AB-INITIO STUDY OF THE ENDOHEDRAL COMPLEXES OF C-60, SI-60, AND GE-60WITH MONOATOMIC IONS - INFLUENCE OF ELECTROSTATIC EFFECTS AND HARDNESS

An ab initio study of endohedral complexes of C-60 and its Si and Ge a nalogues with first, second, third, and fourth row alkali metal and ha logen ions is performed using the multiplicative integral approximatio n and the 3-21 G basis set (3-21+G for the halogen ions). The properti es of the endohedral ions are studied, and complexation energies are c alculated using the counterpoise correction for the basis set superpos ition error. An (approximate) analytical expression for this complexat ion energy is derived within the framework of density-functional theor y. The complexation energy sequences are rationalized using this expre ssion and the properties of the fullerene cages, such as the electrost atic potential within the cage and the average distance of the electro ns from the cage center. Besides electrostatic effects, i.e., the inte raction of the ion with the electrostatic potential within the cage, t he influence of the cage and guest-ion hardnesses on the stabilization energies are also investigated: a local hard and soft acids and bases principle can be invoked to explain the complexation energies within alkali metal and halogen ion series.