J. -P. BLAUDEAU, M. -M. ROHMER, M. BÉNARD, , Laboratoire de Chimie Quantique, UPR 139 du CNRS, Université Louis Pasteur, 67000 Strasbourg, France; N. -E. GHERMANI, Laboratoire de Cristallographie et Modélisation des Matéraux Minéraux et Biologiques, CCM³B, URA 809 du CNRS, Université Henri Poincaré-Nancy, 54506 Vandoeuvre-lès-Nancy, France.

Ab initio Hartree-Fock calculations have been carried out on the octadecavanadate ion [V₁₈O₄₂]^12-( 1), on its protonated derivative [H₄V₁₈O₄₂]^8-( 2), and on the vanadophosphate cluster [V₇O₁₂(O₃PH)₆]^-( 3), taken as a model for [V₇O₁₂(O₃PR)₆]^- ( 3' ). The spheroidal clusters ( 2) and ( 3' ) have been recently characterized as \lq\lq electronically inverse hosts" in the encapsulation complexes Cs₉[X@H₄V₁₈O₄₂]· 12H₂O (X = Br, I) and (Ph₄P)₂[Cl@V₇O₁₂(O₃PPh)₆]. An estimate of the electrostatic potential distribution including the contribution of the lattice potential yields highly positive values of the potential inside the host cavities for ( 2) and ( 3). Those values are larger than the electrostatic potential computed at a vacant chloride site of the CsCl crystal, thus explaining the thermodynamic stability of the encapsulated anions.