Small Group IIa–VIa Clusters and Related Systems: A Theoretical Study of Physical Properties, Reactivity, and Electronic Spectra

Abstract

AbstractIn the monomeric form, the title systems assume unexpected optical properties, and as oligomers they can serve as candidates for molecular devices. In bulk they are attractive in the area of material science. A broad set of quantum chemical methods ranging from density functional theory by the Hartree–Fock method and the Møller–Plesset perturbation theory to the coupled‐cluster method, in connection with nonrelativistic Dunning's basis sets as well as with relativistic SDD basis sets were used. Electronic spectra were analyzed by means of time‐dependent DFT, with the symmetry‐adapted cluster configuration interaction and with the internally contracted multireference configuration interaction. The Douglas–Kroll–Hess quasirelativistic Hamiltonian served as a basis for the estimation of the role of relativistic effects. All the diatomics representing 25 elements of a matrix formed by the combination of group IIa (Be, ..., Ba) with group VIa (O, ..., Po) atoms were characterized by calculated bond length, valence vibration, dipole moment, and the Hartree–Fock frontier orbital energies. Calculated characteristics were confronted with experimental data. The geometry of the oligomers (from dimers through hexamers) was investigated systematically, and the nature of the located stationary points on the respective potential energy surfaces was established. Special attention was paid to the four lightest elements of the group IIa–VIa matrix, i.e. BeO, BeS, MgO, and MgS. As for electronic spectra, the systems of the first column (BeO, ..., BaO), the first row (BeO, ..., BePo), and the systems of the main diagonal (BeO, ..., BaPo) were calculated in the form of monomers and dimers. Analogous calculations were performed for a few group Ia–VIIa, IIIa–Va, IVa–IVa, and IIb–VIa systems. The group IIa–VIa and IIb–VIa diatomics exhibit electronic transitions in the visible region of the spectrum, and the longest wavelength bands are located in the near‐infrared region. MgO represents an extreme with the first band appearing at about 3500 cm–1, which strictly speaking makes the use of the Born–Oppenheimer approximation questionable. Whenever experimental transitions are available, the agreement between the calculated and observed band positions is good. Passing from monomers to oligomers is always associated with a significant hypsochromic shift in the first transition.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)