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701.
Janet E. Del Bene 《Chemical physics letters》1979,64(2):227-229
Ab initio SCF calculations have been performed to investigate the structural and electronic features of the interactions of H+ and Li+ with the oxygen bases H2CO and OH?. The data indicate that the OH interactions are primarily covalent while the LiO interactions are primarily electrostatic, although the LiO interaction in LiOH has considerable covalent character. 相似文献
702.
Out-of-center "primary" electronic distortions are inherent to the oxide fluoride anions of the early d0 transition metals. In the [NbOF5]2- anion, the Nb5+ moves from the center of the octahedron toward the oxide ligand to form a short Nb=O bond and long trans Nb-F bond. The combined results of single-crystal X-ray diffraction and electronic structure calculations indicate that the primary distortion of the [NbOF5]2- anion is affected by the coordination environment that is created by the three-dimensional extended structure. The formation of bonds between an M(L)4(2+) (M = Cd2+, Cu2+; L = 3-aminopyridine, 4-aminopyridine) cation and the oxide and/or trans-fluoride ligands of the [NbOF5]2- anion weakens the pi component of the Nb=O bond. At the same time, hydrogen bond interactions between the equatorial fluorides and the aminopyridine groups both lengthen the equatorial Nb-F bonds and can further reduce the symmetry of the [NbOF5]2- anion. These combined three-dimensional bond network interactions that serve to lengthen the Nb=O bond and thereby decrease the primary distortion of the [NbOF5]2- anion are illustrated in the structures of three new niobium oxide fluoride phases, [4-apyH]2[Cu(4-apy)4(NbOF5)2] (4-apy = 4-aminopyridine), Cd(3-apy)4NbOF5 (3-apy = 3-aminopyridine), and Cu(3-apy)4NbOF5, that were synthesized and characterized using X-ray diffraction. Crystal data for [4-apyH]2[Cu(4-apy)4(NbOF5)2]: tetragonal, space group /4(1)/ acd (No. 142), with a = 20.8745(8) A, c = 17.2929(9) A, and Z= 8. Cd(3-apy)4NbOF5: tetragonal, space group P4(3) (No. 78), with a = 8.4034(4) A, c = 34.933(3) A, and Z = 4. Cu(3-apy)4NbOF5: monoclinic, space group P2(1)/n (No. 14), with a = 8.822(1) A, b = 16.385(3) A, c = 8.902(1) A, beta = 109.270(3) degrees, and Z = 2. 相似文献
703.
Trost BM Gunzner JL Dirat O Rhee YH 《Journal of the American Chemical Society》2002,124(35):10396-10415
The total synthesis of the novel antitumor agent callipeltoside A, as well as several analogues, is accomplished and allows assignment of the stereochemistry not previously established. A convergent strategy is employed wherein the target is dissected into three units-the core macrolactone, the sugar callipeltose, and a cyclopropyl bearing chain. The strategy for the synthesis of the macrolactone derives from employment of diastereoselective aldol reactions that emanate from an 11 carbon piece. The stereochemistry of the latter derives from the chiral pool and two asymmetric reactions-a ketone reduction using CBS-oxazaborolidine and a Pd catalyzed asymmetric allylic alkylation (AAA). The novelty of the latter protocol is its control of regioselectivity as well as absolute configuration. The trisubstituted olefin is generated using an alkene-alkyne coupling to create a trisubustituted olefin with complete control of geometry. The excellent chemo- and regioselectivity highlights the synthetic potential of this new ruthenium catalyzed process. The macrolactonization employs in situ formation of an acylketene generated by the thermolysis of a m-dioxolenone. Two strategies evolved for attachment of the side chain-one based upon olefination and a second upon olefin metathesis. The higher efficiency of the latter makes it the method of choice. A novel one pot olefin metathesis-Takai olefination protocol that should be broadly applicable is developed. The sugar is attached by a glycosylation by employing the O-trichloroacetimidate. This route provided both C-13 epimers of the macrolactone by using either enantiomeric ligand in the Pd AAA reaction. It also provided both trans-chlorocyclopropane diastereomers of callipeltoside A which allows the C-20 and C-21 configuration to be established as S and R, respectively. The convergent nature of the synthesis in which the largest piece, the macrolatone, require only 16 linear steps imparts utility to this strategy for the establishment of the structure-activity relationship. Initial biological testing demonstrates the irrelevance of the chloro substituent and the necessity of the sugar. 相似文献
704.
