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61.
Ronald Breslow David A. Cortés Bernhart Jaun R. David Mitchell 《Tetrahedron letters》1982,23(8):795-798
Chlorination of tricyanocylopropane is observed to lead directly to trichlorotricyanocyclopropane which undergoes a remarkable reductive conversion to hexacyanobenzene. The synthesis of mono- and dichlorotricyanocyclopropane ( and ) and their use as potential precursors for tricyanocyclopropenyl anion is described. 相似文献
62.
Photoelastic coatings were bonded to limestone and granite plates which were then struck on edge with a steel projectile. The resulting fracture zone growth was observed with a reflecting polariscope and recorded with an image converter camera. It is shown that the maximum velocity at which cracks propagate under these circumstances is very much higher than predicted by quasi-steady theories. ‘Apparent’ supervelocity cracks can be generated by shock reflection from bounding interfaces, producing tensile waves of sufficient amplitude to activate and join isolated flaw. Photoelastic coatings are also shown to very much enhance surface-crack visualization subsequent to impact. 相似文献
63.
Frank R. Bernhart 《Journal of Graph Theory》1977,1(3):207-225
A major event in 1976 was the announcement that the Four Color Conjecture (4CC) had at long last become the Four Color Theorem (4CT). The proof by W. Haken, K. Appel, and J. Koch is published in the Illinois Journal of Mathematics, and their two-part article outlines the nature and reliability of the solution. The first section is a readable and informative historical survey. The reminder will appeal chiefly to specialists in graph theory. Although the logic of attack is relatively simple, the need to examine an immense number of individual cases is frustrating. Hopefully this first breakthrough will pave the way for a short elegant proof. For the second section, 1200 hours of computer time was required to verify the 4-color reducibility of nearly 1900 configurations. At this time there is no good way to condense the proof. In this digest we offer an exposition of the main ideas. The first and the last parts are intended for a general audience, but the intermediate sections assume more knowledge of graph theory proper. The usual statement of the 4CC goes as follows: “All maps on the sphere or plane can be colored with four colors so that neighboring regions are never colored alike.” The form in which the 4CT was proved is “There exists an unavoidable set of reducible configurations, relative to triangulations of the plane.” The initial part of our task is to explain how the 4CC comes to be expressed in such jargon. The next step is to show how one finds simple finite sets of unavoidable configurations. Then comes the question of how to prove reducibility, followed by a consideration of the known obstacles to reduction. Our concluding remarks and criticisms include a consideration of prospects for the future. 相似文献
64.
The photochemical formation of bicyclooctenes b from 12,13-didehydrosqualene ( 1 ) upon irradiation at wavelengths longer than 290 nm is reported. The same photoreaction is also observed with trienes 7 and 8 which like 1 have a remote double bond two C-atoms removed from the triene system. A configurational assignment for bicyclooctene 10 obtained from 8 and a mechanism for the observed photochemical reaction are proposed. 相似文献
65.
A methylnickel(II) derivative of coenzyme F430 ( 1 ) was proposed as an intermediate in the enzymic process catalyzed by methyl-CoM reductasc. Indirect evidence points to formation of CH3–F430MII in the reaction of F30M1 (obtained from F430MII ( 2 )) with eleclrophilic methyl donors. The results presented here show, that such a compound does exist. A paramagnetic CD3–NiII derivative 5b of the pentamethyl ester 2 (F430M) of coenzyme F430 was prepared by in situ methylation with (CD3)2Mg and characterized by its isotropically shifted 2H-NMR spectrum. At ?40°, the very broad D-signal of the axially coordinated CD3 group is found at ?490 ppm. Comparison with the 2H- and 1H-NMR spectra of mcthyl(tetramethylcyclam)nickel(II) derivatives 4 ([NiII(CH3))(tmc)]CF3SO3 ( 4a ) is the only isolated CH3–Ni derivative of a N4macrocyclic NiII complex' shows that the large isotropic shift to high field is characteristic for a Me group axially bound to the Ni center. The temperature dependence of the isotropic shift of the CD3–Ni group in both 4b and 5b follows Curie's law and yields 2H hyperfine coupling constants of ?0.65 ( 4b ) and ?0.85 MHz ( 5b ), respectively. The 1H-NMR spectrum indicates that, in contrast to the five-coordinate monochloro complex [NiIICl(tmc)]+, intermolecular exchange of the axial ligand in [NiII(CH3)(tmc)]+ 4a is either slow at the NMR time scale or does not occur at all. 相似文献
66.
Rueping M Mahajan YR Jaun B Seebach D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(7):1607-1615
Two different strategies have been employed for the synthesis of Fmoc-protected beta(3)-homoarginine; the Arndt-Eistert homologation of alpha-arginine and the guanidinylation of beta(3)-homoornithine. Solid-phase beta-peptide synthesis was used for the preparation of beta-heptapeptide 1, which was designed to form a helix stabilized by electrostatic interactions through positively (beta(3)hArg) and negatively charged (beta(3)hGlu) amino acid residues. CD measurements and corresponding NMR investigations in MeOH and aqueous solutions do indeed show that the beta-peptidic 3(14)-helix can be stabilized by salt-bridge formation. 相似文献
67.
