Hybrid classical quantum force field for modeling very large molecules

A coherent computational scheme on a very large molecule in which the subsystem that undergoes the most important electronic changes is treated by a semiempirical quantum chemical method, though the rest of the molecule is described by a classical force field, has been proposed recently. The continuity between the two subsystems is obtained by a strictly localized bond orbital, which is assumed to have transferable properties determined on model molecules. The computation of the forces acting on the atoms is now operating, giving rise to a hybrid classical quantum force field (CQFF ) which allows full energy minimization and modeling chemical changes in large biomolecules. As an illustrative example, we study the short hydrogen bonds and the proton-exchange process in the histidine-aspartic acid system of the catalytic triad of human neutrophil elastase. The CQFF approach reproduces the crystallographic data quite well, in opposition to a classical force field. The method also offers the possibility of switching off the electrostatic interaction between the quantum and the classical subsystems, allowing us to analyze the various components of the perturbation exerted by the macromolecule in the reactive part. Molecular dynamics confirm a fast proton exchange between the three possible energy wells. The method appears to be quite powerful and applicable to other cases of chemical interest such as surface reactivity of nonmetallic solids. © 1996 John Wiley & Sons, Inc.     

Pyirolothieno[1,4]diazepines. Part IV. First synthesis of pyrrolo-[1,2-a]thieno[2,3-f][1,4]diazepine Derivatives

Treatment of 3-(2-formyl-1H-pyrrol-1-yl)-2-thiophenecarboxamide by various nucleophiles like methyl ketones, amines, alcohols, thiols or acetates led to new 5,6-dihydro-4H-pyrrolo[1,2-a]thieno-[2,3-f][1,4]diazepines.     

Probing Ag nanoparticle surface oxidation in contact with (in)organics: an X‐ray scattering and fluorescence yield approach

Characterizing interfacial reactions is a crucial part of understanding the behavior of nanoparticles in nature and for unlocking their functional potential. Here, an advanced nanostructure characterization approach to study the corrosion processes of silver nanoparticles (Ag‐Nps), currently the most highly produced nanoparticle for nanotechnology, is presented. Corrosion of Ag‐Nps under aqueous conditions, in particular in the presence of organic matter and halide species common to many natural environments, is of particular importance because the release of toxic Ag⁺ from oxidation/dissolution of Ag‐Nps may strongly impact ecosystems. In this context, Ag‐Nps capped with polyvinolpyrrolidone (PVP) in contact with a simple proxy of organic matter in natural waters [polyacrylic acid (PAA) and Cl^? in solution] has been investigated. A combination of synchrotron‐based X‐ray standing‐wave fluorescence yield‐ and X‐ray diffraction‐based experiments on a sample consisting of an approximately single‐particle layer of Ag‐Nps deposited on a silicon substrate and coated by a thin film of PAA containing Cl revealed the formation of a stable AgCl corrosion product despite the presence of potential surface stabilizers (PVP and PAA). Diffusion and precipitation processes at the Ag‐Nps–PAA interface were characterized with a high spatial resolution using this new approach.     

Sound propagation above a porous road surface with extended reaction by boundary element method

Acoustic impedance of an absorbing interface is easily introduced in boundary element codes provided that a local reaction is assumed. But this assumption is not valid in the case of porous road surface. A two-domain approach was developed for the prediction of sound propagation above a porous layer that takes into account the sound propagation inside the porous material. The porous material is modeled by a homogeneous dissipative fluid medium. An alternative to this time consuming two-domain approach is proposed by using the grazing incidence approximate impedance in the traditional single-domain boundary element method (BEM). It can be checked that this value is numerically consistent with the surface impedance calculated at the interface from the pressure and surface velocity solutions of the two-domain approach. The single-domain BEM introducing this grazing incidence impedance is compared in terms of sound attenuation with analytical solutions and two-domain BEM. The comparison is also performed with the single-domain BEM using the normal incidence impedance, and reveals a much better accuracy for the prediction of sound propagation above a porous interface.     

Optical scar in a chaotic fiber

We propose to use a multimode optical fiber with a D-shaped cross section as a privileged system to image wavefunctions of a chaotic system. Scar modes are in particular the subject of our investigations. We study their imprints on the statistics of intensity and we show how the introduction of a localized gain region in the fiber is used to perform a selective amplification of scar modes.