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Isabelle Demachy

Publications and source records attributed to Isabelle Demachy.

8 recordsLinked to original sources
PubMed2007

2,2'-Biphosphinines and 2,2'-bipyridines in homoleptic dianionic Group 4 complexes and neutral 2,2'-biphosphinine Group 6 d(6) metal complexes: octahedral versus trigonal-prismatic geometries.

The geometric and electronic structure of formally d(6) tris-biphosphinine [M(bp)(3)](q) and tris-bipyridine [M(bpy)(3)](q) complexes were studied by means of DFT calculations with the B3LYP functional. In agreement with the available experimental data, Group 4 dianionic [M(bp)(3)](2-) complexes (1P-3P for M=Ti, Zr, and Hf, respectively) adopt a trigonal-prismatic (TP) structure, whereas the geometry of their nitrogen analogues [M(bpy)(3)](2-) (1N-3N) is nearly octahedral (OC), although a secondary minimum was found for the TP structures (1N'-3N'). The electronic factors at work in these systems are discussed by means of an MO analysis of the minima, MO correlation diagrams, and thermodynamic cycles connecting the octahedral and trigonal-prismatic limits. In all these complexes, pronounced electron transfer from the metal center to the lowest lying pi* ligand orbitals makes the d(6) electron count purely formal. However, it is shown that the bp and bpy ligands accommodate the release of electron density from the metal in different ways because of a change in the localization of the HOMO, which is a mainly metal-centered orbital in bp complexes and a pure pi* ligand orbital in bpy complexes. The energetic evolution of the HOMO allows a simple rationalization of the progressive change from the TP to the OC structure on successive oxidation of the [Zr(bp)(3)](2-) complex, a trend in agreement with the experimental structure of the monoanionic complex. The geometry of Group 6 neutral complexes [M(bp)(3)] (4P and 5P for M=Mo and W, respectively) is found to be intermediate between the TP and OC limits, as previously shown experimentally for the tungsten complex. The electron transfer from the metal center to the lowest lying pi* ligand orbitals is found to be significantly smaller than for the Group 4 dianionic analogues. The geometrical change between [Zr(bp)(3)](2-) and [W(bp)(3)] is analyzed by means of a thermodynamic cycle and it is shown that a larger ligand-ligand repulsion plays an important role in favoring the distortion of the tungsten complex away from the TP structure.

Journal Article↗
PubMed2006

Molecular simulation studies of water physisorption in zeolites.

We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342-353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si ratio Al ratio in the range 1.53-3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolite-based separation processes.

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PubMed2005

Water condensation in hydrophobic silicalite-1 zeolite: a molecular simulation study.

We report grand canonical Monte Carlo simulations of the gas and liquid phase adsorption of water in silicalite-1 zeolite. Simple but effective models and simulation methods, found useful for studying gas adsorption in nanoporous materials, have been extended to describe the intrusion/extrusion cycle of water in this hydrophobic solid. The picture of water confined to hydrophobic spaces of nanoscopic dimensions that emerges from this study is one of a strongly depleted and highly inhomogeneous fluid.

Journal Article↗
PubMed2005

Cyan fluorescent protein: molecular dynamics, simulations, and electronic absorption spectrum.

The dynamics and electronic absorption spectrum of enhanced cyan fluorescent protein (ECFP), a mutant of green fluorescent protein (GFP), have been studied by means of a 1 ns molecular dynamics (MD) simulation. The two X-ray conformations A' and B' of ECFP were considered. The chromophore was assumed to be neutral, and all titratable residues were taken in their standard protonation state at neutral pH. The protein was embedded in a box of water molecules (and counterions). The first result is that the two conformations A' and B' are found to be stable all along the simulation. Then, an analysis of the hydrogen-bond networks shows strong differences between the two conformations in the surroundings of the nitrogen atom of the indolic part of the chromophore. This is partly due to the imperfection in the beta barrel near the His148 residue, which allows the access of one solvent molecule inside the protein in conformation A'. Finally, quantum mechanical calculations of the electronic transition energies of the chromophore in the charge cloud of the protein and solvent water molecules were performed using the TDDFT method on 160 snapshots extracted every 5 ps of the MD trajectories. It is found that conformations A' and B' exhibit very similar spectra despite different H-bond networks involving the chromophore. This similarity is related to the weak charge transfer involved in the electronic transition and the weak electrostatic field created by ECFP near the chromophore, within the hypotheses made in the present simulation.

Computer Simulation↗
PubMed2003

Why [CpW(CO)3]+ reduces H2 to dihydride.

The [CpW(CO)3]+ complex, with three pi acceptor ligands and a positive charge, is shown to have an unexpected reducing ability towards H2 because of a low lying triplet state energy.

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PubMed1996

Trans or (Unusual) Cis Geometry in d(2) Octahedral Dioxo Complexes. A DFT Study.

Geometry optimization of the cis and the trans isomers of several octahedral dioxo complexes of d(2) electronic configuration are performed using the gradient-corrected density functional theory (B3LYP and, for some key structures, BP86). With only monodentate sigma donor ligands (ReO(2)(NH(3))(4)(+), 7), the usual energy order is found (i.e., the trans isomer is the most stable). Complexes with a chelating bidentate ligand, OsO(2)(OCH(2)CH(2)O)(NH(3))(2) (10) and ReO(2)(HN=CHCH=NH)(NH(3))(2)(+) (11), are used as models for the experimental complexes 5 and 2 in which the arrangement of the O=M=O unit is trans and cis, respectively. Our calculations actually show an inversion of the relative energy of the two isomers in going from 10 to 11: while the trans isomer is found to be the most stable in 10, the unusual cis diamagnetic isomer is favored by about 29 kcal mol(-)(1) in 11. This result is traced to the geometric and electronic properties of the bidentate ligand, in particular an acute bite angle and good pi acceptor character. In complex 14 with a bipyridine chelating ligand (weaker pi acceptor than diaza-1,4-butadiene in 11), this energy difference is, however, reduced to 7.5 kcal mol(-)(1) (partial geometry optimization).

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