95,257 research outputs found
The Functional Form of Angular Forces around Transition Metal Ions in Biomolecules
A method for generating angular forces around -bonded transition
metal ions is generalized to treat -bonded configurations. The theoretical
approach is based on an analysis of a ligand-field Hamiltonian based on the
moments of the electron state distribution. The functional forms that are
obtained involve a modification of the usual expression of the binding energy
as a sum of ligand-ligand interactions, which however requires very little
increased in CPU time. The angular interactions have simple forms involving sin
and cos functions, whose relative weights depend on whether the ligands are
- or -bonded. They describe the ligand-field stabilization energy
to an accuracy of about 10%. The resulting force field is used to model the
structure of small clusters, including fragments of the copper blue protein
structure. Large deviations from the typical square copper coordination are
found when -bonded ligands are present.Comment: Latex source, 9 postscript figure
Adsorption of Ethylene on Neutral, Anionic and Cationic Gold Clusters
The adsorption of ethylene molecule on neutral, anionic and cationic gold
clusters consisting of up to 10 atoms has been investigated using
density-functional theory. It is demonstrated that C2H4 can be adsorbed on
small gold clusters in two different configurations, corresponding to the pi-
and di-sigma-bonded species. Adsorption in the pi-bonded mode dominates over
the di-sigma mode over all considered cluster sizes n, with the exception of
the neutral C2H4-Au5 system. A striking difference is found in the
size-dependence of the adsorption energy of C2H4 bonded to the neutral gold
clusters in the pi and di-sigma configurations. The important role of the
electronic shell effects in the di-sigma mode of ethylene adsorption on neutral
gold clusters is demonstrated. It is shown that the interaction of C2H4 with
small gold clusters strongly depends on their charge. The typical shift in the
vibrational frequencies of C2H4 adsorbed in the pi- and the di-sigma
configurations gives a guidance to experimentally distinguish between the two
modes of adsorption.Comment: 30 pages, 10 figure
Redetermination of para-aminopyridine (fampridine, EL-970) at 150 K
The structure of fampridine (EL-970) or 4-aminopyridine, C₅H₆N₂, has been redetermined at 150 K. The room-temperature structure has been reported previously [Chao & Schempp (1977). Acta Cryst. B33, 1557-1564]. Pyramidalization at the amine N atom occurs in fampridine, with the N atom 0.133 (11) Å from the plane of the three C/H/H atoms to which it is bonded; the interplanar angle between the pyridyl ring and NH2 group is 21 (2)°. Aggregation in the solid state occurs by N-H...N and N-H...[pi](pyridine) interactions with N...N and N...[pi](centroid) distances of 2.9829 (18) and 3.3954 (15) Å, respectively; a C-H...[pi](pyridine) contact completes the intermolecular interactions [C...[pi](centroid) = 3.6360 (16) Å]
Quantum size effect in Pb(100) films: the role of symmetry and implication for film growth
We show from density-functional calculations that Pb(100) thin films exhibit
quantum size effect with a bilayer periodicity in film energies, film
relaxations, and work functions, which originate from different symmetry of the
stacking geometry of odd and even layer films. The bilayer periodicity of the
film energy is argued to survive on a semiconductor substrate, which should
allow the growth of ``magically'' thick even-layer Pb(100) films. Furthermore,
it is found that the quantum well states in a simple metal film can be
classified into -bonded and -bonded states, which quantize
independently
Ab Initio Molecular Dynamics Study of Aqueous Solvation of Ethanol and Ethylene
The structure and dynamics of aqueous solvation of ethanol and ethylene are
studied by DFT-based Car-Parrinello molecular dynamics. We did not find an
enhancement of the structure of the hydrogen bonded network of hydrating water
molecules. Both ethanol and ethylene can easily be accommodated in the
hydrogen-bonded network of water molecules without altering its structure. This
is supports the conclusion from recent neutron diffraction experiments that
there is no hydrophobic hydration around small hydrophobic groups. Analysis of
the electronic charge distribution using Wannier functions shows that the
dipole moment of ethanol increases from 1.8 D to 3.1 D upon solvation, while
the apolar ethylene molecule attains an average dipole moment of 0.5 D. For
ethylene, we identified configurations with -H bonded water molecules,
that have rare four-fold hydrogen-bonded water coordination, yielding
instantaneous dipole moments of ethylene of up to 1 D. The results provide
valuable information for the improvement of empirical force fields, and point
out that for an accurate description of the aqueous solvation of ethanol, and
even of the apolar ethylene, polarizable force fields are required.Comment: 15 pages, 10 figures, 4 tables, revtex4, submitted to J. Chem. Phy
Atomic-scale characterization of nitrogen-doped graphite: Effects of dopant nitrogen on the local electronic structure of the surrounding carbon atoms
We report the local atomic and electronic structure of a nitrogen-doped
graphite surface by scanning tunnelling microscopy, scanning tunnelling
spectroscopy, X-ray photoelectron spectroscopy, and first-principles
calculations. The nitrogen-doped graphite was prepared by nitrogen ion
bombardment followed by thermal annealing. Two types of nitrogen species were
identified at the atomic level: pyridinic-N (N bonded to two C nearest
neighbours) and graphitic-N (N bonded to three C nearest neighbours). Distinct
electronic states of localized {\pi} states were found to appear in the
occupied and unoccupied regions near the Fermi level at the carbon atoms around
pyridinic-N and graphitic-N species, respectively. The origin of these states
is discussed based on the experimental results and theoretical simulations.Comment: 6 Pages, with larger figure
Noninvasive Embedding of Single Co Atoms in Ge(111)2x1 Surfaces
We report on a combined scanning tunneling microscopy (STM) and density
functional theory (DFT) based investigation of Co atoms on Ge(111)2x1 surfaces.
