Structural changes induced by dehydration in the crystalline layered silicate Na-RUB-18: a computational/experimental combined study

Na-RUB-18 layered silicate (Na(8)Si(32)O(64)(OH)(8) center dot 32H(2)O) is prepared following the procedure reported in the literature (ref. 1: K. Kosuge and A. Tsunashima, J. Chem. Soc., Chem. Commun., 1995, 2427) and its significant structural modifications induced by progressive removal of hydration water molecules are studied for the first time by a combined experimental (TGA and variable temperature XRD and FTIR) and theoretical approach. A novel structure of a partially de-hydrated Na-RUB-18 obtained upon heating at 75 degrees C under inert gas flow, which leads to a reversible phase transformation in agreement with XRD and FTIR results, is found. TGA indicates that at this stage, two of the four hydration water molecules per Na ion are lost. Theoretical results based on DFT calculations suggest that the apical water molecules of the solvated octahedrally coordinated sodium ion, forming weak H-bonds with the silicate oxygen atoms, are removed at this stage. As a consequence of the dehydration, the silicate layers get closer, and, similarly to that observed for zeolites, the sodium ions move towards lattice oxygen atoms, to restore their coordination shell. A combination of IR spectroscopy and computational models is used to describe the vibrational properties of silanol/silanolate (SiOH/SiO)(-1) bridges, which are responsible for the proton conduction of Na-RUB-18. The method described can be of general utility to refine structures which are not fully accessible to standard X-ray structure analysis.191726102617European CommunityRegione Piemont

Excitation energy transfer (EET) between molecules in condensed matter: A novel application of the polarizable continuum model (PCM)

We present a quantum-mechanical theory to study excitation energy transfers between molecular systems in solution. The model is developed within the time-dependent (TD) densityfunctional theory and the solvent effects are introduced in terms of the polarizable continuum model (PCM). Unique characteristic of this model is that both "reaction field" and screening effects are included in a coherent and self-consistent way. This is obtained by introducing proper solvent-specific operators in the Kohn-Sham equations and in the corresponding TD scheme. The solvation model exploits the integral equation formalism (IEF) version of PCM and it defines the solvent operators on a molecular cavity modeled on the real three-dimensional (3D) structure of the solute systems. Applications to EET in dimers of ethylene and naphtalene are presented and discussed

Measurement and DFT calculation of Fe(cp)(2) redox potential in molecular monolayers covalently bound to H-Si(100)

The electron transfer to self-assembled molecular monolayers carrying a ferrocene (Fc) center, grafted on a flat Si(100) surface, is a recent subject of experimental investigation. We report here the density functional theory (DFT) ab initio calculation of Fc-silicon hybrid redox potentials. The systems were modeled with a slab of H-terminated Si(100) 1 x 1 and 2 x 1 surfaces: geometries were optimized using the ONIOM method, and solute-solvent interactions were included through the polarizable continuum model (PCM) method. Two new routes for Si functionalization with ethyl- (EtFC) and ethynyl-Fc (EFC) differing only in the unsaturation degree of the anchoring arm have been successfully explored, and the redox potential of the resulting hybrids has been measured by cyclic voltammetry: 0.675 and 0.851 V versus NHE for the EtFC and EFC derivatives, respectively. These values, along with the previously measured potential (0.700 V) for the mono-unsaturated derivative, vinyl-Fc, allow the relation between the unsaturation degree and the adduct redox potential to be studied. The comparison among the measured and computed potentials allows one to discriminate between different adduct isomers for the saturated species and more importantly provides strong indications that the carbon-carbon unsaturation initially present in the molecular arm used for anchoring to the surface is preserved upon addition, in contrast with the commonly accepted reaction mechanism