Search results for "CONSTANT"
showing 10 items of 1718 documents
Berechnung der viskositätszahl und der diffusionskonstante hochpolymerer stoffe mit hilfe eines stäbchenmodells
1953
Die Viskositatszahl linearer Hochpolymerer wird nach zwei Kugelmodellen und einem Stabchenmodell berechnet. Es zeigt sich, das ein kugelsymmetrischer Korper die Reibungsverhaltnisse geknauelter Fadenmolekule nicht richtig erfassen kann. Auf Grund eines Stabchenmodells, dessen Lange der grosten Ausdehnung des Knauels (1,4 hst) entspricht, wird die Gleichung. abgeleitet, in welcher die Konstante Φ2 den universellen Wert 1,3.1023 besitzt, awahrend Φ1 von speziellen hydrodynamischen Verhaltnissen im Knauel abhangt. Bei hoheren Molekulargewichten ist Φ1 ein annahernd konstanter Bruchteil des 2. Terms, so das die vereinfachte Gleichung mit Φ′ = 1,62.1023 annahernde Gultigkeit besitzt. Die Absolut…
Über die Verzweigung des Polyvinylacetats. III. Bestimmung des Verzweigungsgrades und reaktionskinetischer Konstanten durch Messung der Mittelwerte d…
1964
Eine Serie von 11 Polyvinylacetaten wurde unter konstanten Reaktionsbedingungen bis zu verschiedenen Umsatzen polymerisiert. An diesen Praparaten wurden osmotische Messungen und Streulichtmessungen durchgefuhrt. Die Messungen liefern das Zahlen- und das Gewichtsmittel der Polymeren. Aus der Anderung der Mittelwerte mit steigendem Umsatz werden die Ubertragungskonstante Cp zum Polymeren und die Wachstumskonstante kN der endstandigen Doppelbindung berechnet. Die erhaltenen Zahlenwerte sind fur 60°C: Cp = 1,8.10−4, kN/kW = 0,80. Unter Benutzung dieser Zahlenwerte wurde die Zahl der Verzweigungspunkte pro Polymermolekul als Funktion des Umsatzes berechnet. A series of 11 polyvinylacetates was p…
Quantum dynamics of 16O + 36O2 and 18O + 32O2 exchange reactions
2015
We present quantum dynamical investigations of (16)O + (36)O2 and (18)O + (32)O2 exchange reactions using a time-independent quantum mechanical method and an accurate global potential energy surface of ozone [Dawes et al., J. Chem. Phys. 135, 081102 (2011)]. Initial state-selected integral cross sections, rate constants, and Boltzmann averaged thermal rate constants are obtained and compared with earlier experimental and theoretical results. The computed thermal rate constants for the oxygen exchange reactions exhibit a negative temperature dependence, as found experimentally. They are in better agreement with the experiments than the previous studies on the same reactions.
Rietveld Refinement of Tetragonal V4+–ZrO2 Solid Solutions Obtained from Gels by X-ray Powder Diffraction
2002
Abstract The crystal structure of three tetragonal V x Zr 1− x O 2 solid solutions, with x =0.025, 0.05, and 0.075, prepared by heating dried gel precursors at 450°C in air atmosphere, have been determined by Rietveld refinement on the basis of powder X-ray powder diffractometer data. They contain V 4+ cations surrounded by eight oxygens, four at a distance between 2.079 and 2.093 A and another four at longer distances between 2.369 and 2.348 A. The estimation of the crystal average oxygen position from the X-ray lattice parameter of V x Zr 1− x O 2 conform with the relationship proposed by Howard et al. ( J. Am. Ceram. Soc. 81, 241 (1998)).
Kinetic modeling of ion conduction in KcsA potassium channel.
