THE ELECTRONIC AND GEOMETRICAL STRUCTURE OF ALUMINUM FLUORIDE ANIONS ALF(N)(-), N=1-4, AND ELECTRON-AFFINITY OF THEIR NEUTRAL PARENTS

The electronic and geometrical structure of AlF-, AlF2-, AlF3- and AlF 4- as well as their neutral parents are determined at the unrestricted Hartree-Fock (UHF) and second-order unrestricted Moller-Plesset (UMP2 ) levels of theory. The results of the calculations are used for estim ating the adiabatic electron affinity (E.A.) of the neutrals and fragm entation energies of both the neutrals and anions. All these character istics were also recomputed using the UMP2/6-31+G geometry at the fou rth-order UMP4 level of theory. According to the results of the single -, double, triple-, and quadruple-excitation fourth-order Moller-Pless et (MP4SDTQ) calculations, the AlF molecule in the ground state has no positive E.A., AlF2 possesses a rather high E.A. value of 1.90 eV, an d AlF3 has a smaller E.A. of 0.93 eV, whereas the last member in the s eries AlF4 possesses an extremely high E.A. of 7.96 eV. Such a large v alue is related to high stability of the anion towards fragmentation o pposite to the neutral AlF4 whose dissociation energy is computed to b e only 4-5 kcal/mol. It has been predicted that all the anions are rat her stable towards detachment of a fluorine anion, and AlF4- is the mo st stable in the series. It is shown the AlF molecule is able to attac h an additional electron at large interatomic distances and form a sta ble anion. This indicates a possibility of an increasing tendency to f orm resonances when the molecule is excited into a higher vibrational state. Our calculations also show that AlF in the lowest tripler state may attach an additional electron to form a metastable anion in the q uartet state. This anion is predicted to be stable towards both detach ment of the extra electron and dissociation. A similar increasing tend ency to formation of resonance states with an additional electron with vibrational excitation is also predicted for AlF2. Also in this case, at some stretched geometries, the anion in the lowest triplet state b ecomes more stable than the neutral system, However, relaxation of its geometry leads to its decay to the neutral in the doublet state plus a free electron.