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 $T^{3/2}$ 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

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 $T> 0$, 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

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