L. Di Sipio

Semi-empirical molecular orbital theory. The one-centre quantities for the elements of the first and second transition series

Abstract One-centre quantities for the elements of the first and second transition metal series are given which can be used instead of the corresponding one-centre integrals in semi-empirical MO calculations.

Molecular orbital theory for metal-halides complexes. the electronic structure and the spectrum of [PdCl4]2− ion

Abstract The electronic structure of the ground state is studied by semiempirical SCF-MO-LCAO calculations. An assignment of the electronic spectrum is given on the basis of both a configuration interaction calculation and some SCF calculations for lowest ecxited states.

Polarized electronic absorption spectra of the trigonal crystal K2ReF6

Linearly polarized (α, σ, π) electronic absorption spectra have been measured on trigonal crystals of K2ReF6 at temperatures down to 4·2 K. Almost all of the expected magnetic dipole electronic transitions associated with the t 2g /3 configuration have been identified between 9000 and 20000 cm-1. Each electronic transition shows detailed electric dipole vibronic structure which has been analysed. There is evidence for a moderate Jahn-Teller coupling to v 2 and v 5 modes within the trigonal components of the Γ8(2 T 1g ) and Γ8(2 T 2g ) states and for vibronically enhanced resonance coupling between nearby states. The results are compared with previous studies on Re4+ complex ions and with IrF6.

Absorption spectra of tetrahalodioxouranium(IV) complexes at 4.2 K. II. Di(tetramethylammonium)uranyltetrabromide

Abstract Single crystal spectra of Me 4 N) 2 OU 2 Cl 4 have been recorded at 4.2 °K, using polarized radiation. The analysis of the electronic absorption bands shows several progressions in the region 20,000–28,000 cm −1 , some of them strongly polarized. The most intense bands clearly show progressions (R + nv 1 ), where v 1 assumes the mean value of 700 cm −1 and therefore it has been taken as the symmetric stretching wavenumber of the uranyl group in the excited states.

Non-isothermal analysis of the crystallization of the amorphous germanium dioxide

Abstract The amorphous germanium dioxide devitrification has been investigated, with particular regard to the kinetic aspects and to the crystallization mechanism. The amorphous material transforms into the hexagonal dioxide modification, which is metastable at the considered temperatures. This fact is in accordance with the presence of tetrahedrally coordinated Ge in both the glass and the devitri fied material. The crystallization kinetics has been investigated by means of the DTA technique; a rigorous method of analysis has allowed the determination of the activation energy Eact, the Arrhenius preexponential factor A and the morphological index n characteristic of the devitrification reaction. We have so obtained the following values: E = 233 kJmol−1, A = 6.57 · 107s−1 and n = 2.44. These results are not in total accordance with previous studies carried on with similar non-isothermal techniques. Our interpretation is supported also by the morphological study of the GeO2 crystal growth carried out by scanning electron microscopy.

Low-temperature absorption spectra of the MoO2−4 ion in Cs2SO4 and of the ReO−4 ion in RbClO4 and in (C2H5)4NClO4

Abstract Absorption spectra of single crystals of Cs 2 SO 4 doped with MoO 2− 4 and of RbClO 4 and (C 2 H 5 ) 4 HClO 4 doped with ReO − 4 have been measured at the liquid-helium temperature. All spectra show two band systems with pronounced vibrational structures. In T d symmetry they must correspond to 1 T 2 - 1 A 1 charge-transfer electornic transitions. It is likely that in the two band systems there are more than two electronic transitions.