31,088 research outputs found
A generalized operational formula based on total electronic densities to obtain 3D pictures of the dual descriptor to reveal nucleophilic and electrophilic sites accurately on closed-shell molecules
Indexación: Wiley Online Library. Online Version of Record published before inclusion in an issue http://onlinelibrary.wiley.com/doi/10.1002/jcc.24453/fullBy means of the conceptual density functional theory, the so-called dual descriptor (DD) has been adapted to be used in any closed-shell molecule that presents degeneracy in its frontier molecular orbitals. The latter is of paramount importance because a correct description of local reactivity will allow to predict the most favorable sites on a molecule to undergo nucleophilic or electrophilic attacks; on the contrary, an incomplete description of local reactivity might have serio us consequences, particularly for those experimental chemists that have the need of getting an insight about reactivity of chemical reagents before using them in synthesis to obtain a new compound. In the present work, the old approach based only on electronic densities of frontier molecular orbitals is replaced by the most accurate procedure that implies the use of total electronic densities thus keeping consistency with the essential principle of the DFT in which the electronic density is the fundamental variable and not the molecular orbitals. As a result of the present work, the DD will be able to properly describe local reactivities only in terms of total electronic densities. To test the proposed operational formula, 12 very common molecules were selected as the original definition of the DD was not able to describe their local reactivities properly. The ethylene molecule was additionally used to test the capability of the proposed operational formula to reveal a correct local reactivity even in absence of degeneracy in frontier molecular orbitals.http://onlinelibrary.wiley.com/doi/10.1002/jcc.24453/ful
Wobbling of a liquid column between unequal discs
One of the most puzzling results of an experiment on the stability of long liquid columns under microgravity, performed aboard Spacelab-D2 in 1993 and named STACO, aiming at the analysis of deformations of nearly cylindrical liquid columns under several mechanical disturbances, is revisited here. It corresponds to the unexplained breakage of an 85 mm long liquid bridge of low viscosity silicone oil, established between unequal discs of 30 and 28 mm, intended to counterbalance the expected deformation by residual acceleration found in previous flights, and left idle because the vibrations and oscillations to be applied afterwards were not started, for fear of premature breakage. A detailed image analysis is performed to extract the maximum amount of data, to be able to check against available theories for axisymmetric and non-axisymmetric deformations of a liquid column
Effect of van der Waals forces on the stacking of coronenes encapsulated in a single-wall carbon nanotube and many-body excitation spectrum
We investigate the geometry, stability, electronic structure and optical
properties of C24H12 coronenes encapsulated in a single-wall (19,0) carbon
nanotube. By an adequate combination of advanced electronic-structure
techniques, involving weak and van derWaals interaction, as well as many-body
effects for establishing electronic properties and excitations, we have
accurately characterized this hybrid carbon nanostructure, which arises as a
promising candidate for opto-electronic nanodevices. In particular, we show
that the structure of the stacked coronenes inside the nanotube is
characterized by a rotation of every coronene with respect to its neighbors
through van derWaals interaction, which is of paramount importance in these
systems. We also suggest a tentative modification of the system in order this
particular rotation to be observed experimentally. A comparison between the
calculated many-body excitation spectrum of the systems involved reveals a
pronounced optical red-shift with respect to the coronene-stacking gas-phase.
The origin of this red-shift is explained in terms of the confinement of the
coronene molecules inside the nanotube, showing an excellent agreement with the
available experimental evidence
Energy Level Alignment in Organic-Organic Heterojunctions: The TTF-TCNQ Interface
The energy level alignment of the two organic materials forming the TTF-TCNQ
interface is analyzed by means of a local orbital DFT calculation, including an
appropriate correction for the transport energy gaps associated with both
materials. These energy gaps are determined by a combination of some
experimental data and the results of our calculations for the difference
between the TTF_{HOMO} and the TCNQ_{LUMO} levels. We find that the interface
is metallic, as predicted by recent experiments, due to the overlap (and charge
transfer) between the Density of States corresponding to these two levels,
indicating that the main mechanism controlling the TTF-TCNQ energy level
alignment is the charge transfer between the two materials. We find an induced
interface dipole of 0.7 eV in good agreement with the experimental evidence. We
have also analyzed the electronic properties of the TTF-TCNQ interface as a
function of an external bias voltage \Delta, between the TCNQ and TTF crystals,
finding a transition between metallic and insulator behavior for \Delta~0.5 eV
Surface Percolation and Growth. An alternative scheme for breaking the diffraction limit in optical patterning
A nanopatterning scheme is presented by which the structure height can be
controlled in the tens of nanometers range and the lateral resolution is a
factor at least three times better than the point spread function of the
writing beam. The method relies on the initiation of the polymerization
mediated by a very inefficient energy transfer from a fluorescent dye molecule
after single photon absorption. The mechanism has the following distinctive
steps: the dye adsorbs on the substrate surface with a higher concentration
than in the bulk, upon illumination it triggers the polymerization, then
isolated islands develop and merge into a uniform structure (percolation),
which subsequently grows until the illumination is interrupted. This
percolation mechanism has a threshold that introduces the needed nonlinearity
for the fabrication of structures beyond the diffraction limit.Comment: 10 pages, 8 figure
Three-Dimensional Wave Packet Approach for the Quantum Transport of Atoms through Nanoporous Membranes
Quantum phenomena are relevant to the transport of light atoms and molecules
through nanoporous two-dimensional (2D) membranes. Indeed, confinement provided
by (sub-)nanometer pores enhances quantum effects such as tunneling and zero
point energy (ZPE), even leading to quantum sieving of different isotopes of a
given element. However, these features are not always taken into account in
approaches where classical theories or approximate quantum models are
preferred. In this work we present an exact three-dimensional wave packet
propagation treatment for simulating the passage of atoms through periodic 2D
membranes. Calculations are reported for the transmission of He and He
through graphdiyne as well as through a holey graphene model. For
He-graphdiyne, estimations based on tunneling-corrected transition state theory
are correct: both tunneling and ZPE effects are very important but competition
between each other leads to a moderately small He/He selectivity. Thus,
formulations that neglect one or another quantum effect are inappropriate. For
the transport of He isotopes through leaky graphene, the computed transmission
probabilities are highly structured suggesting widespread selective adsorption
resonances and the resulting rate coefficients and selectivity ratios are not
in agreement with predictions from transition state theory. Present approach
serves as a benchmark for studies of the range of validity of more approximate
methods.Comment: 4 figure
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