710 research outputs found
Theoretical studies of Si and GaAs surfaces and initial steps in the oxidation
Using ab initio quantum chemical methods (generalized valence bond), we examine (i) the electronic states of Si (111) and GaAs (110) surface, (ii) the relaxation of the Si (111) surface, (iii) the reconstruction of the GaAs surface, (iv) the initial steps in the chemisorption of O_2 on Si (111), and (v) the bonding of O atom to Ga and As centers
Persistence to high temperatures of interlayer coherence in an organic superconductor
The interlayer magnetoresistance of the organic metal \cuscn is
studied in fields of up to 45 T and at temperatures from 0.5 K to 30 K. The
peak in seen in in-plane fields, a definitive signature of
interlayer coherence, remains to s exceeding the Anderson criterion for
incoherent transport by a factor . Angle-dependent magnetoresistance
oscillations are modeled using an approach based on field-induced quasiparticle
paths on a 3D Fermi surface, to yield the dependence of the scattering rate
. The results suggest that does not vary strongly over
the Fermi surface, and that it has a dependence due to electron-electron
scattering
Unraveling the Wacker Oxidation Mechanisms
The mechanisms for the aqueous PdCl2 mediated olefin oxidation reaction (the Wacker process) have been studied with density functional theory, with special emphasis on determining competitive pathways that explain the product distribution's dependence on reaction conditions. Surprisingly, our results indicate that the previously suggested inner-sphere rate-determining step for this process is incompatible with the experimental observations. We describe three key steps, all with barriers between 22.7 and 23.3 kcal/mol. These results, together with literature experimental data, were used to construct a model that explains the observations in the Wacker process. We find that the rate-determining step under low [Cl^-] conditions is not hydroxypalladation as generally believed, but intermolecular isomerization after a lower-energy water-catalyzed internal nucleophilic attack. The pathway under high [Cl^-] leading to anti-addition aldehyde products is only accessible when CuCl_2 is available to selectively stabilize associative chloride exchange. The controversial switch in mechanisms is caused by both this selective stabilization from CuCl_2, and the prerequisite dissociation of Cl^- prior to internal attack. Finally, we suggest that the previously published rate expression for the Wacker process under high [Cl^-] is incomplete and should be replaced with a two-term expression, featuring one term first-order and one term second- (or higher) order in [CuCl_2]
Theoretical Studies of Electron Transfer in Metal Dimers: XY+→X+Y, Where X, Y=Be, Mg, Ca, Zn, Cd
The electronic matrix element responsible for electron exchange in a series of metal dimers was calculated using ab initio wave functions. The distance dependence is approximately exponential for a large range of internuclear separations. A localized description, where the two nonorthogonal structures characterizing the electron localized at the left and right sites are each obtained self‐consistently, is found to provide the best description of the electron exchange process. We find that Gaussian basis sets are capable of predicting the expected exponential decay of the electronic interactions even at quite large internuclear distances
A Push-Button Molecular Switch
The preparation, characterization, and switching mechanism of a unique single-station mechanically switchable hetero[2]catenane are reported. The facile synthesis utilizing a “threading-followed-by-clipping” protocol features Cu^(2+)-catalyzed Eglinton coupling as a mild and efficient route to the tetrathiafulvalene-based catenane in high yield. The resulting mechanically interlocked molecule operates as a perfect molecular switch, most readily described as a “push-button” switch, whereby two discrete and fully occupied translational states are toggled electrochemically at incredibly high rates. This mechanical switching was probed using a wide variety of experimental techniques as well as quantum-mechanical investigations. The fundamental distinctions between this single-station [2]catenane and other more traditional bi- and multistation molecular switches are significant
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Measurements of the total CO₂ concentration and total alkalinity in seawater collected during the SONNE Cruise 102, May-June 1995
Oxidative Aliphatic C-H Fluorination with Fluoride Ion Catalyzed by a Manganese Porphyrin
Despite the growing importance of fluorinated organic compounds in drug development, there are no direct protocols for the fluorination of aliphatic C-H bonds using conveniently handled fluoride salts. We have discovered that a manganese porphyrin complex catalyzes alkyl fluorination by fluoride ion under mild conditions in conjunction with stoichiometric oxidation by iodosylbenzene. Simple alkanes, terpenoids, and even steroids were selectively fluorinated at otherwise inaccessible sites in 50 to 60% yield. Decalin was fluorinated predominantly at the C2 and C3 methylene positions. Bornyl acetate was converted to exo-5-fluoro-bornyl acetate, and 5α-androstan-17-one was fluorinated selectively in the A ring. Mechanistic analysis suggests that the regioselectivity for C-H bond cleavage is directed by an oxomanganese(V) catalytic intermediate followed by F delivery via an unusual manganese(IV) fluoride that has been isolated and structurally characterized
Test for interlayer coherence in a quasi-two-dimensional superconductor
Peaks in the magnetoresistivity of the layered superconductor
-(BEDT-TTF)Cu(NCS), measured in fields T applied
within the layers, show that the Fermi surface is extended in the interlayer
direction and enable the interlayer transfer integral (
meV) to be deduced. However, the quasiparticle scattering rate is
such that , implying that
-(BEDT-TTF)Cu(NCS) meets the criterion used to identify
interlayer incoherence. The applicability of this criterion to anisotropic
materials is thus shown to be questionable.Comment: 5 pages, 4 figure
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