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41.
This paper reports the study of backbone cleavages in the collision-induced negative-ion mass spectra of the [M - H](-) anions of some synthetic modifications of the bioactive amphibian peptide citropin 1 (GLFDVIKKVASVIGGL-NH(2)). The peptides chosen for study contain no amino acid residues which could effect facile side-chain cleavage, i.e. Ser (-CH(2)O, side-chain cleavage) and Asp (-H(2)O) are replaced by Ala or Lys. We expected that such peptides should exhibit standard and pronounced peaks due to alpha cleavage ions (and to a lesser extent beta cleavage ions) in their collision-induced negative-ion spectra. This expectation was realised, but the spectra also contained peaks formed by a new series of cleavage anions. These are produced following cyclisation of the C-terminal CONH(-) moiety at carbonyl functions of amide groups along the peptide backbone; effectively transferring the NH of the C-terminal CONH(-) group to other amino acid residues. We have called the product anions of these processes beta' ions, in order to distinguish them from standard beta ions. Some beta' ions also fragment directly to some other beta' ions of smaller mass. The reaction coordinates of alpha,beta and beta' backbone processes have been calculated at the HF/6-31G*//AM1 level theory for simple model systems. The initial cyclisation step of the beta' sequence is barrierless and exothermic. Subsequent steps have a maximum barrier of +40 kcal mol(-1), with the overall reaction being endothermic by some 30 kcal mol(-1) at the level of theory used. These calculations take no account of the complexity of the conformationally flexible peptide system, and it is surprising that each of the two reacting centres can 'find' each other in such a large system. 相似文献
42.
Double [2,3] sigmatropic rearrangements of bis(propargyl sulfenates) to bis(allenic sulfoxides) and of bis(propargyl sulfinates) to bis(allenic sulfones) are shown to be a convenient and effective method for the preparation of conjugated diallene systems bearing two electron withdrawing trihalomethyl sulfoxide or sulfone substituents either on C-1 and C-6, or on C-3 and C-4. Such substituents are further shown to facilitate cyclization to bis(methylene)cyclobutenes, and to stabilize the latter. The electron withdrawing group substitution on the exocyclic methylene extremities proved more effective than similar substitution on the endocyclic double bond. 相似文献
43.
Smith AB Pitram SM Gaunt MJ Kozmin SA 《Journal of the American Chemical Society》2002,124(49):14516-14517
High chemoselectivity can be achieved in the addition of lithium dithiane anions to vinyl epoxides exploiting the steric nature of the dithiane substituent. Unencumbered dithiane anions afford SN2 adducts, whereas sterically encumbered anions lead primarily to SN2' adducts. Furthermore, the SN2' addition occurs syn to the vinyl epoxide. 相似文献
44.
B. Venkateswara Reddy K. V. N. Suresh Reddy J. Sreeramulu G. V. Kanumula 《Chromatographia》2007,66(1-2):111-114
A rapid, specific reversed phase HPLC method has been developed for simultaneous determination of olanzapine and fluoxetine
in their formulations. Chromatographic separation of these two pharmaceuticals was carried out on an Inertsil C18 reversed phase column (150 mm × 4.6 mm, 5 μm) with a 40:30:30 (v/v/v) mixture of 9.5 mM sodium dihydrogen phosphate (pH adjusted to 6.8 ± 0.1 with triethylamine), acetonitrile and methanol as
mobile phase. The flow rate 1.2 mL min−1 and the analytes are monitored at 225 nm. Paroxetine was used as internal standard. The assay results were linear from 25
to 75 μg mL−1 for olanzapine (r
2 ≥ 0.995) and 100–300 μg mL−1 for fluoxetine (r
2 ≥ 0.995), showed intra- and inter-day precision less than 1.0%, and accuracy of 97.7–99.1% and 97.9–99.0%. LOQ was 0.005
and 0.001 μg mL−1 for olanzapine and fluoxetine, respectively. Separation was complete in less than 10 min. Validation of the method showed
it to be robust, precise, accurate and linear over the range of analysis. 相似文献
45.
[structures: see text] A range of cis-fused hexahydro-1-benzofuran-3(2H)-one or tetrahydro-2H-cyclopenta[b]furan-3(3aH)-one ring systems was synthesized by the rhodium(II) acetate catalyzed reaction of alpha-diazo carbonyl compounds in the presence of various oxygen, nitrogen, and sulfur nucleophiles. A double-nucleophilic addition was possible by using an excess of alpha-diazo ketone. These reactions proceeded with complete diastereoselectivity, and the stereochemistry was confirmed by the single-crystal X-ray analysis. This process discloses the first example of tandem cyclization-nucleophilic addition reaction. 相似文献
46.
47.
The ab initio calculation of the title compound was carried out at HF as well as DFT level of theory. The full geometry optimization of the ligand was carried out using 6-31G(d) basis set. The results obtained were correlated with the single crystal X-ray data, also reported in this paper, shows close resemblance between these two. The influence of electron correlation effects also was studied by carrying out geometry optimization at the MP2 level. The attempts were also made to ascertain the most stable tautomer of the said compound. 相似文献
48.
KailasaSuresh Kumar Kanchi Suvardhan Dasari Rekha B. Jayaraj Pattium Chiranjeevi 《Helvetica chimica acta》2005,88(5):1022-1027
A facile, rapid, and sensitive spectrophotometric method for the determination of propoxur in insecticidal formulations, fortified water, vegetables, agricultural wastewater, and agricultural soil samples has been elaborated. The proposed method is based on the hydrolysis of propoxur under basic conditions, followed by instantaneous azo coupling of the resulting 2‐isopropoxyphenol with the anilines 2a – c . This yielded the orange‐red chromophore 3a (λmax=at 470 nm), the pale‐red coupling product 3b (490 nm), or the red derivative 3c (478 nm), which are stable for 46 h, 38 h, and 24 h, respectively, and could be readily analyzed spectrophotometrically. 相似文献
49.
Devendra K. Rastogi Suresh K. Sahni Vidya B. Rana Satendra K. Dua 《Transition Metal Chemistry》1978,3(1):56-60
Summary Dimeric manganese(II)and iron(II)complexes, (ML)2, derived from benzoyl hydrazones ofo-hydroxyaryl aldehydes and ketones arc described and characterised by elemental analyses and by conductance, molecular weight, magnetic, electronic and i.r. spectral measurements. The dimeric nature of the complexes is revealed by i.r. spectra which show bands atca. 885 Mn2+ and 820 cm–1 Fe2+, characteristic of ring vibrations. The i.r. spectra reveal the terdentate nature of the ligands. The electronic spectra in dimethylformamide are consistent with the tetrahedral nature of the complexes. The appreciable lowering in
eff is attributed to the presence of exchange interactions between two paramagnetic atomsvia oxygen bridges.Reprints of this article are not available. 相似文献
50.
D'Souza F Chitta R Gadde S Zandler ME McCarty AL Sandanayaka AS Araki Y Ito O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(15):4416-4428
Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine. 相似文献