The role of aluminum substitution on the stability of substituted polyhedral oligomeric silsesquioxanes

A first principles density functional theory calculation has been carried out to study the energetics and structural properties of substituted polyhedral oligomeric silsesquioxane (POSS) as a function of the POSS cage size. The substitution of Si atom in the POSS molecules has been performed in three different ways, namely (i) both Si and an adjacent H atom are replaced by an Al atom, (ii) only a Si atom is replaced by an Al atom, and (iii) a Si atom is replaced by an Al and a Na/K atom. For the first and third kind of substitution, the net spin state is the same (i.e. closed shell), while the structures corresponding to the second kind of substitution are characterized by an excess spin (i.e. open shell). The structures of substituted POSS for the first and second kind of substitution are greatly distorted and always energetically less stable than the parent structure, while for the third kind of substitution, the stability depends on the cage size of POSS molecule, as also shown in a previous work (Asaduzzaman et al. [1]).