ABINITIO PREDICTION OF THE STRUCTURE AND STABILITIES OF THE HYPERMAGNESIUM MOLECULES - MG2O, MG3O, AND MG4O

The concept of hypermetalation, cbaracterized by molecules with unprecedented stoichiometries, is extended to the magnesium-oxygen combinations, Mg2O, Mg3O, and Mg4O. Their equilibrium geometries and fundamental frequencies were calculated at the HF/6-31G* and correlated MP2 (full)/6-31G* levels. Extensive searches of possible structures and electronic states were carried out. The global minima are as follows: linear Mg2O (D(infinity h), both singlet 1-SIGMA(g)+ and the triplet 3-SIGMA(u)- states), planar Mg3O (D3h, 3A1'), and Mg4O forms slightly distorted from the square planar arrangement (D2d, 1A1 and planar MgOMg3 (C(s), 1A1) structures have nearly the same energy). These Mg2O, Mg3O, and Mg4O species are stable with regard to all possible decomposition pathways. Representative dissociation energies are the following: Mg2O, 74.5 kcal/mol into MgO + Mg at QCISD (T)/6-311+G(3df)+ZPE; Mg3O, 26.7 kcal/mol into Mg2O + Mg at QCISD(T)/6-311+G*+ZPE; and Mg4O, 7.9 kcal/mol into Mg3O + Mg at MP4SDTQ/6-311+G*+ZPE. Magnesium-magnesium bonding contributes significantly to the stability of Mg3O and Mg4O.