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21.
Victor X. Jin Donal H. Macartney Erwin Buncel 《Journal of inclusion phenomena and macrocyclic chemistry》2005,53(3-4):197-203
A series of novel bischelate bridging ligands, CH3NH(CH2)2N(CH3)(CH2) n N(CH3)(CH2)2NHCH3 (n = 9, 10, 11, and 12) were synthesized as hydrochloride salts and characterized by elemental analyses, electrospray mass spectrometry, and 1H and 13C NMR spectroscopy. These ligands form [2]pseudorotaxanes with α-cyclodextrin (α-CD) and the stability constants have been determined from 1H NMR titrations in D2O. The kinetics and mechanism of the assembly and dissociation of a [2]pseudorotaxane in which α-CD has been threaded by the CH3NH2(CH2)2N(CH3)(CH2)12N(CH3)(CH2)2NH2CH 3 2+ ligand were determined in aqueous solution using 1H NMR spectroscopy. A weak inclusion of the dimethylethylenediamine end group precedes the passage of the α-CD onto the hydrophobic dodecamethylene chain. 相似文献
22.
We report on the catalytic effects by alkali-metal ions in the ethanolysis of p-nitrophenyl diphenyl phosphate, in continuation of our studies on alkali-metal ion catalysis and inhibition in nucleophilic displacement reactions at carbon, phosphorus and sulfur centres. The following selectivity order of catalytic reactivity was observed for nucleophilic displacement at the phosphorus center with p-nitrophenoxide as leaving group: Li+ > Na+ > K+ > Cs+. A minor reaction pathway with phenoxide leaving was also found. The kobs data have been dissected into reaction pathways by free ions (kEtO) and by ion pairs (kMOEt), with the latter being dominant, in a 4-membered transition-state. Further analysis is given in terms of initial-state and transition-state stabilization by the alkali-metal ions in terms of the Eisenman model (electrostatic interaction vs. desolvation). Results of ab-initio MO calculations are presented based on interaction between M+ and a model bipyramidal phosphorane intermediate and compared with the sulfurane analogue. 相似文献
23.
A two-step protocol for the first chemoselective cleavage of 2-hydroxy acid amides has been developed. Mesylation of the model substrate 2-(hydroxypropionylamino)-4-methylpentanoic acid methyl ester (11) followed by treatment with N-ethylthiourea (13) allows cleavage of 2-hydroxy acid amides under smooth conditions. Successful application of this methodology to the open-chain transesterification product 15 (methylester) of the cyclic heptadepsipeptide HUN-7293, a potent inhibitor of inducible cell adhesion molecule expression, delivered the corresponding hexapeptide 18 with unprotected N-terminus in 70-75% yield. This result demonstrates that the protocol developed even works in the presence of an ester and several methylated and unmethylated amide bonds. Finally, a sequence of ligation of methyl D-dehydroglutaminate (20) to the C-terminus of the saponification product 21, followed by the degradation protocol and ring closure, allowed chemical "point mutation" at the DGCN site affording the aza analogue of HUN-7293 (24) in 15% overall yield. To the best of our knowledge this is the first report on chemoselective cleavage of 2-hydroxy acid amides. 相似文献
24.
Ernst Schaumann Erwin Kausch Karl-Heinz Klaska Rolf Klaska Otto Jarchow 《Journal of heterocyclic chemistry》1977,14(5):857-860
The crystal and molecular structures of the cycloadduct from isopropyl[α-(dimethylthio-carbamoyl)isopropyl]carbodiimide and methyl isothiocyanate were determined by single-crystal X-ray methods. The product was established to be 2-(1-isopropyl-3,5-dimethyl-4,6-dithioxo-hexahydro-1,3,5-triazine-2-ylidenamino)-N,N-dimethylthioisoburyramide, in which contrary to expectations based on hybridisation the heterocyclic ring adopts a boat conformation. 相似文献
25.
Erwin Mues 《manuscripta mathematica》1978,23(4):325-341
In this paper it is discussed for which entire functions g the homogeneous differential polynomial gg-ag2 has no zeros. It is shown that g(z)=exp (z+) are the only transcendental entire functions with this property if a1. The case a=1 is indeed exceptional. This result settles a question of Hayman. 相似文献
26.
