The application of MIM method to spiroconjugated systems

The Molecules in Molecules method has been applied to spiroconnected systems. Energy, polarisation and intensity of electronic transitions of spiro(5,5)undeca-1,4,6,9-tetraene-3,8-dione have been compared with experimental data and PPP results. Through a configuration analysis, PPP wave functions have been interpreted in terms of MIM configurations. The results show that the method is conveniently applicable to spiroconnected systems.

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Zusammenfassung Die Molecules in Molecules-Methode wurde auf Spiro-verbundene Systeme angewendet. Es wurden Energie, Polarisation und die Intensität der elektronischen Übergänge von Spiro(5,5)undeca1, 4,6,9-Tetraene-3,8-dione mit den experimentellen Werten und PPP-Ergebnissen verglichen. PPP-Eigenfunktionen wurden in MIM-Konfigurationen ausgedrückt. Die Ergebnisse zeigen, daß die Methode für Spiro-verbundene Systeme verläßlich ist.

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Résumé La méthode molecules in molecules a été appliquée à molécules spiroconjugées. On a comparé les énergies, la polarisation et l'intensité des transitions électroniques de spiro (5,5)undeca-1,4,6,9-tetraene-3, 8-dione avec les données expérimentales et les données PPP. Parmis un Configuration Analysis on a interprété les PPP fonctions d'onde en termes de MIM configurations. Les résultats indiquent que la méthode est applicable aux systèmes spiroconjugées.

     

Quasi-classical trajectories study of the chemisorption of H2 on the Pt(111) surface: effect of surface motion

The effect of energy exchange on the dynamics of the chemisorption of H₂ on Pt (111) is studied within a LEPS potential quasi-classical trajectories treatment. The Einstein model is assumed for the solid. The rigidity of the surfaces is shown to be an acceptable approximation.     

Surface models and reaction barrier in Eley-Rideal formation of H2 on graphitic surfaces

The exothermic, collinearly-dominated Eley-Rideal hydrogen formation on graphite is studied with electronic structure and quantum dynamical means. In particular, the focus is on the importance of the model used to describe the graphitic substrate, in light of the marked discrepancies present in available literature results. To this end, the collinear reaction is considered and the potential energy surface is computed for a number of different graphitic surface models using Density Functional Theory (DFT) for different dynamical regimes. Quantum dynamics is performed with wavepacket techniques down to the cold collision energies relevant for the chemistry of the interstellar medium. Results show that the reactivity at moderate-to-high collision energies sensitively depends on the shape of the PES in the entrance channel, which in turn is related to the adopted surface model. At low energies we rule out the presence of any barrier to reaction, thereby highlighting the importance of quantum reflection in limiting the reaction efficiency.     

Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. I. H-chemisorbed case

Collision induced (CI) processes involving hydrogen atoms on a graphite surface are studied quantum mechanically within the rigid, flat surface approximation, using a time-dependent wave packet method. The Eley-Rideal (ER) reaction and collision induced desorption (CID) cross sections are obtained with the help of two propagations which use different sets of coordinates, a "product" and a "reagent" set. Several adsorbate-substrate initial states of the target H atom in the chemisorption well are considered, and CI processes are studied over a wide range of projectile energy. Results show that (i) the Eley-Rideal reaction is the major reactive outcome and (ii) CID cross sections do not exceed 4 A2 and present dynamic thresholds for low values of the target vibrational quantum number. ER cross sections show oscillations at high energies which cannot be reproduced by classical and quasiclassical trajectory calculations. They are related to the vibrational excitation of the reaction products, which is a rather steep decreasing function of the collision energy. This behavior causes a selective population of the low-lying vibrational states and allows the quantization of the product molecular states to manifest itself in a collisional observable. A peak structure in the CID cross section is also observed and is assigned to the selective population of metastable states of the transient molecular hydrogen.     

Ab initio valence bond calculations. VII. HF,HF+, and H2F+

Ab initio valence bond calculations for the ground and excited states of HF and HF⁺ are presented. Total energies, equilibrium geometries, dissociation energies, dipole moments, and spectroscopic constants for HF and HF⁺ have been calculated. The photoelectron spectrum of HF has been examined and interpreted by means of the valence bond formalism. The ground state of the protonated species H₂F⁺ has been investigated.     

Chemistry at surfaces: from ab initio structures to quantum dynamics

Recent years have witnessed an ever growing interest in theoretically studying chemical processes at surfaces. Apart from the interest in catalysis, electrochemistry, hydrogen economy, green chemistry, atmospheric and interstellar chemistry, theoretical understanding of the molecule–surface chemical bonding and of the microscopic dynamics of adsorption and reaction of adsorbates are of fundamental importance for modeling known processes, understanding new experimental data, predicting new phenomena, controlling reaction pathways. In this work, we review the efforts we have made in the last few years in this exciting field. We first consider the energetics and the structural properties of some adsorbates on metal surfaces, as deduced by converged, first-principles, plane-wave calculations within the slab-supercell approach. These studies comprise water adsorption on Ru(0001), a subject of very intense debate in the past few years, and oxygen adsorption on aluminum, the prototypical example of metal passivation. Next, we address dynamical processes at surfaces with classical and quantum methods. Here the main interest is in hydrogen dynamics on metallic and semi-metallic surfaces, because of its importance for hydrogen storage and interstellar chemistry. Hydrogen sticking is studied with classical and quasi-classical means, with particular emphasis on the relaxation of hot–atoms following dissociative chemisorption. Hot atoms dynamics on metal surfaces is investigated in the reverse, hydrogen recombination process and compared to Eley–Rideal dynamics. Finally, Eley–Rideal, collision-induced desorption, and adsorbate-induced trapping are studied quantum mechanically on a graphite surface, and unexpected quantum effects are observed.     

Quantum effects in an exoergic, barrierless reaction at high collision energies

The exoergic Eley-Rideal hydrogen recombination on graphite surfaces is known to produce vibrationally hot product molecules. Realistic quantum scattering calculations at normal incidence over a wide range of collision energies show that the degree of vibrational excitation of the reaction product is a steep decreasing function of the collision energy. This results in selective population of the lower-lying vibrational levels and gives rise to an oscillating structure in the total reaction cross-sections at high energies, which cannot be reproduced by classical and quasi-classical trajectory calculations. An analogous quantum structure appears in the total collision-induced desorption cross-sections and is assigned to selective population of the metastable states of the transient molecular hydrogen.