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1.
Previous studies using dideuterium as a solute have demonstrated the importance for orientation of the interaction between the solute molecular quadrupole moment and the average electric field gradient present in liquid crystals. With the aim of learning about additional orientational mechanisms, we have studied the temperature dependence of the 2H-NMR spectra of the liquid crystal 5CB-d19 as the solute in three liquid crystal mixtures: 55 wt % 1132/EBBA, 56·5 wt % 1132/EBBA and 70 wt % 5CB/EBBA. In these mixtures, the contribution from the environment to the average electric field gradient at the 2H nucleus of dideuterium is zero. The spectra of 5CB-d19 in the mixtures 55 wt % 1132/EBBA and 56·5 wt % 1132/EBBA are equivalent, but are different from those in 70 wt % 5CB/EBBA. The spectra of 5CB in 55 wt % 1132/EBBA and 70 wt % 5CB/EBBA are analysed using two different models for the short range potential, and param eters of the models are used to compare the potentials in the different mixtures. For a given spectral splitting of the chain C1 deuteron, the reduced short range potential is the same in all three mixtures studied. The spectral differences observed are a consequence of different nematic-isotropic phase transition temperatures combined with the effect of trans/gauche-isomerization in the hydrocarbon chain. 相似文献
2.
The molecular structure of TTF dissolved in nematic liquid crystalline solvents has been determined from the proton magnetic resonance including couplings due to 13C in natural abundance. The molecule is puckered in a boat conformation with the SCHCHS planes making a dihedral angle of 13 ± 2° with the S2C CS2 plane. The other structural parameters obtained are rCH = 1.085 ± 0.014 Å and the angel CCH = 123.7 ± 1.5°. 相似文献
3.
The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D2 and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for hydrogen and several other solutes. 相似文献
4.
From proton magnetic resonance spectra of TTF and TCNQ partially oriented in nematic liquid crystal solvents the ratio of length to width of the proton rectangle is 3.124 ± 0.03 for TTF and 1.765 ± 0.024 for TCNQ. Values of JCH obtained for TTF dissolved in d6-acetone, are JCH = 187.9 ± 0.3 Hz, JCCH = 6.7 ±0.1 Hz and JCSCH = 9.0 ±0.2 Hz. 相似文献
5.
The recent publication by Matsumoto1 of a synthesis of Coleon U2(1) prompts us to present our own preparation of this poly-hydroxy diterpene as the tri-O-methyl (12a) and tetra-O-methyl ethers (12b). In a previous communication3 we outlined our approach which is aimed at several similar natural products such as Coleon C4 (1b), Lycoxanthol5 (2), etc. and differs considerably from that of the Japanese group. Formally at least, the two syntheses start with the same material, (+) ferruginol methyl ether 5a. In our case the latter was prepared by recorded methods from methyl O-methyl podocarpate after introducing the iso-propyl sidechain6 (→4), transforming the C.4 methoxycarbonyl residue to a methyl7 (→5a, ferruginol methyl ether) which was oxidised at the benzylic position of the B ring to give sugiol methyl ether 5b. 相似文献
6.
D.M. Burnell John Zasadzinski R.J. Noer E.L. Wolf G.B. Arnold 《Solid State Communications》1982,41(9):637-641
New experimental estimates are given of the separate electron-phonon (λph) and paramagnon (λS) contributions to mass renormalization for the transition metals Nb and V, from recent tunneling results. Our analysis, which is the first of its kind, is based on the recent demonstration of Daams, Mitrovic, and Carbotte of a simple rescaling of the Eliashberg equations using an effective coupling parameter . The results provide a consistent picture of superconductivity in these transition elements from tunneling, in substantially improved agreement with other measurements. We conclude that spin fluctuations reduce the electron-phonon Tc values in Nb and V by 25% and 45%, respectively. 相似文献
7.
The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, tau, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of tau indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers. 相似文献
8.
Erica S. Burnell Abdur-Rehman Irshad Alan T.L. Lee Eric J. Thomas 《Tetrahedron》2018,74(47):6737-6748
Oxidation of a 15-methylenebicyclo[9.3.1]pentadeca-3,7-dien-14-ol with a 10-phenylsulfonyl substituent provided a 14-oxobicyclo[9.3.1]pentadeca-1(15),3,7-triene-15-carbaldehyde that on reduction with DIBAL-H was converted into a 15-hydroxymethylbicyclo[9.3.1]pentadeca-1(15),3,7-triene-14-ol with the relative configuration at C14 required for incorporation into a synthesis of phomactin A. The oxidation and reduction of an analogous 3,4-epoxide that lacked the 10-phenylsulfonyl group gave a diol with the opposite relative configuration at C14. However, the TPAP oxidation and DIBAL-H reduction of a 14-hydroxy-15-methylene-3,4-epoxide that still had the 10-phenylsulfonyl group gave a diol with the required configuration at C14 although the expected spontaneous participation of the 14-hydroxyl group in an intramolecular epoxide ring-opening was only observed under Lewis acidic conditions. 相似文献
9.
Xidos JD Gosse TL Burke ED Poirier RA Burnell DJ 《Journal of the American Chemical Society》2001,123(23):5482-5488
A computational examination of the four modes of addition in the Diels-Alder reactions of 3-substituted cyclopropene derivatives (substituents: BH(2), CH(3), SiH(3), NH(2), PH(2), OH, SH, F, and Cl) with butadiene have been carried out at the B3LYP/6-31++G(d)//HF/6-31++G(d) level. The degree of stabilization of these derivatives at the ground state correlates with the electronegativity of the substituent. This attenuation of reactivity and differences in steric interactions are the only factors needed to explain both the high facial selectivity and the differences in the endo-exo selectivity seen in these reactions. Furthermore, evidence is presented that indicates that stabilization by an interaction involving the syn C-3 hydrogen of cyclopropene and butadiene is small or irrelevant in controlling the endo-exo selectivity of the Diels-Alder reaction. 相似文献
10.
[reaction: see text] The synthesis of the tetracyclic ring system of the kempane diterpenes was achieved through the highly regio- and stereoselective Diels-Alder reaction of an isopropenyl-diene with 2,6-dimethyl-p-benzoquinone, addition of an allyl group, and ring-closing metathesis of the isopropenyl and allyl groups. 相似文献