeptide natural products. 相似文献
63.
Summary The rhodium(I) carboxylates,trans-RhO2CR(CO)(PPh3)2 (R = C6F5, C6Cl5,p-HC6F4,m-HC6F4,o-HC6F4,p-McOC6F4, 4,5-H2C6F3, 3,5-H2C6F3, or 2,6-F2C6H3, have been prepared by reaction of RhH(CO)(PPh3)3 with the appropriate polyhalogenobenzoic acids in ethanol and/or by reaction oftrans-RhCl(CO)(PPh3)2 with the appropriate thallous carboxylates in benzene. Decarboxylations with formation of polyhalogenoarylrhodium(I) compounds,trans-RhR(CO)(PPh3)2 (R = C6F5, C6Cl5,p-HC6F4,m-HC6F4,p-MeOC6F4, 4,5-H2C6F3 or 3,5-H2C6F3), have been achieved either by decomposition of the corresponding rhodium(I) carboxylates in pyridine or by reaction oftrans-RhCl(CO)(PPh3)2 and the thallous carboxylates in pyridine, but the derivatives R =o-HC6F4 or 2,6-F2C6H3 could not be obtained by this method. The rate of decarboxylation decreased in the sequence R = C6F5 >p-MeOC6F4 >p-HC6F4 >m-HC6F4 > 4,5-H2C6F3 > 3,5-H2C6F3.Part 1, ref. 10.Preliminary communication, ref. 9. 相似文献
64.
Simon R. Drake Brian F. G. Johnson Jack Lewis 《Journal of organometallic chemistry》1988,340(3):C31-C36
The heteronuclear clusters [Os10C(CO)24(MPR3nm− (n = 1, m = 1; 2: M = Au; 3: M = Ag; 4: M = Cu; 5 n = 2, m = 0, M = Ag) have been prepared. These clusters undergo molecular rearrangements in solution, and two isomeric forms of 2, 3, 4, 5 and 6 have been identified. This interconversion is thought to involve a cap [lrarr2] edge bridge [lrarr2] cap pathway. 相似文献
65.
66.
Xuejun Pan Dan Xie Neil Gilkes David J. Gregg Jack N. Saddler 《Applied biochemistry and biotechnology》2005,124(1-3):1069-1079
Pretreatment of Douglas-fir by steam explosion produces a substrate containing approx 43% lignin. Two strategies were investigated
for reducing the effect of this residual lignin on enzymatic hydrolysis of cellulose: mild alkali extraction and protein addition.
Extraction with cold 1% NaOH reduced the lignin content by only approx 7%, but cellulose to glucose conversion was enhanced
by about 30%. Before alkali extraction, addition of exogenous protein resulted in a significant improvement in cellulose hydrolysis,
but this protein effect was substantially diminished after alkali treatment. Lignin appears to reduce cellulose hydrolysis
by two distinct mechanisms: by forming a physical barrier that prevents enzyme access and by non-productively binding cellulolytic
enzymes. Cold alkali appears to selectively remove a fraction of lignin from steam-exploded Douglas-fir with high affinity
for protein. Corresponding data for mixed softwood pretreated by organosolv extraction indicates that the relative importance
of the two mechanisms by which residual lignin affects hydrolysis is different according to the pre- and post-treatment method
used. 相似文献
67.
68.
Jack C. Kim Sang-Duk Bae Ji-A Kim Soon-Kyu Choi 《Journal of heterocyclic chemistry》1998,35(3):531-533
A Regioselective synthesis of (R)-11-hydroxyaporphine 2 directly from (R)-10,11-dihydroxyaporphine ((R)-apomorphine, 1 ) is described for the first time. The isopropylidene ketal ring of 10,11-(isopropyl-idenyldioxy)aporphine 5 obtained by the isopropylidenation of apomorphine was regioselectively opened by ten equivalents of trimethylaluminum to give (R)-10-hydroxy-11-tert-butyloxyaporphine 6 . The free 10-hydioxyl position of 6 was triflated with N-pbenyltrifluoromethanesulfonimide and potassium carbonate under reflux to give (R)-10-[(trifluoromethyl)sulfonyloxy]-11-tert-butyloxyaporphine 7 . The reduced product, 11-tert-butyloxyaporphine 8 was prepared from 7 by a palladium-catalyzed hydrogenolysis. The ether cleavage of (R)-11-tert-butyloxyaporphine with 48% hydrobromic acid afforded the desired (R)-11-hydroxyaporphine 2 in good yield. 相似文献
69.
Towards the aim of creating a functional mimic of isopenicillin N synthase, a small molecule designed to coordinate around iron(II) and model the enzyme active site has been prepared in nine synthetic steps from 2,6-bis(hydroxymethyl)pyridine, (S)-(+)-mandelic acid and pivaldehyde. One aspartate, two histidines and a water ligand in the natural enzyme are replaced by an α-hydroxy acid, pyridine and aniline in the model compound. Additionally, a free thiol designed to simulate the enzyme substrate, δ-(l-α-aminoadipoyl)-l-cysteinyl-d-valine, is linked to the ligand by a three carbon chain. We postulate that in the presence of molecular oxygen, the complex formed between this synthetic ligand and iron(II) will display oxidative chemistry similar to that observed in the active site of isopenicillin N synthase. 相似文献
70.
Lin-Zhi Chen Jack M. Miller 《Journal of the American Society for Mass Spectrometry》1992,3(4):451-459
A mass spectrometer fast atom bombardment source has been used to synthesize, in the gas phase, the ion-molecule complexes of transition-metal ions (Ni+, CO+, Fe+, and Mn+) with α- or β-unsaturated alkenenitriles, RCH=CHCN (R=H, CH3, and C2H5) and CH3CH=CHCH2CN, and 2-methyl glutaronitrile. The metastable ion fragmentations of the complexes are monitored in the first held-free region by B/E linked scans. Surprisingly, an intense HCN loss via an intermediate (C n H2n ?2)?M+?(HCN) is observed for the complexes of the alkenenitriles. The metal ions significantly affect the fragmentation processes. The coexistence of both end-on and side-on coordination modes is suggested to explain the fragmentations. 相似文献