Rémy Fortrie

Size-Dependent Catalytic Activity of Supported Vanadium Oxide Species: Oxidative Dehydrogenation of Propane

Possible reaction pathways for the oxidative dehydrogenation of propane by vanadium oxide catalysts supported on silica are examined by density functional theory. Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes. The reaction proceeds in two subsequent steps. In a first step, hydrogen abstraction from propane by a vanadyl (O═V) group yields a propyl radical bound to a HOV(IV) surface site. Propene is formed by a second hydrogen abstraction, either at the same vanadia site or at a different one. V(V)/V(IV) redox cycles are preferred over V(V)/V(III) cycles. Under the assumption of fast reoxidation, microkinetic simulations show that the first step is rate-determining and yields Arrhenius barriers that are lower for dimers (114 kJ/mol at 750 K) than for monomers (124 kJ/mol). The rate constants predicted for a mixture of monomers and dimers are 14% larger (750 K) than for monomers only, although the increase remains within experimental uncertainty limits. Direct calculations of energy barriers also yield lower values for dimeric species than for monomeric ones. Reactivity descriptors indicate that this trend will continue also for larger oligomers. The size distribution of oligomeric species is predicted to be rather statistical. This, together with the small increase in the rate constants, explains that turnover frequencies observed for submonolayer coverages of vanadia on silica do not vary with the loading within the experimental uncertainty limits.

Synthesis of chromophores combining second harmonic generation and two photon induced fluorescence properties

The design of new chromophores presenting simultaneous SHG and TPEF properties is reported.

Excitonically Coupled Oligomers and Dendrimers for Two-Photon Absorption

The first part of this review presents the state of the art on two-photon absorbing molecules. Earlyworks concerned the optimization of dipolar push–pull molecules. Recently, the synthesis of linearand branched centrosymmetrical quadrupolar molecules led to higher nonlinearities, but still far from reachingthe fundamental limits of molecular two-photon cross-sections.

Vibronic Quasi-Free Rotation Effects in Biphenyl-Like Molecules. TD−DFT Study of Bifluorene†

In this paper, we investigate the vibronic shape of the lowest UV-visible absorption band of biphenyl-like systems, using the bifluorene molecule as a workhorse. The molecule is here regarded as a one-dimensional two-level system, whose ground and excited states are simulated with time-dependent density functional theory and semiempirical methods. The vibrational Schrödinger equation is then numerically solved along the torsional coordinate, and the vibronic shape of the absorption band is modeled. Comparisons with the harmonic approximation, with or without the Franck-Condon approximation, are performed. This study confirms that a vibronic effect is most likely responsible for the strong dissymmetry of the lowest UV-visible absorption band of biphenyl-like systems and that, for such systems, the experimental data should be extracted using the whole absorption band, instead of a Gaussian fit on the first part of the band, as it is often done when a superposition between several electronic transitions is suspected.

Nonuniform temperature dependence of the reactivity of disordered VOx/κ+Al2O3(001) surfaces : A density functional theory based Monte Carlo study

Periodic density functional theory (DFT) calculations concerning VO(x)/kappa-Al(2)O(3)(001) surfaces are used for parametrizing Monte Carlo simulations performed on a mesoscopic scale surface sample (41.95x48.80 nm(2)). New structural and chemical information are then obtained that are not accessible from DFT calculations: segregation, short- and long-range ordering, and the effect of the temperature on the reducibility. The reducibility of the surface is investigated for locating chemical potential regions where the catalyst is especially reactive for oxidation reactions. Comparison with the V(2)O(5)(001) surface at catalytic conditions (800 K, 1 bar) is performed. The reducibility exhibits an unexpectedly strong temperature dependence.

Bare Histidine–Serine Models: Implication and Impact of Hydrogen Bonding on Nucleophilicity

A new family of 2-hydroxyalk(en/yn)ylimidazoles has been evaluated as serine-histidine bare dyad models for the ring-opening reaction of L-lacOCA, a cyclic O-carboxyanhydride. These models were selected to unravel the implication of intramolecular hydrogen bonding and to substantiate its influence on the nucleophilicity of the alcohol moiety, as it is suspected to occur in enzyme active sites. Although designed to exclusively facilitate the preliminary step of proton transfer during the studied ring-opening reaction, these minimalistic models depicted a measureable increase in reactivity relative to the isolated fragments. A couple of reliable experimental and theoretical methods have been developed to readily monitor the strength of the intramolecular hydrogen bond in dilute solution. Results show that the folded conformers are the most nucleophilic species because of the intramolecular hydrogen bond.