C. Girardet

Potential surfaces and adsorption energy of Xe, CH4, N2, CO, NH3 and CH3F molecules on a MgO substrate

Abstract A semi-empirical potential expression is used to determine the minimum energy surface experienced by various molecules adsorbed on the (100) face of MgO. The position and orientation of the molecules in the favourable adsorption sites and the surface corrugation are calculated. The molecular motions are then considered and the adsorption energy is compared to experimental values, when available, of the isosteric heat of adsorption, for temperatures less than 100 K. These quantities (site, corrugation and energy) are very sensitive to the magnitude of the electrostatic field due to the ionic substrate. This field in turn depends on the charges of the substrate ions, and it is shown that effective charges must be introduced to conveniently interpret the available data.

Endohedral and exohedral adsorption in C60: An analytical model

A simple description of the interaction potential between a small molecule and a fullerene 60 cage is discussed on the basis of the continuum approximation. General conditions are obtained for the validity of this continuum approximation and for the confinement mechanism of the molecule inside (or outside) the cage. Application to the insertion of alkali metal ions and rare gas atoms shows that these conditions can adequately interpret the equilibrium positions of inside (outside) adducts, the adsorption energy and the collision experiments between the ions and atoms and the C60 cage.

Theoretical study of the monolayer structures of CO adsorbed on NaCl(100)

Abstract Interaction potential calculations are performed to interpret recent experimental results on the geometry of CO monolayers adsorbed on clean NaCl(100) substrate deduced from polarized Fourier transform infrared spectroscopy and helium beam scattering. Two isoenergetical structures are calculated: the (1 × 1) geometry with the molecular axes standing upright or inclined with respect to the surface normal and the (2 × 1) geometry with the molecular axes inclined and tilted anti-parallel. A statistical analysis of the energy minima for the CO monolayer is performed and it is shown that the transition from the (2 × 1) to the (1 × 1) structure can proceed in a very small energy range ( ~ a few meV) leading to a rearrangement of the molecular centers of mass above the Na sites with a concomitant redistribution of the molecular axis orientations. These results are in close agreement with experiments indicating a phase transition at 35 K from the (2 × 1) to the (1 × 1) phase.

Influence of molecular adsorption on the dielectric properties of a single wall nanotube: a model sensor.

Recent measurements of the resonance frequency of a copper disk covered with carbon nanotube bundles have shown characteristic resonance shifts during exposure with various gas molecules. The shifts were interpreted as the change of the dielectric permittivity of the system forming the sensor due to the electric properties of the adsorbed molecules. Starting from a simplified sensor model formed by one single wall nanotube, we develop a self-consistent approach to describe the variation of the linear dielectric susceptibility of the tube at the atomic scale when molecules are adsorbed at its external surface. The sensitivity of this model sensor is tested as a function of the apolar or polar nature of the admolecules, their adsorption geometry, their concentration, and the characteristics of the tube (length, diameter,...). The comparison with data on dielectric constant changes vs adsorption, coming from measurements of the resonance frequency shifts, displays striking agreement for most of the molecular species considered.

Geometry and dynamics of formic and acetic acids adsorbed on ice

Energy optimization at 0 K and constrained molecular dynamics simulations at 250 K have been carried out to study adsorption and incorporation of formic and acetic acids on/in ice. The results show that the adsorption and incorporation processes are highly influenced by the formation of two H-bonds between the carboxyl function and two water molecules. The free energy profiles indicate that the two acid molecules are strongly trapped at the ice surface and that the incorporation of formic acid is favored when compared to acetic acid. These data are discussed within the context of tropospheric conditions.

A molecular dynamics study of the structure of water layers adsorbed on MgO(100)

Molecular dynamics simulations are performed at various temperatures (150-300 K) and coverages (1-3 layers) on the adsorption of water on a clean MgO(100) surface using semiempirical potentials. At the monolayer coverage, a number of very stable (m×n) structures are obtained which differ only by the mutual orientations of the molecules. The p(3×2) phase observed above 180 K in low-energy electron diffraction (LEED) and helium atom scattering (HAS) experiments is shown to be the most stable at 200 K and above this temperature. It contains six inequivalently oriented molecules which lie flat above the cation sites with the hydrogens pointing approximately along the Mg rows. When the water coverage increases, a layer of icelike hexagonal structure within which the water molecules are hydrogen bonded is formed above the stable monolayer. This overlayer, which is stable at 150 K, is not hydrogen bonded to the stable monolayer. At 300 K it tends to break up and to aggregate into a 3D ice structure with strong h...

Adsorption of polar molecules on substrates with strong electric surface fields: from aggregates to monolayers

Abstract The adsorption of ammonia and water molecules on MgO (100) and NaCl (100) substrates, which generate strong electric surface fields, is studied for the first steps of aggregation (M) N N=2, …, 5 and for the monolayer. Potential calculations show that water molecules form planar aggregates on both substrates and, at higher coverage, commensurate (2x2) monolayers on MgO or (4x2) bilayers on NaCl. In contrast, the competition between the electrostratic interactions due to the substrate and the lateral molecular interactions leads to a specific behavior of ammonia admolecules: on MgO they do not aggregate and can only arrange in incomplete monolayers, while on NaCl they form 3D aggregates with a tendency to 3D crystallization at higher coverage.

Adsorption of NH3 on MgO(100): a comparative study of ab initio and semi-classical calculations

Abstract Three ab initio approaches (i.e., molecular Hartree-Fock, periodic crystal Hartree-Fock and embedded perturbed cluster Hartree-Fock) were used to determine the adsorption site and energy of ammonia molecules on an ideal MgO(100) surface. Each approach is discussed by comparing it with previous semi-classical calculations and with data from laser-induced thermal desorption and low-energy electron diffraction experiments. It is shown that the latter two ab initio methods offer complementary information and their results are fairly consistent with the available data.

Gas-induced variation in the dielectric properties of carbon nanotube bundles for selective sensing

There is an increasing demand for robust, miniaturized sensors with ppm or parts per 109(ppb) sensing capability, and high selectivity to different chemical or biological species. Here we show that trace amounts (ppb) of gases or organic solvent vapors can be detected with high selectivity and sensitivity using single-walled carbon nanotube bundles in a resonator configuration. The enhanced sensing properties result from a change in the effective dielectric properties of the resonator when exposed to different gas environments. A theoretical model is described which computes resonant frequency shifts that are in remarkable agreement with corresponding experimental shifts exhibited by the resonator when exposed to different gas molecules. This work demonstrates a gas-sensing platform with superior sensitivity and selectivity for gas detection, and presents advantages in terms of portability and recovery time. In particular, the sensing platform does not require functionalized carbon nanotubes to enhance sp...

Targeted molecular dynamics of an open-state KcsA channel

Pore opening of KcsA channel is studied using targeted molecular dynamics simulations. Conformational changes of the protein are determined, starting from the crystallized refined 2.0 A structure (pdb 1K4C) determined in x-ray experiments and arriving to the open-state structure constructed on the basis of electron paramagnetic resonance spectroscopic data (pdb 1JQ1). Our results corroborate the essential role played by the terminal residues located on the transmembrane helices M2 which were not taken into account at that time. The aperture mechanism of the channel appears to be ziplike. A small constraint (approximately equal to 5 x 10(-2) kcal mol(-1) A(-2) per C(alpha)) applied to the terminal residues located on the intracellular side is sufficient to initialize the pore opening at the innermost part of the gate, but additional constraint must be applied to definitely complete the pore aperture. The open structure is proved to be a metastable state since releasing the constraint leads to another relaxed open conformation which seems to reach stability.