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61.
62.
Alex. Polizu und Hildegard Greger 《Fresenius' Journal of Analytical Chemistry》1966,223(4):247-260
Zusammenfassung Die oxydierende Wirkung, die drei verschiedene Oxydationsmittel [Kaliumdichromat, Cer(IV)-sulfat und Vanadium(V)-oxid] auf die zu Diaminophenolen reduzierten Dinitrophenole (2,4-Dinitrophenol, 2-Methyl-4,6-dinitrophenol und 2-sek.-Butylphenol) ausüben, wird untersucht.Die Untersuchungen erstrecken sich in allen Fällen sowohl auf die Reduktions- als auch auf die Oxydationsphase. Die Form der Spektralkurven und die zeitliche Entwicklung der Farben werden beschrieben. Es zeigte sich, daß Vanadium(V)-oxid den anderen Oxydationsmitteln überlegen ist.
Summary A comparative study is presented on the oxydative effect of potassium dichromate, cerium(IV)-sulphate and vanadic acid on reduced dinitrophenols to form diaminophenols (2,4-dinitrophenol, 2-methyl-4,6-dinitrophenol and 2-sec. butyl-4,6-dinitrophenol).The reduction and oxydation are studied for each oxidizing agent and for each dinitro compound. They are followed by spectral curves and by the control of time stability of the colour. The data obtained show that vanadic acid is superior to the other oxidants.相似文献
63.
Wästerby P Orädd G Lindblom G 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2002,157(1):156-159
The anisotropy, D(parallel)/D( perpendicular ), of water diffusion in fully hydrated bilayers of dimyristoylphosphatidylcholine at 29 degrees C has been measured by pulsed magnetic field gradient (pfg) NMR. By using NMR imaging hardware to produce magnetic field gradients in an arbitrary direction with respect to a stack of macroscopically aligned lipid bilayers, translational diffusion of water was measured as a function of the angle between the direction of the magnetic field gradient and the normal of the lipid membrane. The observed diffusion coefficient is found to depend strongly on this angle. The anisotropy cannot be accurately determined due to the very small value of D( perpendicular ), but a lower limit of about 70 can be estimated from the observed diffusion coefficients. The results are discussed in terms of the relatively low permeability of water across the lipid bilayer, instrumental limitations, and/or possible defects in the lamellae. 相似文献
64.
[reaction: see text] Transient silenes that are strongly influenced by reversed Si[double bond]C bond polarization are formed upon heating of tris(trimethylsilyl)silylamides. The silenes are trapped with 2,3-dimethyl-1,3-butadiene to quantitatively yield only one of the possible diastereomers of the functionalized cyclic allylsilanes. 相似文献
65.
Otmar Hofer Ulrike G. Wagner Harald Greger 《Monatshefte für Chemie / Chemical Monthly》1988,119(10):1143-1153
The crystal structures of two diastereomeric tetrahydrofurofuran lignans, diasesartemin (1) and episesartemin-B (2), were determined. The former one is of the classical axial-axial substitution type (the two aryl moieties of the 1,4-diaryl-tetrahydro-1H,3H-furo[3,4-c]furan in ax-ax position), the latter one belongs to the axial-equatorial type. Analysis of the X-ray results revealed that in both cases the substituents are attached in pseudo-equatorial positions due to a flip of the (slightly twisted) envelope conformation of the oxolane five-rings. In1 the oxygen atoms of the two five rings point against each other (overall boat conformation of the tetrahydrofurofuran system orendo-endo topology), in2 one ring is folded towards and the other one is folded away from the second ring (overall boat-chair conformation of the tetrahydrofurofuran system orendo-exo topology): as a consequence,all aryl substituents adopt a pseudo-equatorial position. The terms ax-ax, ax-eq, and eq-eq used throughout in the literature are therefore misleading as far as the true conformations are concerned. Analysis of the aromatic short wavelength CD couplet of1 by means of the coupled oscillator model shows that the X-ray derived geometry is compatible with the CD data in solution.
Röntgenstrukturanalyse von Tetrahydrofurofuran-Lignanen
Zusammenfassung Es wurde die Kristallstruktur von zwei diastereomeren Tetrahydrofurofuran-Lignanen, Diasesartemin (1) und Episesartemin-B (2), bestimmt. Ersteres gehört dem klassischen axial-axial-substituierten Typ an (dabei sind die zwei Arylsubstituenten des 1,4-Diaryl-tetrahydro-1H,3H-furo[3,4-c]furans in ax-ax-Position), das zweite gehört zum axial-equatorial-Typ. Die Röntgenstrukturanalyse zeigte, daß die Aryl-Substituenten immer in pseudo-equatorialen Positionen stehen, da die Oxolan-Fünfringe eine entsprechende (leicht getwistete) Briefumschlag-Konformation einnehmen. In1 zeigen die Sauerstoffatome der beiden Ringe zueinander (insgesamt eine Boot-Konformation des Tetrahydrofurofuransystems bzw.endo-endo-Topologie), in2 ist ein Ring in Richtung zum, der andere Ring jedoch weg vom jeweils zweiten gegenüberliegenden gefaltet (insgesamt eine Boot-Sessel-Anordnung in der Tetrahydrofurofuran-Einheit bzw.endo-exo-Topologie): Eine Folge davon ist, daß alle Arylsubstituenten eine pseudo-equatoriale Position einnehmen. Die in der Literatur durchgehend verwendete Bezeichnung ax-ax, ax-eq oder eq-eq ist daher bezüglich der tatsächlichen Konformation mißverständlich. Die Analyse der kurzwelligen aromatischen CD-Banden von1 mittels des Modells der gekoppelten Oszillatoren zeigte, daß die Konformation aus der Röntgenstrukturanalyse mit den CD-Daten in Lösung in Übereinstimmung ist.相似文献
66.