Erwin Brüning 《Communications in Mathematical Physics》1978,58(2):167-194
The problem of uniqueness of monotone continuous linear extensions of $$T_{(2N)} = \{ 1,T_1 ,...,T_{2N} \} \in E'_{(2N)} = \prod\limits_{n = 0}^{2N} {E'_n } $$ is solved. A characterization of a relativistic QFT in terms of finitely many VEV's is derived. All results are illustrated by an explicit discussion of the extension problem for special cases ofT (4)={1,0,T 2,T 3,T 4}. This discussion contains explicitly necessary and sufficient conditions onT (4) for the existence of minimal extensions and some convenient sufficient conditions. 相似文献
27.
K[AlH(CH3)3] is formed by decomposition of K[Al(CH3)3SiH3] in diethyl ether. The compound crystallizes in the orthorhombic space group Cmca with the lattice parameters a = 9.647, b = 17.332, c = 7.711 Å. There are eight formula units in the unit cell. Analogously to K[Al(CH3)4] the hydrido compound contains isolated anions with a tetrahedral coordination of aluminium by hydrogen and three methyl groups. 相似文献
28.
Prof. Dr. Erwin Weiss 《Angewandte Chemie (International ed. in English)》1993,32(11):1501-1523
The investigation of the reactivity and structure of organometallic compounds of alkali metals has experienced a blustering development in the last decades. This class includes compounds that are especially important for our understanding of chemical bonding and also quite simple, for example methyl alkali metal complexes, whose structures have been unequivocally determined. Organometallic compounds of alkali metals (and also magnesium) generally exist as ion aggregates whose properties can be significantly modified through solvation by, for example, ether or amines. Important advances in the synthesis of new compounds, especially those of the heavier alkali metals, have been based on these results. It was long believed that the alkali metals had little tendency to undergo coordination and that their coordination chemistry would offer few surprises. This picture has now changed completely. Results from crystal structure investigations have revealed a variety of often surprising structure types (rings, heterocubanes, chains, layers, etc.) not only with the organometallic compounds but also with the amides, imides, alkoxides, phenoxides, enolates, and even halides. A comparison reveals interesting similarities between compounds that appear to be so different and leads to a general classification of the structure types possible with C, N, O, and halo ligands. 相似文献
29.
Werner Marty Pramesh N. Kapoor Hans-Beat Bürgi Erwin Fischer 《Helvetica chimica acta》1987,70(1):158-170
The bidentate diphosphine ligand, 3,3′-oxybis[(dipenylphosphino)methylbenzene] ( 1 ) forms monomeric, trans-square-planar complexes MX2( 1 ) (M = Ni, Pd, Pt; X = Cl?, Br? I?, and, in part, N, NCS?, CN?, NO) as well as Pt(H)Cl( 1 ), Pt(H)Br( 1 ), and RhCl(CO)( 1 ). Polymeric species have been observed with substitutionally inert metal centres: trans-[PtCl2( 1 )]2 and cis-[PtCl2( 1 )]n (mean value of n ≈ 4–5) 31P-NMR, and selected IR and UV/VIS parameters are reported. Ligand 1 shows a marked preference for trans-spanning and monomeric chelate formation, despite its various degrees of freedom of internal rotation in the lignad backbone. The readily available ligand 1 as well as analogues with other donor atoms, therefore, appear useful in most potential applications of trans-spanning chelate ligands. The crystal structure of AgCl( 1 )·0.5 (CH3)2C?O·0.39 C6H12 (space group C2/c,a = 21.02 Å, b = 14.57 Å, c = 24.79 Å, β = 99.77°, V = 7531.4 Å3, Z = 8) confirms the presence of three-coordinate Ag( I ), with a coordination intermediate between a trigonal-planar and a T-shaped geometry (P-Ag-P = 145.61(8)°). 相似文献
30.
Treatment of 1,2-epoxy-3-ketosteroids with tosylhydrazine and sodium borohydride in methanol lead to mixtures consisting of (i) ring A seco-acetylenic alcohols, (ii) methoxyhydrins formed by opening of the epoxide ring following the carbonyl reduction, and (iii) an epoxy-3,3-di- methylketal. For the corresponding 4-methylene derivative, the ring A fragmentation product was found to be the conjugated en-yne 8a. Reduction of these epoxyketones with sodium borohydride alone affords mixtures of cis- and trans epoxyalcohols; the cis derivatives were unaffected by heating with sodium borohydride in methanol, whereas the trans were converted into the corresponding diaxial methoxyhydrins. 相似文献