Gianfranco La Manna

DFT studies on catalytic properties of isolated and carbon nanotube supported Pd9 cluster–I: adsorption, fragmentation and diffusion of hydrogen

The processes of adsorption, fragmentation and diffusion of hydrogen on a small palladium cluster have been investigated by means of DFT and DFT/MM approaches. These studies have been performed by considering a D(3h) symmetry Pd(9) in the isolated state as well as when supported on a portion of single-walled armchair(6,6) carbon nanotube. The hydrogen fragmentation process easily occurs on the bare Pd(9) cluster, involving energy barriers of 25-35 kJ mol(-1) and the drop in spin multiplicity on passing from the reactant to the product. The atomic hydrogen diffuses through the cluster atoms with energy barriers, which do not exceed 20 kJ mol(-1), with some positions clearly identifiable as the most stable. In the case of the palladium supported system, which is a better model to simulate experimental conditions, calculations predict that the hydrogen fragmentation barrier is reduced by ca. 15 kJ mol(-1), with respect to that of the unsupported system, while the energetics of the diffusive process is not significantly affected by the support, if the reduction of the number of sites available in the same palladium cluster, as well as their geometry, are taken into account.

AB INITIO CONFORMATIONAL STUDY OF 2,2':5',2''-TERTHIOPHENE

Abstract The conformers of 2,2′:5′,2″-terthiophene were determined through ab initio MO full geometry optimization with a 6-31G * basis set. Different minima were found corresponding to twisted anti, anti, syn, anti and syn, syn structures. An estimate of the interconversion rate between conformers was performed.

Ab initio study on the photoisomers of a nitro-substituted spiropyran

Structural and spectroscopic properties of the photoisomers of a nitro-substituted spiropyran have been investigated by performing ab initio molecular orbital (MO) calculations both in vacuo and in hexafluoro-2-propanol solution. Full geometry optimisation of the closed form and of the transoid conformations of the open form has been carried out. Dipole moments of both photoisomers have been determined, the ratio of which agrees with recent experimental results. Net atomic charges have also been determined according to three different approaches.

SEMIEMPIRICAL CALCULATIONS ON THE INTERACTION BETWEEN DIMETHYLTIN(IV) AND DNA MODEL SYSTEM

Abstract The interaction between a dinucleotide triphosphate duplex (DD), mimicking the DNA molecule, and the dimethyltin(IV) cation is studied by the semiempirical PM3 method. The results show that the interaction can occur involving the tin atom and the electron-donor centres of DD, requiring in some cases the presence of water molecules. In particular, the binding of the dimethyltin(IV) moiety with two adjacent phosphate oxygen atoms is allowed by the presence of water molecules coordinating to the tin atom. In this case the tin environment shows a geometry in agreement with 119 Sn Mossbauer and X-ray data.

Computational studies on surface reaction mechanisms: ethylene hydrogenation on platinum catalysts

A new computational approach that combines kinetic and thermodynamic methods to mimic surface reactions is reported. Thermodynamics is mainly used to calculate, from experimental and/or quantum chemistry data, the parameters to be used in kinetic studies performed by Monte Carlo simulations. Monte Carlo algorithms are formulated in terms of the time dependent version. An innovative statistical approach is presented for cutting down the computational time required for the simulations. The reported discontinuity in the Arrhenius plot for the hydrogenation of ethylene on Pt/silica catalysts, and other peculiar properties of the above reaction on platinum catalysts, are extensively discussed as an example of reliability of the method. A new approach to metal catalysts and their role in catalytic reactions is also presented.

Theoretical evaluation of structures and energetics involved in the hydrogenation of hydrocarbons on palladium surfaces

Abstract Quantum mechanical calculations were employed to deepen information about the basic properties of the mechanism occurring in the palladium catalytic hydrogenation involving surface hydrogen species of unsaturated hydrocarbons. Activation energy values of elementary events implicated in the whole reaction were computed by potential energy surface analyses, to determine the corresponding occurrence probabilities of the same events. Different surface hydrogen species were pointed out and their relevance in the overall surface process was discussed together with their geometrical and chemical characteristics.

The interaction of deoxyribonucleic acid with methyltin(IV) moieties in solution studied by small-angle X-ray scattering, circular dichroism and UV spectroscopy

The nature of calf thymus DNA in aqueous solution, in the presence of the organotin(IV) species Sn IV Me 3 , Sn IV Me 2 and Sn IV Me, possibly partly hydrolysed and/or hydrated, was investigated by small-angle X-ray scattering (SAXS), ultraviolet spectroscopy (UV) at different temperatures and circular dichroism (CD). The results are compared with those of previous 119 Sn Mossbauer studies on condensed DNA phases. The effects of tin-phosphate oxygen bonding on the DNA melting profile in DNA-Sn IV Me systems are in agreement with previous reports on DNA-Main Group metal ion interactions. The structure and conformation of the DNA double helix are not influenced by Sn IV Me n species, even in experimental conditions near the onset of DNA condensation. Gel electrophoresis indicates that DNA is not degraded by Sn IV Me n ; the X-ray radii of gyration of the DNA cross-section and its mass per unit length, determined by SAXS from aqueous solutions, are constant and independent of the nature and concentration of the Sn IV Me n species present in the solutions. The invariance of the interparticle distances in solution at very low ionic strengths, in the presence of increasing amounts of Sn IV Me n compounds, even approaching DNA precipitation, suggests that the condensation process involves isolated DNA chains.

Kinetic study of 1,5-cyclooctadiene hydro-isomerization on Pd/pumice catalysts

A new route for obtaining 1,4-cyclooctadiene by catalytic isomerization of 1,5-cyclooctadiene at low temperature in presence of H2 in a three-phase reactor is presented. Wei, Prater and Silvestri methods are used for the kinetic studies. A comparison of the activity-selectivity patterns of the Pd/pumice catalysts with Pd on silica and Pt and Pd–Pt on pumice catalysts is performed. Although Pt catalysts are not active in the hydro-isomerization of 1,5-cyclooctadiene, the presence of small amount of Pt in Pd/pumice catalysts increases the yield to 1,4-cyclooctadiene. Besides the catalyst characteristics, temperature and H2 pressure influence the production of 1,4-cyclooctadiene. A mechanism of the process is proposed.

ONIOM study on the equilibrium geometries of some cyclopeptides

Abstract The geometries of two octacyclopeptides, cyclo[( d -AmP- l -AmP) 4 ] (AmP=α-aminopentanoic acid) and cyclo[( d -Ala- l -Phe) 4 ] were obtained by DFT and ONIOM methods. The resulting data show a substantial agreement with a computing time about three times lower in the case of the ONIOM procedure. This can be exploited in the study of polymeric structures set up by cyclopeptidic monomeric units.

Ab initio calculations and vibrational spectroscopy on the phenylenediamine isomers

Abstract Molecular orbital calculations at HF and MP2 levels have been performed using the 6-31G ∗∗ basis set for full geometry optimization of the phenylenediamine isomers. Our results show that only a transoid conformer is found for o -phenylenediamine, whereas cis and trans conformers exist for m - and p -phenylenediamine. Vibrational normal modes have been also analyzed for the gas phase and in chloroform solution, and compared with experimental data we have obtained using FTIR spectroscopy.