When deposited on cold surfaces, individual Co atoms have a limited diffusivity
on the atomically flat areas and apparently reside on top of the upper
pi-bonded chain rows exclusively. Voltage-dependent STM imaging reveals a
highly anisotropic electronic perturbation of the Ge surface surrounding these
Co atoms and pronounced one-dimensional confinement along the pi-bonded chains.
DFT calculations reveal that the individual Co atoms are in fact embedded in
the Ge surface, where they occupy a quasi-stationary position within the big
7-member Ge ring in between the 3rd and 4th atomic Ge layer. The energy needed
for the Co atoms to overcome the potential barrier for penetration in the Ge
surface is provided by the kinetic energy resulting from the deposition
process. DFT calculations further demonstrate that the embedded Co atoms form
four covalent Co-Ge bonds, resulting in a Co4+ valence state and a 3d5
electronic configuration. Calculated STM images are in perfect agreement with
the experimental atomic resolution STM images for the broad range of applied
tunneling voltages.Comment: 19 pages, 15 figures, 3 table
Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes
We measure electronic conductance through single conjugated molecules bonded
to Au metal electrodes with direct Au-C covalent bonds using the scanning
tunneling microscope based break-junction technique. We start with molecules
terminated with trimethyltin end groups that cleave off in situ resulting in
formation of a direct covalent sigma bond between the carbon backbone and the
gold metal electrodes. The molecular carbon backbone used in this study consist
of a conjugated pi-system that has one terminal methylene group on each end,
which bonds to the electrodes, achieving large electronic coupling of the
electrodes to the pi-system. The junctions formed with the prototypical example
of 1,4-dimethylenebenzene show a conductance approaching one conductance
quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls
with two to four phenyl units show a hundred-fold increase in conductance
compared with junctions formed with amine-linked oligophenyls. The conduction
mechanism for these longer oligophenyls is tunneling as they exhibit an
exponential dependence of conductance with oligomer length. In addition,
density functional theory based calculations for the Au-xylylene-Au junction
show near-resonant transmission with a cross-over to tunneling for the longer
oligomers.Comment: Accepted to the Journal of the American Chemical Society as a
Communication
Ab-initio study of model guanine assemblies: The role of pi-pi coupling and band transport
Several assemblies of guanine molecules are investigated by means of
first-principle calculations. Such structures include stacked and
hydrogen-bonded dimers, as well as vertical columns and planar ribbons,
respectively, obtained by periodically replicating the dimers. Our results are
in good agreement with experimental data for isolated molecules, isolated
dimers, and periodic ribbons. For stacked dimers and columns, the stability is
affected by the relative charge distribution of the pi orbitals in adjacent
guanine molecules. pi-pi coupling in some stacked columns induces dispersive
energy bands, while no dispersion is identified in the planar ribbons along the
connections of hydrogen bonds. The implications for different materials
comprised of guanine aggregates are discussed. The bandstructure of dispersive
configurations may justify a contribution of band transport (Bloch type) in the
conduction mechanism of deoxyguanosine fibres, while in DNA-like configurations
band transport should be negligible.Comment: 21 pages, 6 figures, 3 tables, to be published in Phys. Rev.
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