The Role of Lone Pairs in Heteroatomic Chemistry. Graphical Analysis and Ab Initio Calculations of Oxirane and Aziridine Ring Opening

The »overlap graph«, where vertices represent atomic orbitals and edges, their in-phase or out-of-phase overlaps, suggests that, . in the thermal electrocyclic opening of oxirane, the second oxygen lone pair has little effect on the allowed conrotatory mode but lowers the activation energy of the forbidden disrotatory mode. This effect does not exist in the electrocyclic opening of aziridine, which is one reason why the energy gap between the allowed and forbidden processes should remain substantial. In the course of an aziridine ring opening, the nitrogen\u27s hydrogen may remain in the plane of the heavy atoms or may move out into either of the two half-spaces defined by this plane. Three different disrotatory pathways are thus conceivable. They are compared by a perturbational treatment using simple Hiickel theory. Localization of the transition states on these three reaction paths - i. e. determination of their »early« or »late« character, as measured by the magnitude of the rotation of the two methylene groups, for example - can be made with the help of »natural correlation diagrams«. This provides a simple criterion for predicting the breakdown of the perturbational treatment, when pathways of similar energies are compared: if one transition state is much earlier (later) than the other(s), no safe conclusion can be reached. These qualitative arguments are fully supported by ab initio calculations (ST0-3G + (3 X 3) CI and 4-31G + extensive CI). Substituent effects and possible cases of violation of orbital symmetry requirements are discussed

Molecular Signature of Polyoxometalates in Electron Transport of Silicon-based Molecular Junctions

Polyoxometalates (POMs) are unconventional electro-active molecules with a great potential for applications in molecular memories, providing efficient processing steps onto electrodes are available. The synthesis of the organic-inorganic polyoxometalate hybrids [PM$_{11}$O$_{39}$(Sn(C$_6$H$_4$)C$\equiv$C(C$_6$H$_4$)N$_2$)]$^{3-}$ (M = Mo, W) endowed with a remote diazonium function is reported together with their covalent immobilization onto hydrogenated n-Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers contacted with a mercury drop as a top electrode confirms their homogeneity. Adjustment of the current-voltage curves with the Simmons equation gives a mean tunnel energy barrier of 1.8 eV and 1.6 eV, for the Silicon-Molecules-Metal (SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M = Mo), respectively. This follows the trend observed in the electrochemical properties of POMs in solution, the polyoxomolybdates being easier to reduce than the polyoxotungstates, in agreement with lowest unoccupied molecular orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus clearly identifiable in the solid-state electrical properties and the unmatched diversity of POM molecular and electronic structures should offer a great modularity

Décomptes électroniques dans les complexesde métaux de transition(2): Pour aller plus loin

International audienceC et artiCle, qui fait suite au précédent, se propose d'aller plus loin dans les décomptes élec-troniques, en considérant notamment des ligands susceptibles d'interagir avec le métal par l'intermédiaire de plusieurs orbitales. Le premier exemple traité est celui des ligands tétra-hydroborate dont la géométrie de coordination dépend du nombre d'électrons transférés au métal. Suit l'analyse des ligands chlorure dans des complexes tels que TiCl 4 et CrCl 6 ; quelques notions de théorie des groupes sont ici nécessaires pour établir les décomptes électroniques. Les phénomènes de rétrodonation sont ensuite étudiés ainsi que les conséquences sur les décomptes d'électrons. La dernière partie est consacrée à l'étude des complexes moléculaires de l'hydrogène ; les possibles conséquences de la rétrodonation dans la catalyse homogène sont présentées pour conclure cet article

Décomptes électroniques dans les complexesde métaux de transition(1)

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THEORETICAL STUDY OF CHLORINATION REACTION OF NITROBENZENE FROM DFT CALCULATIONS

The geometric parameters of stationary points on the potential surface energy of the chlorination reaction of nitrobenzene in the presence of Aluminium chloride as catalyst were investigated theoretically by hybrid DFT (Density Functional Theory) calculations in order to determine his general reaction mechanism in gas phase and in solution. The results obtained by DFT have been compared with CCSD(T) method which is the most powerful post-Hartree Fock method in terms of inclusion of dynamic correlation. Although the electrophilic substitution reaction is widely taught in most courses in organic chemistry, the mechanism has been very few studied theoretically. The results obtained in gas phase are consistent with the traditional description of these reactions: the orientation of this substitution in meta position depends on the stability of a reaction intermediate (Wheland said). Without taking in consideration the reactants and products, six stationary points are found on the potential surface energy of this reaction. The reaction has also been studied in the presence of solvent and we’ve noted that the influence of solvent decreases the electrostatic attraction on the Wheland complex, but the general reaction mechanism remains unchanged in solution

Apprendre la chimie avec les orbitales OrbiMol, une boîte à outils pédagogique

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Transport and Phototransport in ITO Nanocrystals with Short to Long-Wave Infrared Absorption

Nanocrystals are often described as an interesting strategy for the design of low-cost optoelectronic devices especially in the infrared range. However the driving materials reaching infrared absorption are generally heavy metalcontaining (Pb and Hg) with a high toxicity. An alternative strategy to achieve infrared transition is the use of doped semiconductors presenting intraband or plasmonic transition in the short, mid and long-wave infrared. This strategy may offer more flexibility regarding the range of possible candidate materials. In particular, significant progresses have been achieved for the synthesis of doped oxides and for the control of their doping magnitude. Among them, tin doped indium oxide (ITO) is the one providing the broadest spectral tunability. Here we test the potential of such ITO nanoparticles for photoconduction in the infrared. We demonstrate that In2O3 nanoparticles presents an intraband absorption in the mid infrared range which is transformed into a plasmonic feature as doping is introduced. We have determined the cross section associated with the plasmonic transition to be in the 1-3x10-13 cm2 range. We have observed that the nanocrystals can be made conductive and photoconductive due to a ligand exchange using a short carboxylic acid, leading to a dark conduction with n-type character. We bring further evidence that the observed photoresponse in the infrared is the result of a bolometric effect

The one year fate of iron oxide coated gold nanoparticles in mice

Safe implementation of nanotechnology and nanomedicine requires an in-depth understanding of the life cycle of nanoparticles in the body. Here, we investigate the long-term fate of gold/iron oxide heterostructures after intravenous injection in mice. We show these heterostructures degrade in vivo and that the magnetic and optical properties change during the degradation process. These particles eventually eliminate from the body. The comparison of two different coating shells for heterostructures, amphiphilic polymer or polyethylene glycol, reveals the long lasting impact of initial surface properties on the nanocrystal degradability and on the kinetics of elimination of magnetic iron and gold from liver and spleen. Modulation of nanoparticles reactivity to the biological environment by the choice of materials and surface functionalization may provide new directions in the design of multifunctional nanomedicines with predictable fate