Otmar Hofer Géza Szabó Harald Greger 《Monatshefte für Chemie / Chemical Monthly》1986,117(10):1219-1222
The isolation of (E)-2-hydroxy-4-methoxycinnamic acid (1) fromArtemisia dracunculus L. supports strongly the assumption that this compound is an intermediate in the biosynthesis of 7-methoxycoumarin (herniarin,3). The structure of the UV-unstable compound1 was derived from1H-NMR data and by comparison of the stable dimethylated derivative with synthetic (E)-2,4-dimethoxycinnamic acid methyl ester (2).Dedicated to Prof. Dr.Kurt L. Komarek on the occasion of his 60th birthday. 相似文献
67.
Johann Schinnerl Elisabeth Kaltenegger Thomas Pacher Srunya Vajrodaya Otmar Hofer Harald Greger 《Monatshefte für Chemie / Chemical Monthly》2005,136(9):1671-1680
Summary. Three new pyrroloazepine type alkaloids, stichoneurines A and B and 6-hydroxycroomine were isolated from the lipophilic root extracts of Stichoneuron caudatum and Stemona tuberosa collected in Thailand together with the already known croomine, tuberostemonine, and tuberostemonine A. The structures were elucidated by spectroscopic methods including H/H-COSY, HMQC, and HMBC. Information on the relative stereochemistries and conformational behaviour was obtained by analysis of the NOESY spectra. The formation of pyrroloazepine alkaloids in the genus Stichoneuron is reported for the first time and supports its affiliation to the family Stemonaceae. The occurrence of two different types of alkaloids, of the tuberostemonine and croomine series, in different geographical provenances of Stemona tuberosa is of special chemosystematic interest and may contribute to a more natural species delimitation within that complex group. 相似文献
68.
Nolting D Ottosson N Faubel M Hertel IV Winter B 《Journal of the American Chemical Society》2008,130(26):8150-8151
The photoelectron spectra of aqueous imidazole are presented, and the N 1s and C 1s binding energies are assigned aided by density functional theory calculations. The chemical equivalency of the two nitrogens of the cationic form is directly identified by the occurrence of a single N 1s photoelectron peak, which results from the delocalization of the positive charge over the molecule as a consequence of the Cv symmetry of the system. In contrast to NMR measurements, the photoemission process is faster than the rapid proton exchange in the aqueous environment, making the pseudoequivalent nitrogens of the neutral state clearly distinguishable with a N 1s binding energy shift of 1.7 eV. 相似文献
69.
Bonhommeau S Ottosson N Pokapanich W Svensson S Eberhardt W Björneholm O Aziz EF 《The journal of physical chemistry. B》2008,112(40):12571-12574
The local electronic structure of Fe(III) and Fe(II) ions in different alcohol solutions (methanol, ethanol, propan-1-ol) is investigated by means of soft X-ray absorption spectroscopy at the iron L 2,3-edge. The experimental spectra are compared with ligand field multiplet simulations. The solvated Fe(III) complex is found to exhibit octahedral symmetry, while a tetragonal symmetry is observed for Fe(II). A decrease in the solvent polarity increases the charge transfer from the oxygen of the alcohol to the iron ions. This conclusion is supported by Hartree-Fock calculations of the Mulliken charge distribution on the alcohols. A larger charge transfer is further observed from the solvent to Fe(III) compared to Fe(II), which is connected to the higher positive charge state of the former. Finally, iron ions in solution are found to prefer the high-spin configuration irrespective of their oxidation state. 相似文献
70.
Sangeeta Yadav Ouissam ElBakouri Kjell Jorner Hui Tong Christian Dahlstrand Miquel Sol Henrik Ottosson 《化学:亚洲杂志》2019,14(10):1870-1878
Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed‐shell singlet state (normally ground state, S0) and lowest ππ* triplet state (T1 or T0), as given by Hückel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T1 and S0 states. This effect is caused by a reversal in the dipole moment when going from S0 to T1 as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S0 reduces (enhances) aromaticity in T1, allowing for rationalizations of the triplet state energies (ET) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted ET. As a means to increase the π‐electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6‐dicyanopentafulvenes can be a route to on‐demand formation of a fulvenium dication with a triplet ground state (T0). The five‐membered ring of this species is markedly Baird‐aromatic, although less than the cyclopentadienyl cation known to have a Baird‐aromatic T0 state. 相似文献