The theory of atoms in molecules as a tool to investigate the reactivity of tetraphosphacubane

Abstract

Bader's theory of atoms in molecules is used to rationalize the gas-phase reactivity of tetraphosphacubane vs, H+, Li+, Na+, and Be2+. For this purpose we have used MP2 densities obtained at the 6-31G(d,p) level. The characteristics of the C—P bonds of tetraphosphacubane are discussed. The Laplacian of its electron charge density shows that both phosphorus and carbon atoms are active centers for electrophilic substitutions. This is consistent with the fact that both phosphorus and carbon protonated species are minima of the potential energy surface. The strong charge redistribution associated with carbon protonation explains the enhanced stability of the carbon protonated species with respect to the phosphorus protonated one. The Laplacian field also shows the existence of a cavity inside the cage surrounded by high electronic density that can stabilize a cation of the appropriate size. Our results confirm that Li+ and Be2+ fulfil this requirement and the corresponding complexes, where the cation is located inside the cage, are minima of the corresponding potential energy surface. Na+ is far too large and a similar structure is a saddle point of the potential energy surface. Key words: atoms-in-molecules theory, tetraphosphacubane, reactivity, cationization.