158 research outputs found
Guaiacol as a new reagent for the spectrophotometric determination of uranium
Guiacol, i.e. o-hydroxyanisole, gives a distinct color reaction with U(VI)
suitable for spectrophotometric determination of the metal. The complex formed in the reaction has an
absorption maximum at 352 nm. Optimum pH for the color development ranges from 6.5 to 8.5. The molar
absorptivity and Sandell's sensitivity of the method were found to be 3.75×103
l.mol-1.cm-1 and 0.063 μg.cm-2, respectively. Many anions and
cations do not interfere up to 100 ppm. The method has been made very specific by selective
extraction of U(VI) with TBP from a mixture of different cations and anions in the presence of 60%
NH4NO3 as salting out agent followed by developing the color in the non-aqueous
phase by adding quaiacol in methanol at pH 6.5 to 8.5 An amount as low as 30 μg of uranium (VI)
per 10 ml of the solution could be satisfactorily determined with an RSD of ±2.0%. The method
was applied to rock samples after U(VI) had been extracted from a sample solution into 25% TBP in
hexane. Results obtained by the new method compare very well with those of conventional fluorimetric
and radiometric assays. The features of the method include excellent precision, rapidity, good
selectivity, and ease of performance
Tuning ion coordination preferences to enable selective permeation
Potassium (K-) channels catalyze K+ ion permeation across cellular membranes
while simultaneously discriminating their permeation over Na+ ions by more than
a factor of a thousand. Structural studies show bare K+ ions occupying the
narrowest channel regions in a state of high coordination by all 8 surrounding
oxygen ligands from the channel walls. As in most channels, the driving force
for selectivity occurs when one ion is preferentially stabilized or
destabilized by the channel compared to water. In the common view of mechanism,
made vivid by textbook graphics, the driving force for selectivity in K-
channels arises by a fit, whereby the channel induces K+ ions to leave water by
offering an environment like water for K+, in terms of both energy and local
structure. The implication that knowledge of local ion coordination in a liquid
environment translates to design parameters in a protein ion channel, producing
similar energetic stabilities, has gone unchallenged, presumably due in part to
lack of consensus regarding ion coordination structures in liquid water.
Growing evidence that smaller numbers and different arrangements of ligands
coordinate K+ ions in liquid water, however, raises new questions regarding
mechanism: how and why should ion coordination preferences change, and how does
that alter the current notions of ion selectivity? Our studies lead to a new
channelcentric paradigm for the mechanism of K+ ion channel selectivity.
Because the channel environment is not liquid-like, the channel necessarily
induces local structural changes in ion coordination preferences that enable
structural and energetic differentiation between ions.Comment: Main manuscript: 12 pages, 6 figures. Supplementary information: 10
pages, 7 figure
Consequences of local gauge symmetry in empirical tight-binding theory
A method for incorporating electromagnetic fields into empirical
tight-binding theory is derived from the principle of local gauge symmetry.
Gauge invariance is shown to be incompatible with empirical tight-binding
theory unless a representation exists in which the coordinate operator is
diagonal. The present approach takes this basis as fundamental and uses group
theory to construct symmetrized linear combinations of discrete coordinate
eigenkets. This produces orthogonal atomic-like "orbitals" that may be used as
a tight-binding basis. The coordinate matrix in the latter basis includes
intra-atomic matrix elements between different orbitals on the same atom.
Lattice gauge theory is then used to define discrete electromagnetic fields and
their interaction with electrons. Local gauge symmetry is shown to impose
strong restrictions limiting the range of the Hamiltonian in the coordinate
basis. The theory is applied to the semiconductors Ge and Si, for which it is
shown that a basis of 15 orbitals per atom provides a satisfactory description
of the valence bands and the lowest conduction bands. Calculations of the
dielectric function demonstrate that this model yields an accurate joint
density of states, but underestimates the oscillator strength by about 20% in
comparison to a nonlocal empirical pseudopotential calculation.Comment: 23 pages, 7 figures, RevTeX4; submitted to Phys. Rev.
Multiband tight-binding theory of disordered ABC semiconductor quantum dots: Application to the optical properties of alloyed CdZnSe nanocrystals
Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a
broad range of applications, as their spectrum and thus their excitation gap
can be tailored by variation of their size. Additionally, nanocrystals of the
type ABC can be realized by alloying of two pure compound semiconductor
materials AC and BC, which allows for a continuous tuning of their absorption
and emission spectrum with the concentration x. We use the single-particle
energies and wave functions calculated from a multiband sp^3 empirical
tight-binding model in combination with the configuration interaction scheme to
calculate the optical properties of CdZnSe nanocrystals with a spherical shape.
In contrast to common mean-field approaches like the virtual crystal
approximation (VCA), we treat the disorder on a microscopic level by taking
into account a finite number of realizations for each size and concentration.
We then compare the results for the optical properties with recent experimental
data and calculate the optical bowing coefficient for further sizes
Effect of graphene oxide loading on TiO2: Morphological, optical, interfacial charge dynamics-A combined experimental and theoretical study
Ab-initio molecular dynamical study of a single transition metal atom on fullerene C
We report the first-principles Car-Parrinello molecular dynamics study of the
behaviour of a single transition metal Ta atom on fullerene C60, at different temperatures, and for both neutral and charged clusters. We seek to characterise the
motion of the lone Ta metal atom on the C60 surface, contrasting its behaviour both with that of three Ta atoms, as well as with a single alkali metal atom on the cage surface.
Our earlier simulations on C60Ta3 had revealed that the Ta atoms on the surface of
the fullerene are affected by a rather high mobility, and that the motion of these atoms is highly correlated due to Ta-atom-Ta-atom attraction. Earlier, experimental studies of a single metal atom (K, Rb) on the surface of a C60 molecule had led to the
inference that at room temperature the metal atom skates freely over the surface, the first direct evidence for which was presented by us in earlier first principles molecular dynamical simulations
Ab-initio molecular dynamical study of a single transition metal atom on fullerene C60: the case of Ta
Maxwell-Stefan Diffusion And Dynamical Correlation In Molten LiF-KF: A Molecular Dynamics Study
In this work our main objective is to compute Dynamical correlations, Onsager coefficients and Maxwell-Stefan (MS) diffusivities for molten salt LiF-KF mixture at various thermodynamic states through Green-Kubo formalism for the first time. The equilibrium molecular dynamics (MD) simulations were performed using BHM potential for LiF-KF mixture. The velocity autocorrelations functions involving Li ions reflect the endurance of cage dynamics or backscattering with temperature. The magnitude of Onsager coefficients for all pairs increases with increase in temperature. Interestingly most of the Onsager coefficients has almost maximum magnitude at the eutectic composition indicating the most dynamic character of the eutectic mixture. MS diffusivity hence diffusion for all ion pairs increases in the system with increasing temperature. Smooth variation of the diffusivity values denies any network formation in the mixture. Also, the striking feature is the noticeable concentration dependence of MS diffusivity between cation-cation pair, DLi-K which remains negative for most of the concentration range but changes sign to become positive for higher LiF concentration. The negative MS diffusivity is acceptable as it satisfies the non-negative entropy constraint governed by 2nd law of thermodynamics. This high diffusivity also vouches the candidature of molten salt as a coolant
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