3,388 research outputs found
Spin-dependent charge recombination along para-phenylene molecular wires
We have used an efficient new quantum mechanical method for radical pair
recombination reactions to study the spin-dependent charge recombination along
PTZ--Ph--PDI molecular wires. By comparing our
results to the experimental data of E. Weiss {\em et al.} [J. Am. Chem. Soc.
{\bf 126}, 5577 (2004)], we are able to extract the spin-dependent (singlet and
triplet) charge recombination rate constants for wires with . These
spin-dependent rate constants have not been extracted previously from the
experimental data because they require fitting its magnetic field-dependence to
the results of quantum spin dynamics simulations. We find that the triplet
recombination rate constant decreases exponentially with the length of the
wire, consistent with the superexchange mechanism of charge recombination.
However, the singlet recombination rate constant is nearly independent of the
length of the wire, suggesting that the singlet pathway is dominated by an
incoherent hopping mechanism. A simple qualitative explanation for the
different behaviours of the two spin-selective charge recombination pathways is
provided in terms of Marcus theory. We also find evidence for a magnetic
field-independent background contribution to the triplet yield of the charge
recombination reaction, and suggest several possible explanations for it. Since
none of these explanations is especially compelling given the available
experimental evidence, and since the result appears to apply more generally to
other molecular wires, we hope that this aspect of our study will stimulate
further experimental work.Comment: 12 pages, 10 figure
Nuclear quantum effects in water exchange around lithium and fluoride ions
We employ classical and ring polymer molecular dynamics simulations to study
the effect of nuclear quantum fluctuations on the structure and the water
exchange dynamics of aqueous solutions of lithium and fluoride ions. While we
obtain reasonably good agreement with experimental data for solutions of
lithium by augmenting the Coulombic interactions between the ion and the water
molecules with a standard Lennard-Jones ion-oxygen potential, the same is not
true for solutions of fluoride, for which we find that a potential with a
softer repulsive wall gives much better agreement. A small degree of
destabilization of the first hydration shell is found in quantum simulations of
both ions when compared with classical simulations, with the shell becoming
less sharply defined and the mean residence time of the water molecules in the
shell decreasing. In line with these modest differences, we find that the
mechanisms of the exchange processes are unaffected by quantization, so a
classical description of these reactions gives qualitatively correct and
quantitatively reasonable results. We also find that the quantum effects in
solutions of lithium are larger than in solutions of fluoride. This is partly
due to the stronger interaction of lithium with water molecules, partly due to
the lighter mass of lithium, and partly due to competing quantum effects in the
hydration of fluoride, which are absent in the hydration of lithium.Comment: 12 pages, 8 figure
Asymmetric recombination and electron spin relaxation in the semiclassical theory of radical pair reactions
We describe how the semiclassical theory of radical pair recombination
reactions recently introduced by two of us [D. E. Manolopoulos and P. J. Hore,
J. Chem. Phys. 139, 124106 (2013)] can be generalised to allow for different
singlet and triplet recombination rates. This is a non-trivial generalisation
because when the recombination rates are different the recombination process is
dynamically coupled to the coherent electron spin dynamics of the radical pair.
Furthermore, because the recombination operator is a two-electron operator, it
is no longer sufficient simply to consider the two electrons as classical
vectors: one has to consider the complete set of 16 two-electron spin operators
as independent classical variables. The resulting semiclassical theory is first
validated by comparison with exact quantum mechanical results for a model
radical pair containing 12 nuclear spins. It is then used to shed light on the
spin dynamics of a carotenoid-porphyrin-fullerene (CPF) triad containing
considerably more nuclear spins which has recently been used to establish a
'proof of principle' for the operation of a chemical compass [K. Maeda et al.,
Nature 453, 387 (2008)]. We find in particular that the intriguing biphasic
behaviour that has been observed in the effect of an Earth-strength magnetic
field on the time-dependent survival probability of the photo-excited C+PF-
radical pair arises from a delicate balance between its asymmetric
recombination and the relaxation of the electron spin in the carotenoid
radical
Mean-Field Theory of Water-Water Correlations in Electrolyte Solutions
Long-range ion induced water-water correlations were recently observed in
femtosecond elastic second harmonic scattering experiments of electrolyte
solutions. To further the qualitative understanding of these correlations, we
derive an analytical expression that quantifies ion induced dipole-dipole
correlations in a non-interacting gas of dipoles. This model is a logical
extension of Debye-H\"uckel theory that can be used to qualitatively understand
how the combined electric field of the ions induces correlations in the
orientational distributions of the water molecules in an aqueous solution. The
model agrees with results from molecular dynamics simulations and provides an
important starting point for further theoretical work
BlogForever D2.4: Weblog spider prototype and associated methodology
The purpose of this document is to present the evaluation of different solutions for capturing blogs, established methodology and to describe the developed blog spider prototype
The Generation of Fullerenes
We describe an efficient new algorithm for the generation of fullerenes. Our
implementation of this algorithm is more than 3.5 times faster than the
previously fastest generator for fullerenes -- fullgen -- and the first program
since fullgen to be useful for more than 100 vertices. We also note a
programming error in fullgen that caused problems for 136 or more vertices. We
tabulate the numbers of fullerenes and IPR fullerenes up to 400 vertices. We
also check up to 316 vertices a conjecture of Barnette that cubic planar graphs
with maximum face size 6 are hamiltonian and verify that the smallest
counterexample to the spiral conjecture has 380 vertices.Comment: 21 pages; added a not
BlogForever D5.3: User Questionnaires and Reports
This report presents the feedback gathered from third party users during the BlogForever Case Studies. Therefore, the research framework is defined and the case studies results are presented, followed by a summary of conclusions and remarks
BlogForever D5.2: Implementation of Case Studies
This document presents the internal and external testing results for the BlogForever case studies. The evaluation of the BlogForever implementation process is tabulated under the most relevant themes and aspects obtained within the testing processes. The case studies provide relevant feedback for the sustainability of the platform in terms of potential usersâ needs and relevant information on the possible long term impact
Litz wire loss performance and optimization for cryogenic windings
Litz wires operating in a cryogenic environment can potentially improve both the efficiency and power density of electrical machines and passive components. However, due to the low resistivity and high magnetic fields, eddy-current losses may become significant in cryogenically cooled windings, especially in airgap winding arrangements or in the case of significant slot leakage fields, unless the litz wire parameters are carefully chosen. A framework for litz wire loss performance optimization and experimental characterisation at cryogenic temperatures is provided. An optimum operating temperature for minimum loss is derived based on analytical expressions, which highlights the role of litz wire parameters, current density and external field. The proximity loss model, used to calculate the optimum operating temperature, is validated experimentally. Two test rigs with different magnetic cores were designed and built. Copper and aluminium litz wires with a strand diameter down to 0.1 mm were tested in a liquid nitrogen bath with a uniform harmonic external magnetic field up to 0.5 T peak and a frequency up to 1 kHz. Measurements show good agreement with the theoretical results and confirm that the proposed model can be confidently used during the preliminary design of cryogenic windings
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