Électronégativité

Résumé

vignette|Dipôle permanent de la molécule d'eau. En chimie, l'électronégativité d'un atome est une grandeur physique qui caractérise sa capacité à attirer le doublet d'électrons partagés lors de la formation d'une liaison chimique avec un autre atome. La différence d'électronégativité entre ces deux atomes détermine la nature de la liaison : liaison covalente apolaire lorsque la différence est nulle ou faible, liaison covalente polaire quand la différence est moyenne, et liaison ionique quand la différence est tellement forte qu'un des atomes a attiré complètement, ou presque, les électrons de la liaison. Dans le dernier cas les atomes sont devenus des ions et portent des charges électriques entières, ou presque. La notion d'électronégativité, qui décrit le comportement des électrons dans une liaison chimique, ne doit pas être confondue avec celle d'affinité électronique. Le concept d’électronégativité est introduit pour la première fois par Jöns Jacob Berzelius en 1835. Par la suite,

Source officielle

https://fr.wikipedia.org/wiki/%C3%89lectron%C3%A9gativit%C3%A9

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Nitranilic acid hexahydrate, a novel benchmark system of the Zundel cation in an intrinsically asymmetric environment: spectroscopic features and hydrogen bond dynamics characterised by experimental and theoretical methods

Nitranilic acid (2,5-dihydroxy-3,6-dinitro-2,5-cyclohexadiene-1,4-dione) as a strong dibasic acid in acidic aqueous media creates the Zundel cation, H5O2+. The structural unit in a crystal comprises (H5O2)(2)(+)(2,5-dihydroxy-3,6-dinitro-1,4-benzoquinonate)(2-) dihydrate where the Zundel cation reveals no symmetry, being an ideal case for studying proton dynamics and its stability. The Zundel cation and proton transfer dynamics are studied by variable-temperature X-ray diffraction, IR and solid-state NMR spectroscopy, and various quantum chemical methods, including periodic DFT calculations, ab initio molecular dynamics simulation, and quantization of nuclear motion along three fully coupled internal coordinates. The Zundel cation features a short H-bond with the O center dot center dot center dot O distance of 2.433(2) angstrom with an asymmetric placement of hydrogen. The proton potential is of a single well type and, due to the non-symmetric surroundings, of asymmetric shape. The formation of the Zundel cation is facilitated by the electronegative NO2 groups. The employed spectroscopic techniques supported by calculations confirm the presence of a short H-bond with a complex proton dynamics.

Royal Soc Chemistry2014

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The dependence of the lattice parameter on dopant concentration in Ce1-xMxO2 (M = Sn and Ti) solid solutions is not linear. A change towards a steeper slope is observed around x similar to 0.35, though the fluorite structure (space group Fm3m) is preserved up to x = 0.5. This phenomenon has not been observed for Ce1-xZrxO2 solid solutions showing a perfectly linear decrease of the lattice parameter up to x = 0.5. In order to understand this behavior, the oxidation state of the metal ions, the disorder in the oxygen substructure and the nature of metal-oxygen bonds have been analyzed by XPS, Sn-119 Mossbauer spectroscopy and X-ray absorption spectroscopy. It is observed that the first Sn-O coordination shell in Ce1-xSnxO2 is more compact and less flexible than that of Ce-O. The Sn coordination remains symmetric with eight equivalent, shorter Sn-O bonds, while Ce-O coordination gradually splits into a range of eight non-equivalent bonds compensating for the difference in the ionic radii of Ce4+ and Sn4+. Thus, a long-range effect of Sn doping is hardly extended throughout the lattice in Ce1-xSnxO2. In contrast, for Ce1-xZrxO2 solid solutions, both Ce and Zr have similar local coordination creating similar rearrangement of the oxygen substructure and showing a linear lattice parameter decrease up to 50% Zr substitution. We suggest that the localized effect of Sn substitution due to its higher electronegativity may be responsible for the deviation from Vegard's law in Ce1-xSnxO2 solid solutions.

Royal Soc Chemistry2016

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Adsorption of HPDX and CaHPOx (x=1, ...,4) molecules on anatase TiO2 (001) surfaces

Michel Posternak

perform DFT calculations within the COSMO framework to study the adsorption properties of the CaHPOx (x = 1,...,4) phosphate family, and of the related HPOx acids, on a paradigmatic anatase (001) surface. Of particular interest is CaHPO4, a precursor of hydroxyapatite, which forms during osseointegration of titanium implants. Our results, obtained within the COSMO framework, are based on total energy calculations, and are interpreted in terms of electronegativity differences between the relevant system components. They show that adsorption of HPOx and in particular of HPO4, is highly enhanced by the presence of Ca adatoms on the TiO2 substrate, a result which is in agreement with our previous work on the wettability of anatase surfaces. We also study the adsorption of CaHPOx molecules on TiO2 surfaces, and show that molecular adsorption is strongly favoured for x = 4, while for x = 1, 2 dissociative adsorption of HPOx molecules and Ca adatoms prevails.

ELSEVIER SCIENCE BV2019

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