274 research outputs found
The genesis of the quantum theory of the chemical bond
An historical overview is given of the relevant steps that allowed the
genesis of the quantum theory of the chemical bond, starting from the
appearance of the new quantum mechanics and following later developments till
approximately 1931. General ideas and some important details are discussed
concerning molecular spectroscopy, as well as quantum computations for simple
molecular systems performed within perturbative and variational approaches, for
which the Born-Oppenheimer method provided a quantitative theory accounting for
rotational, vibrational and electronic states. The novel concepts introduced by
the Heitler-London theory, complemented by those underlying the method of the
molecular orbitals, are critically analyzed along with some of their relevant
applications. Further improvements in the understanding of the nature of the
chemical bond are also considered, including the ideas of one-electron and
three-electron bonds introduced by Pauling, as well as the generalizations of
the Heitler-London theory firstly performed by Majorana, which allowed the
presence of ionic structures into homopolar compounds and provided the
theoretical proof of the stability of the helium molecular ion. The study of
intermolecular interactions, as developed by London, is finally examined.Comment: amsart, 34 pages, 2 figure
Modification of the Bloch law in ferromagnetic nanostructures
The temperature dependence of magnetization in ferromagnetic nanostructures
(e.g., nanoparticles or nanoclusters) is usually analyzed by means of an
empirical extension of the Bloch law sufficiently flexible for a good fitting
to the observed data and indicates a strong softening of magnetic coupling
compared to the bulk material. We analytically derive a microscopic
generalization of the Bloch law for the Heisenberg spin model which takes into
account the effects of size, shape and various surface boundary conditions. The
result establishes explicit connection to the microscopic parameters and
differs significantly from the existing description. In particular, we show
with a specific example that the latter may be misleading and grossly
overestimates magnetic softening in nanoparticles. It becomes clear why the
usual dependence appears to be valid in some nanostructures, while
large deviations are a general rule. We demonstrate that combination of
geometrical characteristics and coupling to environment can be used to
efficiently control magnetization and, in particular, to reach a magnetization
higher than in the bulk material.Comment: 7 pages, 4 figure
Majorana and the theoretical problem of photon-electron scattering
Relevant contributions by Majorana regarding Compton scattering off free or
bound electrons are considered in detail, where a (full quantum) generalization
of the Kramers-Heisenberg dispersion formula is derived. The role of
intermediate electronic states is appropriately pointed out in recovering the
standard Klein-Nishina formula (for free electron scattering) by making
recourse to a limpid physical scheme alternative to the (then unknown) Feynman
diagram approach. For bound electron scattering, a quantitative description of
the broadening of the Compton line was obtained for the first time by
introducing a finite mean life for the excited state of the electron system.
Finally, a generalization aimed to describe Compton scattering assisted by a
non-vanishing applied magnetic field is as well considered, revealing its
relevance for present day research.Comment: latex, amsart, 10 pages, 1 figur
Young people’s education choices and progression to higher education: A comparison of A-Level and Non-A-Level students in Key Stage 5, their subject choices and transitions to university
A note on the analogy between superfluids and cosmology
A new analogy between superfluid systems and cosmology is here presented,
which relies strongly on the following ingredient: the back-reaction of the
vacuum to the quanta of sound waves. We show how the presence of thermal
phonons, the excitations above the quantum vacuum for , enable us to
deduce an hydrodynamical equation formally similar to the one obtained for a
perfect fluid in a Universe obeying the Friedmann-Robertson-Walker metric.Comment: Accepted for publication in Modern Physics Letters
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Now is the time for a comfort congress
When buying a bed, a chair or a car, taking the train, holding a hand tool or flying across the ocean,
comfort comes into play. Users interact with products and rate their experience. Therefore,
designers and manufacturers of products such as seats, cars, beds, hand tools, and production lines
strive for optimal comfort. If we look at some trends like “attention to health”, “ageing workforce
(and population)”, “environmental awareness and sustainability” and “attention to well-being”,
(dis)comfort is an important consideration (Vink & Hallbeck, 2012). This means that in our daily
lives we are confronted with comfort
Two steps one pot process for the conversion of dimethylfuran to pyrrole compounds with almost null E factor
The replacement of the oil-based chemicals with those derived from biomasses is one of the most exciting challenges of the last decades. For example, 1,4-dicarbonyl compounds have a great importance in chemical synthesis, thanks to their high chemoselectivity and there is an increasing interest for preparing them from biomasses. In particular, 2,5-hexanedione could be synthesized starting from lignocellulosic sources, through the acid-ring opening reaction of 2,5-dimethylfuran as the bio-based feedstock.[1] The reaction of 2,5-hexanedione and a generic primary amine leads to pyrrole compounds. Many examples have been reported by some of the authors.[2]
In this work a sustainable process for the preparation of pyrrole compounds starting from a bio-based reagent has been developed. The selected starting material was 2,5-dimethyl furan.
In this work, the ring opening reaction of 2,5-dimethylfuran was optimized by tuning parameters such as the amount of water, type and amount of acid, time and temperature. 2,5-hexanedione was obtained with a high yield (95%) without the need of purification. Then, different primary amines, in particular biosourced, have been used to prepare a variety of pyrrole compounds, with high yield (at least 90%) and with high carbon efficiency, without producing waste. The pyrrole compounds have then been used for the functionalization of a nanosized graphite, promoting the exfoliation to few layers graphene
A bio-sourced molecule as carbon black coupling agent in rubber compounds with low hysteresis
The prime application of rubber composites is represented by tire compounds. To achieve
the desired tire performances an equilibrium between dynamic rigidity and hysteresis must
be acquired. Amorphous precipitated silica is the preferred reinforcing filler to have low
energy dissipations and thus low fuel consumption. Indeed, silica is characterized by nano
dimensions and by the possibility of establishing chemical bonds with rubber chains allowing
the achievement of high hysteresis at low temperatures, to promote wet traction, and low
hysteresis at medium-high temperatures, for low fuel consumption.
Carbon black (CB) is the main filler for tire compounds, but it does not have functional groups
able to promote chemical bonds with the rubber matrix, though it would be highly desirable.
In this work, a pyrrole compound (PyC) containing a thiol group, and which can be
synthesized starting from bio-based building blocks was used to functionalize CB by the socalled
“pyrrole methodology”. The thiol group was expected to react with the sulphurbased
crosslinking system and/or with rubber chains, thus forming strong bonds with the
rubber matrix.
Results
The synthesis of the PyC and the functionalization reaction were characterized by high atom
efficiency. A poly(styrene-co-butadiene) copolymer from anionic solution polymerization was
used as the main rubber for the compound preparation. The crosslinked composite material
filled with functionalized CB revealed substantial improvements with respect to the
composite with pristine CB, in particular: high rigidity and low hysteresis at high temperature.
Composite properties were even comparable to those of silica-based rubber composites.
The formation of the expected rubber-filler chemical bond via the thiol group of the selected
PyC was confirmed studying such functionalizing agent in a squalene-based model
compound. The results here reported pave the way to CB-based rubber composites with a
low environmental impact
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