2005
KcsA constitutes a potassium channel of known structure that shows both high conduction rates and selectivity among monovalent cations. A kinetic model for ion conduction through this channel that assumes rapid ion transport within the filter has recently been presented by Nelson. In a recent, brief communication, we used the model to provide preliminary explanations to the experimental current-voltage J-V and conductance-concentration g-S curves obtained for a series of monovalent ions (K(+),Tl(+), and Rb(+)). We did not assume rapid ion transport in the calculations, since ion transport within the selectivity filter could be rate limiting for ions other than native K(+). This previous wor…
On the Formation of Droperidol Solvates: Characterization of Structure and Properties
2014
A solvate screening and characterization of the obtained solvates was performed to rationalize and understand the solvate formation of active pharamaceutical ingredient droperidol. The solvate screening revealed that droperidol can form 11 different solvates. The analysis of the crystal structures and molecular properties revealed that droperidol solvate formation is mainly driven by the inability of droperidol molecules to pack efficiently. The obtained droperidol solvates were characterized by X-ray diffraction and thermal analysis. It was found that droperidol forms seven nonstoichiometric isostructural solvates, and the crystal structures were determined for five of these solvates. To b…
Reaction of complex ligands: Part 95. Chromium tricarbonyl complexes of polysubstituted naphthohydroquinones: regioselective synthesis via [3+2+1]-be…
2003
Abstract A series of polysubstituted naphthohydroquinoid tricarbonyl chromium complexes were prepared by chromium mediated [3+2+1]-benzannulation of Fischer-type carbene complexes with alkynes. The kinetics and the thermodynamic data of the η6-η6-haptotropic rearrangements were investigated by in-situ NMR spectroscopy. The free activation energies ΔG# range from 23 to 26 kcal mol−1 and only slightly depend on the bulk and the donor/acceptor properties of the substituents. Equilibrium of metal migration was observed for naphthohydroquinone complexes bearing methoxy substituents at the non-hyroquinoid ring (ring A). In complexes bearing methyl groups or only hydrogen substituents at ring A th…
Organometallic nucleophiles. Mechanism of halide displacement at saturated carbon by 2-pyridyl and 4-Pyridyl complexes [M(dmtc)(C5H4N-Cn)(L)] (M Pd…
1992
Abstract A mechanistic study is reported of nucleophilic halide substitution by pyridyl complexes [M(dmtc)C 5 H 4 N- C 2 )(L)] (M Pd or Pt; L PMe 3 , PEt 3 or PPh 3 ) and [Pd(dmtc)C 5 H 4 N- C 4 )(L)] (L PMe 3 or PPh 3 ) on organic halides XCH 2 R (X Cl or Br; R CHCH 2 , COMe, Ph, or CN) in various solvents, yielding the pyridylium derivatives [M(dmtc)1-CH 2 R)C 5 H 4 N- C 2 (L)] + and [Pd(dmtc)(1-CH 2 R)(C 5 H 4 N- C 4 (L)] + , respectively. The kinetics obey a second-order rate law: rate k 2 [XCH 2 R][Complex]. A similar rate law is observed for the analogous reactions involving 4-dimethyl-aminopyridine (4-dmapy) as the nucleophile. The effects of solvent and leaving group, a…
A supramolecular heteropolyoxopalladate {Pd15} cluster host encapsulating a {Pd2} dinuclear guest: [Pd(II)2⊂{H7Pd(II)15O10(PO4)10}](9-).
2011
A high-nuclearity polyoxopalladate compound, [Pd(II)(2)⊂{H(7)Pd(II)(15)O(10)(PO(4))(10)}](9-) {Pd(II)(17)}, comprising a {Pd(15)} host occupied by a {Pd(2)} guest and the parent pristine "empty" [H(7)Pd(II)(15)O(10)(P(V)O(4))(10)](13-) {Pd(15)} cluster have both been prepared and characterized by single-crystal X-ray crystallography, (31)P NMR, CSI-MS, and XPS. The encapsulated {Pd(2)} has a short Pd(II)-Pd(II) distance within the {Pd(15)} host. Solution studies indicate that the empty host and filled guest complex are in equilibrium with each other, and UV titrations revealed a binding constant of ca. 10(3) for the guest Pd(II) ions, with a binding stoichiometry of almost 2.
Classical and quantum aspects of electric-magnetic duality rotations in curved spacetimes
2018
It is well known that the source-free Maxwell equations are invariant under electric-magnetic duality rotations, $\mathrm{F}\ensuremath{\rightarrow}\mathrm{F}\mathrm{cos}\ensuremath{\theta}+^{\ensuremath{\star}}\mathrm{F}\mathrm{sin}\ensuremath{\theta}$. These transformations are indeed a symmetry of the theory in the Noether sense. The associated constant of motion is the difference in the intensity between self-dual and anti-self-dual components of the electromagnetic field or, equivalently, the difference between the right and left circularly polarized components. This conservation law holds even if the electromagnetic field interacts with an arbitrary classical gravitational background.…