Nicolaou MG Wolfe JL Schowen RL Borchardt RT 《The Journal of organic chemistry》1996,61(19):6633-6638
The chemical stability studies of amides of 3-(3',6'-dioxo-2',4'-dimethyl-1',4'-cyclohexadienyl)-3,3-dimethylpropionic acid (Qa) [Qop(a-j)] were conducted in order to determine the utility of this redox-sensitive system as a potential prodrug promoiety or redox-sensitive protecting group in organic synthesis. This study showed that quinone propionic amides of aniline derivatives [Qop(a-d)] underwent rapid degradation in mildly acidic conditions (pH 4.5-6.0) to yield degradation products resulting from the intramolecular 1,2- or 1,4- conjugate addition of the amide nitrogen to the quinone ring. This conjugate addition was found to be specific base-catalyzed and independent of the para substituent on the aromatic ring of the amine. The predominant route of degradation yielded a five-membered ring spirolactam. By altering the nature of the amine component of the amide, these degradation reactions were prevented. Amides of Qa other than those of the aniline type [Qop(e-j)] were found to be substantially more stable and were thus proposed as the more suitable candidates for this potential redox-sensitive prodrug system and redox-sensitive protecting group for amines and alcohols in organic synthesis. 相似文献
705.
Del Bene JE Elguero J Alkorta I Mó O Yañez M 《The journal of physical chemistry. A》2005,109(10):2350-2355
Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond ((1)J(X)(-)(H) and (1h)J(H)(-)(Y)) and two-bond ((2h)J(X)(-)(Y)) spin-spin coupling constants in AH:XH:YH(3) complexes, where A and X are (19)F and (35)Cl and Y is either (15)N or (31)P. The changes in both one- and two-bond spin-spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X-H-Y hydrogen bond. The proton-shared character of the X-H-Y hydrogen bond increases in the order XH:YH(3) < ClH:XH:YH(3) < FH:XH:YH(3). This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X-Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X-H and X-Y spin-spin coupling constants ((1)K(X)(-)(H) and (2h)K(X)(-)(Y)) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. (1h)K(H)(-)(Y) is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH(3) and FH:FH:NH(3). In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in (1h)K(H)(-)(Y). The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH(3) rather than PH(3) is the proton acceptor. 相似文献
706.
Janet Heine Barnett 《Periodica Mathematica Hungarica》1995,30(1):27-36
It is consistent that $\kappa \to (\kappa ,{\text{ }}\left( {_{\omega _1 }^\alpha } \right))^2 $ holds in the random extension. 相似文献
707.
Mino R. Caira Luigi R. Nassimbeni Janet L. Scott Fumio Toda 《Journal of chemical crystallography》1994,24(8):545-552
The crystal structures and thermal analyses of the host compound, N,N,N,N-tetracyclohexylfumaride, witho-cresol,p-cresol and water are reported and compared to that of them-cresol complex previously reported in an attempt to elucidate the reasons for host selectivity. 相似文献
708.
Robert W. Reed Zuowei Xie Janet Manning Rajeev Mathur Robert Bau Christopher A. Reed 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract The brominated carborane, Br6-CB11 H6- (structure 1) is the most recent candidate for the least coordinating anion. It has wide application to the chemistry of reactive cations. The closest approach to date to the long sought silylium ion, R3Si+, utilized this anion in the structurally characterized compound (i-Pr3Siδ+)(Br6-CB11H6 δ?)1 (structure 2). 相似文献
709.
Smith and Weissman introduced a M4 class of processes which are very flexible models for temporally dependent multivariate extreme value processes. However all variables in these M4 models are asymptotically dependent and what this paper does is to extend this M4 class in a number of ways to produce classes of models which are also asymptotically independent. We shall study properties of the proposed models. In particular, asymptotic dependence indexes, coefficients of tail dependence, and extremal indexes are derived for each case. 相似文献