Lewis structure

Summary

Lewis structures, also called Lewis dot formulas, Lewis dot structures, electron dot structures, or Lewis electron dot structures (LEDs) – are diagrams that show the bonding between atoms of a molecule, as well as the lone pairs of electrons that may exist in the molecule. A Lewis structure can be drawn for any covalently bonded molecule, as well as coordination compounds. The Lewis structure was named after Gilbert N. Lewis, who introduced it in his 1916 article The Atom and the Molecule. Lewis structures extend the concept of the electron dot diagram by adding lines between atoms to represent shared pairs in a chemical bond. Lewis structures show each atom and its position in the structure of the molecule using its chemical symbol. Lines are drawn between atoms that are bonded to one another (pairs of dots can be used instead of lines). Excess electrons that form lone pairs are represented as pairs of dots, and are placed next to the atoms. Although main group elements

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Le cours comporte deux parties. Les bases de la thermodynamique des équilibres et de la cinétique des réactions sont introduites dans l'une d'elles. Les premières notions de chimie quantique sur les électrons et les liaisons chimiques, exemplifiées en chimie organique, sont présentées dans l'autre.

This course aims to teach essential notions of the structure of matter, chemical equilibria and reactivity. Classes and exercises provide the means to analyze and solve, by reasoning and calculation, novel problems of geeneral chemistry.

L'objectif de ce cours est de fournir les connaissances et le moyen de comprendre, au niveau moléculaire, le fonctionnement des réactions chimiques organiques. Pendant le cours, l'étudiant réfléchira et résoudra de nouveaux problèmes.

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The Cas A-Like Snr 1E 0102.2-7219 In The Small Magellanic Cloud: An Asymmetric Bipolar Explosion

Frédéric Vogt

We have used the Wide Field Spectrograph on the 2.3 m telescope at Siding Spring Observatory to map the [O III] lambda 5007 dynamics of the young oxygen-rich supernova remnant 1E 0102.2-7219 in the Small Magellanic Cloud. From the resultant data cube, we have been able to reconstruct the full three-dimensional structure of the system of [O III] filaments. From this we are able to deduce that the ejecta trace an asymmetric bipolar structure. The redshifted gas contains fainter clumps but with higher velocities than the blueshifted gas. The major axis of the explosion is inclined at similar to 40 degrees to the line of sight. This structure shows that the supernova explosion has been influenced both by the rotation of the progenitor star and, presumably, by a global rotational instability following core collapse.

2010

Cyanine dyes represent a class of ionic functional materials used by a broad heterogeneous science community contain-ing biologists, chemists, physicists and materials scientists. Their diverse application opportunities are ranging from fluorescence markers, nonlinear optics, CD-R data storage, photography and organic electronics. Therefore the development of cyanine dyes and their applications in chemistry, engineering, medicine and pharmacology is growing continuously. A typical leitmotif of a cyanine dye contains two nitrogen heterocyclic rings joined by a conjugated chain of carbon atoms. Due to the synthetic approach, quaternary salt precursors are required resulting in ionic compounds. Such chromophores are positively charged and require a counter anion to maintain the overall charge neutrality. Several reports were presenting certain anions as efficiency enhancers in cyanine dye based organic electronic devices. Despite their long presence in science, there is a lack of knowledge behind the mechanisms causing these efficiency enhancements. Furthermore, in the field of organic electronics, the processability of cyanine dyes towards electronic devices was restricted to solution-based methods. In the present thesis work the three main challenges in cyanine dye research, namely: efficient ion exchange procedures, mechanisms behind organic electronics figures of merit enhancement as well as the introduction of physical vapour deposition method have been addressed. An efficient halide-for-anion exchange method with the potential for industry scale up was introduced. This allowed modifying a cationic cyanine chromophore with organic or inorganic anions yielding six salts. The following investigations of these six anions by either varying organic moieties or negative charge delocalisation paved the way towards a generalized concept describing the semiconducting properties of this cationic cyanine chromophore. The lattice energy was established as a universal parameter describing cyanine salt semiconducting properties on a universal energy scale. Such decoupling of an ionic structure from its Lewis formula allows future predictions of semiconducting properties which can be widened to all organic ionic functional materials. Furthermore weakly coordinating anion influence on the volatility of a cyanine chromophore established physical vapour deposition as thin film fabrication method. Using co-evaporation of the cyanine dye and the fullerene C60 a suitable bulk heterojunction morphology was obtained. This resulted in the fabrication of the first fully vacuum deposited cyanine dye bulk heterojunction device.

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We describe a beam splitter for polar neutral molecules. An electrostatic hexapole initially confines and guides a supersonic expansion of ammonia, and it then smoothly transforms into two bent quadrupole guides, thus splitting the molecular beam in two correlated fractions. This paves the way towards molecular beam experiments wherein one beam is modified through interactions with, e.g. a laser beam or another molecular beam, while the other one remains unmodified and serves as a reference. Because both beams originate from the same parent beam, such differential experiments can dramatically enhance the sensitivity. The highly complex electrode structure required for the beam splitter would be very difficult to build by traditional means. Instead, we introduce a new method of production: 3D printing of a plastic piece, followed by electroplating. The 3D printed piece can take any desired shape and, since the entire structure can be printed as a single piece, provides inherently precise alignment. Electroplating produces atomically smooth metal surfaces that allow the application of electric potentials. The simple and powerful fabrication method introduced here opens a plethora of new avenues for research, far beyond molecular beams experiments, since 3D printing imposes practically no limitations on possible shapes, and the metal-plating produces chemically robust, conductive construction elements. It has the added advantage of dramatically reduced production cost and time: all components used in the present study were printed within less than 48 hours; electroplating is completed in one day, and the bottle neck of the entire process was the shipping to and from the plating company.

2017

EPFL2018