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1.
Jonathan M. Levin Lawrence L. Wald Marc J. Kaufman Marjorie H. Ross Luis C. Maas Perry F. Renshaw 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,130(2):292-295
Residual effects of an initial bolus of gadolinium contrast agent have been previously demonstrated in sequential dynamic susceptibility contrast MR experiments. While these residual effects quickly reach a saturation steady state, their etiology is uncertain, and they can lead to spurious estimates of hemodynamic parameters in activation experiments. The possible influence ofT1effects is now investigated with experiments in whichT1weighting is varied as well as with serial regionalT1measurements. Little evidence for significant residualT1effects is found, suggesting instead that susceptibility effects underlie these observations. An initial saturation dose of contrast agent minimizes this effect. 相似文献
2.
K. Jungmann P. E. G. Baird J. R. M. Barr C. Bressler P. F. Curley R. Dixson G. H. Eaton A. I. Ferguson H. Geerds V. W. Hughes J. Kenntner S. N. Lea F. Maas M. A. Persaud G. zu Putlitz P. G. H. Sandars W. Schwarz W. T. Toner M. Towrie G. Woodman L. Zhang Z. Zhang 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,21(3):241-243
The 12 S 1/2(F=1)?22 S 1/2(F=1) energy interval in muonium has been investigated by Doppler-free two-photon laser spectroscopy. The transition was observed by uniquely identifying and counting the muons released after the photoionization of the 2S state by the same laser field. The measured transition frequency of 2455 528016(58)(43) MHz is in good agreement with QED calculations. The experiment can be interpreted as a test of the Lamb shift contributions at the 1% level. The method is also well suited for a precise determination of the mass of the positive muon in a muoniumhydrogen isotope shift measurement. 相似文献
3.
The diagnostic value of an interpretation system for vibrational spectra has been investigated in relation to the number of structural units the system can cope with. For five structural units the correlated wavenumber intervals are derived from three infrared and three Raman files by the CRISE program. The interpretative value of each interval in relation to the intensity threshold is discussed. For most intervals a decrease is found on increasing the number of structural units. 相似文献
4.
The CRISE computer program is used to correlate wavenumber regions and 6 structural elements containing carbon, hydrogen and oxygen on the basis of 2 standard files with 549 infrared and Raman spectra. The degree of correlation, including score percentages and interfering percentages, is established for different types of intervals in relation to various intensity thresholds. Specific regions (score 100%, interference 0%) proved to be rare, whereas pseudo-specific regions (score < 100%, interference 0%) are normally present. The usefulness of selective regions (score 100%, interference > 0%) is doubtful. The infrared and Raman results for a structural element can differ appreciably, yet neither technique is clearly superior for interpretative purposes. 相似文献
5.
The 3-trifluoromethylsulfonyloxy-1-hexeniminium triflate 1 reacts with two equivalents of an aliphatic nitrile or of benzonitrile to give the 4-(2-diethylaminovinyl)pyrimidinediium bis(triflates) 4 , which can be deprotonated to give the monoprotonated or neutral pyrimidines 5 and 6 , respectively. When the related 1-phenyl-substituted iminium salt 7 is heated in acetonitrile at 140°, 1,5-cyclization of the cation leading to indane derivative 8 competes with formation of the pyrimidinium salt 9 . X-ray crystal structure determination reveals significant differences in the bond lengths of mono-and diprotonated 4-(2-diethylaminovinyl)pyrimidines 5a and 4a . 相似文献
6.
G. Maas 《无机化学与普通化学杂志》1977,432(1):203-210
Crystal and Molecular Structure of μ-5-Pyrimidinecarboxylato(O,O′)-di-μ-hydroxo-bis-[triamminecobalt(III)]perchlorate μ-5-Pyrimidinecarboxylato(O,O′)-di-[μ-hydroxo-bis(triamminecobalt)(III)]perchlorate, [(NH3)3Co-μ(OH, OH, C5H4N2O2)Co(NH3)3](ClO4)4, crystallizes in the orthorhombic space group Ibca with a = 12.686, b = 13.079, c = 31.785 Å and Z = 8 formula units. The complex cation adopts C2 symmetry, but no mirror plane is present. The Co? Co separation in the binuclear complex is 2.794 Å. The four-membered ring defined by the two Co atoms and the bridging oxygens is folded along the O? O axis, the interplanar angle being 154°. One of the ClO4 tetrahedra is disordered over two sites in the crystal. 相似文献
7.
M. D. Thompson J. S. Bradshaw S. F. Nielsen C. T. Bishop F. T. Cox P. E. Fore G. E. Maas R. M. Izatt J. J. Christensen 《Tetrahedron》1977,33(24):3313-3316
A series of macrocyclic ether-esters has been prepared by treating various glycols with adipoyl chloride and various substituted malonyl, succinyl and glutaryl chlorides. The prepared compounds include: 15-ethyl- and 15-phenyl-1,4,7,10,13-pentaoxacyclohexadecane-14,16-dione (5 and 6); 15-methyl-, 15-phenyl-, cis-cyclohexo-[o]-and benzo-[o]-1,4,7,10,13-pentaoxacycloheptadecane-14,17-dione (7–10); trans,trans-1,4,7,10,13,18,21,24,27,30-de-caoxacyclotetratriacontane-15, 32-diene-14,17,31,34-tetraone (11); 1,4,7,10,13-pentaoxacyclooctadecane-14,18-dione (12); 15,15,16,16,17,17-hexafluoro- and 16-methyl-1,4,7,10,13-pentaoxacyclooctadecane-14,18-dione (13 and 14); 1,4,7,10-tetraoxacyclohexadecane-11,16-dione (15); and 1,4,7,10,13-pentaoxacyclononadecane-14,19-dione (16). 相似文献
8.
Maas G 《Chemical Society reviews》2004,33(3):183-190
The transition-metal catalysed cyclopropanation of olefinic bonds using diazo compounds as a carbene source is among the best developed and most useful transformations available to the synthetic organic chemist. Nevertheless, the quest for new catalyst/ligand systems continues in order to further extend the scope of this method and to identify more economical catalytic systems. In this tutorial review, several different ruthenium complexes are presented which have recently emerged as suitable catalysts for carbenoid cyclopropanation. For the model reaction--cyclopropanation of styrene(s) with diazoacetates--and also for some intramolecular cyclopropanation reactions highly remarkable results in terms of catalyst efficiency, product yields, dia- and enantioselectivity have been reported. 相似文献
9.
The crystal structure of Di-μ-sulfato-μ-hydroxo-bis[triamminecobalt(III)] sulfate 8-hydrate has been determined from three-dimensional x-ray data collected by counter techniques. The structure was refined using 2515 independent reflections and the refinement converged to a conventional R factor (on F) of 3.8%. The compound crystallizes in the monoclinic space group C—P2/a, a = 14.122(9), b = 9.858(2), c = 18.81(2) Å, β = 139.3(4), Z = 2, dobsd = 2.086 g/cm3 and dcalc = 2.14 g/cm3. Within the cation two bidentate SO4-ligands form bridges between two cobalt atoms. There are two types of S? O bonds (1.50 Å endocyclic, 1.45 Å exocyclic). 相似文献
10.