M. Simonetta

The molecular structure of biphenyl in the gas and solid phases

‘Molecular mechanics’ has been used to calculate the geometry of biphenyl in the gas and crystalline phases. The geometry of the isolated molecule is mainly determined by a balance of π-electron and non-bonded energies, while in the crystal the most important forces are the intermolecular C … H attractions. Reasonable agreement is obtained with electron diffraction, x-ray and thermal data.

Valence‐Bond Theory for Simple Hydrocarbon Molecules, Radicals, and Ions

Valence‐bond (VB) theory is formulated in a general form, to be applied to states of any multiplicity, with inclusion of both spin and orbital degeneracy. Structures are related to products of spin functions, the leading terms, and a one‐to‐one correspondence is established between these products and extended Rumer diagrams. A relationship is also found with genealogical spin functions. The leading terms can be directly used in the calculation of energy eigenvalues and eigenvectors. Applications to simple hydrocarbon systems are shown, and the results are compared with experimental data.

Benzene adsorption on the Rh(111) metal surface: A theoretical study

Abstract The chemisorption of benzene on the Rh(111) surface is studied in the general framework of the extended Huckel theory, with optimized Rh parameters. MO arguments are used to discuss the main features of the electronic interaction of the aromatic system with the surface. Binding energy curves for adsorption on the most likely surface sites are computed, and the effect of the CH bond back-bending and of tilting of the benzene plane are examined. The most favorable chemisorption geometry is found when the C-atom ring is parallel to the surface and the center of the ring is above a threefold hollow site, but H atoms are farther away from the surface than C atoms. The optimum CRh distance compares well with those found in metal complex molecules. The on-top site is unfavorable for benzene chemisorption. The results of tight-binding calculations on infinite slabs agree with, and support, those of simple cluster calculations. A slight “Kekule distortion” and a small activation barrier to chemisorption are predicted.

An investigation of the geometry of the CH+5 ion by the CNDO method

Abstract The geometry of the CH+5 ion has been investigated employing the semiempirical LCAO SCF (CNDO/2) method. A configuration with Cs symmetry has been found to be the most stable.

An extended hückel study of adsorption of acetylene on Pt(111)

Abstract The parameterization of Extended Huckel MO Theory for the study of the interactions between hydrocarbons and Pt is analysed in detail. Single-ζ and double-ζ d functions of variable diffuseness are tested. The stability of acetylene and many molecular fragments and atoms on the surface is discussed in terms of binding energies and hydrocarbon—surface bond stretching force constants. Pt…H non-bonded repulsions are seen to be relevant, and a formula is proposed to describe these interactions. The metal is shown to offer a large catalytic effect in the reactions of rearrangement of acetylene, and, depending on the temperature and the availability of free hydrogen, H 3 CC or H 3 CCH are shown to be the most likely surface species. The geometries of adsorption compare rather favourably with LEED intensity studies. The use of EHT in the calculation of density of states curves and electronic spectra is discussed.

A conformational study of phenyl- and naphthyl-naphthalenes

Abstract The energies of 1-phenyl-naphthalene, 2-phenyl-naphthalene, 1-(1′-naphthyl-)naphthalene, 1-(2′-naphthyl-)naphtha]ene, and 2-(2′-naphthyl-)naphthalene have been calculated as a function of the twist angle. The total energy was obtained as a sum of the π-electron energy, nonbonded interactions and the energy of stretching for the inter-ring bond. For the most interesting conformations, the positions and the intensities of the first electronic bands have been calculated by the MIM method, and compared with experimental spectra. The possibility of using the spectroscopic results in conformational analysis is discussed.

Extended Hückel study of the chemisorption of acetylene on the Pt(III) surface

Abstract The ease of approach of an acetylene molecule to the various possible sites for chemisorption on the Pt(III) surface was studied by extended Huckel calculations. The results are compared with the outcome of LEED intensity analysis.

Ab-initio valence bond calculations

Ab initio valence bond calculations have been performed for the ground and excited states of H2S, H2S+, H2S-, SH, SH+, SH-. The dependence of calculated properties such as energy, ionization potential, dissociation energy, and dipole moment on the starting basis set, on the amount of configuration interaction and on the number of electrons explicitly included in the calculation is studied. It is found that freezing the electron core is a satisfactory approximation leading to a drastic reduction of computation time while preserving the quality of the results as compared with experiment and MO calculations. In addition, the geometry of the ground state of H2S was optimized.

Valence Bond Calculations of Spin Density Distribution in Benzyl Radical

The a priori valence bond method in the σ‐π approximation has been applied to benzyl radical with inclusion of an increasing number of ionic structures. When all structures are included, the hyperfine coupling constants calculated from spin densities through the McConnell relationship are in excellent agreement with experimental results. Comparison with other theoretical results is also included.

A full dynamical leed analysis of the NI(111) P(2 × 2)-C2H2 structure

Abstract The p(2 × 2) structure formed upon adsorption of acetylene on the Ni(111) surface, for 0.5 L exposure at 250 K, is studied by a full dynamical LEED analysis based on the comparison of only five I7ndash;V profiles at normal incidence. Among the several models tested, the most probable one is that with the adsorbed molecule parallel to the surface plane in a μ-bridging bonding site. The NiC distance is equal to 2.1 A.