Dieter Seebach Karl Gademann JüRg V. Schreiber Jennifer L. Matthews Tobias Hintermann Bernhard Jaun Lukas Oberer Ulrich Hommel Hans Widmer 《Helvetica chimica acta》1997,80(7):2033-2038
β-Hexapeptides 1–5 and a β-dodecapeptide 6 with sequences containing two different types of β-amino acids (aliphatic proteinageous side chains in the 2- or in the 3-position) have been prepared. CD (Fig. 1) and NMR measurements indicate that, with one exception, the secondary structures formed by these new β-peptides differ from those of isomers studied previously. Detailed NMR analysis of the β-hexapeptide 5 (with alternating β2,β3-building blocks) and molecular-dynamics simulations have produced a minimum energy conformation (Fig. 2,b)which might be described as a novel irregular helix containing ten- and twelve-membered H-bonded rings. This demonstrates the great structural variability of β-peptides, since three different helical secondary structures have been discovered to date. 相似文献
68.
Auffrant A Jaun B Jarowski PD Houk KN Diederich F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(12):2906-2911
A variety of 1,1,4,4-tetraal kynylbutatrienes and 1,4-dialkynylbutatrienes was synthezized by dimerization of the corresponding gem-dibromoolefins. Both (1)H and (13)C NMR spectroscopy indicated that the di- and tetraalkynylated butatrienes are formed as a mixture of cis and trans isomers. Variable temperature NMR studies evidenced a facile cis-trans isomerization, thus preventing the separation of these isomers by gravity or high-performance liquid chromatography (HPLC). For 1,1,4,4-tetraalkynylbutatrienes, the activation barrier deltaG( not equal ) was measured by magnetization transfer to be around 20 kcal mol(-1), in the range of the barrier for internal rotation about a peptide bond. Unlike the tetraalkynylated [3]cumulenes, 1,4-dialkynylbutatrienes are more difficult to isomerize and could, in one case, be obtained isomerically pure. Based on experimental data, the rotational barrier DeltaG( not equal ) for 1,4-dialkynylbutatrienes is estimated to be around 25 kcal mol(-1). The hypothesis of a stabilizing effect of the four alkynyl substituents on the proposed but-2-yne-1,4-diyl singlet diradical transition state of this cis-trans isomerization is further supported by a computational study. 相似文献
69.
Andreas Pfaltz Bernhard Jaun Alexander Fassler Albert Eschenmoser Rolf Jaenchen Hans Harald Gilles Gabriele Diekert Rudolf K. Thauer 《Helvetica chimica acta》1982,65(3):828-865
Factor F430 from Methanogenic Bacteria: Structure of the Porphninoid Ligand System A structure is proposed for F430M, a non-cristalline methanolysis product of isolates of the nickel-containing, porphinoid factor F430 from Methanobacterium thermoautotrophicum. Crucial to the structure determination are five incorporation experiments with M. thermoautotrophicum (strain Marburg) in which the specifically mono-13C-labeled biosynthetic precursors (2-13C), (3-13C), (4-13C)-, (5-13C) ALA (ALA = δ-amino-levulinic acid) and L-(methyl-13C)methionine were incorporated into F430 with high efficiency. The 13C-NMR,-spectra of the specifically labeled F430M samples derived therefrom, together with the UV./VIS. spectral data of F430M, contain all the information necessary for the deduction of the constitution of the F430M chromophore, assuming the established pattern of porphinoid biosynthesis to be operative in F430 biosynthesis. 1H-NMR. spectroscopy and, in particular, 1H-NMR.-NOE-difference spectroscopy corroborates and completes the constitutional assignments and, furthermore, makes possible an almost complete derivation of the molecule's relative configuration. Schemes 3 and 4 summarize the results of 1H-NMR. spectroscopy, presenting them within the context of the proposed structure for F430M. The assignment of absolute configuration implied in the formula is given preference because of F430M's very close structural and (assumed) biosynthetic relationship to sirohydrochlorin and vitamin B12 (with respect to ring C, the assignment is based on degradative evidence). According to the proposed structure, the nickel complex F430M possesses an uroporphinoid (Type III) ligand skeleton with an additional carbocyclic ring and a chromophore system not previously encountered among natural porphinoids. It can be considered to be a (tetrahydro) derivative of the corphin system, combining structural elements of both porphyrins and corrins. 相似文献
70.
Finazzo C Harmer J Bauer C Jaun B Duin EC Mahlert F Goenrich M Thauer RK Van Doorslaer S Schweiger A 《Journal of the American Chemical Society》2003,125(17):4988-4989
Methyl-coenzyme M reductase (MCR) catalyzes the reaction of methyl-coenzyme M (CH3-S-CoM) with coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. At the active site, it contains the nickel porphinoid F430, which has to be in the Ni(I) oxidation state for the enzyme to be active. How the substrates interact with the active site Ni(I) has remained elusive. We report here that coenzyme M (HS-CoM), which is a reversible competitive inhibitor to methyl-coenzyme M, interacts with its thiol group with the Ni(I) and that for interaction the simultaneous presence of coenzyme B is required. The evidence is based on X-band continuous wave EPR and Q-band hyperfine sublevel correlation spectroscopy of MCR in the red2 state induced with 33S-labeled coenzyme M and unlabeled coenzyme B. 相似文献