Giancarlo De Alti

Normal coordinate treatment and molecular constants for fluoro-, chloro- and bromo-forms

Abstract By a least-square iteration process and following the analytical criteria recently proposed by A ldous and M ills the recalculation of the quadratic potential energy function for fluoro-, chloro- and bromo-forms in a modified valence force field approximation was carried out, adopting actual interbond angles. The assignment of ν 2 fundamental for CF 3 H necessary to obtain convergence was found to be different from that used in previous calculations. The values of stretch—bend interaction constants which lead to a well-conditioned convergence suggest that the hybrid orbital force field approximation is applicable for the present molecules. Coriolis coupling constants, rotation distorsion constants, mean-square amplitudes of vibration and thermodynamic properties were also calculated.

Photoelectron spectra and electronic structure of furan homologues computed by the ms scf Xα method

Abstract The ionization energies of furan, thiophene, selenophene and tellurophene are evaluated by the MS Xα method. From the results obtained and a comparison with previous calculations an assignment or experimental spectra is proposed. An analysis of the nature of the molecular orbitals is presented.

On the validity of the Xα method in the computation of ionization potentials. A comparison of the results for small molecules

Abstract Xα calculations of the ionization potentials were performed on a series of small molecules by the MS-Xα (tangent and overlapping spheres, and LCAO-Xα methods. The results are compared with the experimental values and the results of previous accurate calculations. It is inferred that the Xα formalism is rather effective for the calculation of ionization potentials and possibly superior to the Koopmans theorem. Indeed in all considered cases the ordering inversions supplied by Koopmans theorem are removed in the Xα predictions. The LCAO-Xα method seems to be the most accurate of the Xα methods examined.

An MS-Xα study of the photoelectron spectra and bonding in the fluorosilanes

Abstract Calculations with the MS-Xα method, with and without overlap, STO 3G ab initio and CNDO/2, with different basis sets, were performed for the fluorosilane series. The valence ionization potentials evaluated with the transition state formalism are in essential agreement with the assignment previously proposed for the photoelectron spectra. Various inversions are predicted by the other methods. The electronic structure, orbital population and silicon d participation to the bond are discussed.

General valence force constants for group (V) B hydrides

Abstract Different sets of force constants for the group (V) B hydrides are calculated using different combinations of experimental frequencies, corrected for the anharmonicity, of wholly hydrogenated, deuterated and tritiated derivatives. Harmonic frequencies, variance, potential energy distribution, rotational distortion and Coriolis coupling constants are calculated from every set. A “best” set of force constants for each representative molecule is proposed.

Potential energy constants, mean-square amplitudes of vibration and rotational distortion constants for C2F4, C2Cl4 and C2Br4

Abstract By adopting the potential energy function of G.V.F.F. type the principal force constants for each of the perhaloethylenes C 2 F 4 , C 2 Cl 4 and C 2 Br 4 were refined through a best-fit iteration process. The carbon—carbon and carbon—halogen stretching force constants are discussed considering the possible effects of halogen atoms on the molecular structure. The mean-square parallel and perpendicular amplitudes of vibration at 298·16°K were computed. The rotational distortion constants were evaluated on the basis of a first-order perturbation method.

Ionized states of fluoromethanes computed by the SCF-MS-Xα method

Abstract A multiple scattering X α calculation was employed to compute the ionization potentials for all energy states relative to some fluorinated methanes. Overall agreement with the results of previous ab initio calculations was obtained and the earlier ordering was substantially confirmed. However some inversions within the same experimental band are predicted.

Semi-empirical and ab initio calculations on cyclopentene ring puckering

Abstract The ring puckering potential energy function relative to cyclopentene is calculated using the standard CNDO/2 method optimizing all geometrical parameters. Ab initio calculations are performed upon selected geometries and the equilibrium conformations are discussed.

MS Xα and LCAO Xα calculations of ionization potentials. A comparison with Koopmans' theorem and accurate calculations

Abstract Ionization potentials of small molecules are calculated by the MS Xα and LCAO Xα methods. The results are compared with the experimental values and the results of previous accurate calculations. The Xα method proves to be rather accurate and potentially superior to the HF method. Indication is found of a possible failure of the method in the fluoromethanes. The limitation of the MS Xα method is discussed.

CNDO calculations of ring puckering potential energy

Abstract The ring puckering potential energy functions are calculated for 2,3-dihydrofuran, 2,5-dihydrofuran, cyclopentene and cyclopent-3-enone using the standard CNDO/2 method. The equilibrium conformations are discussed and the ring puckering force constants as well as the dipole moments are evaluated. A more detailed analysis of the influence of angular deformations on the potential function of the cyclopentene molecule is performed.