Ab initio calculations of X@A(12)H(12)(2-) closo alane and gallane anions with X atoms of inert gases or halogens inside the icosahedral [Al-12] and [Ga-12] clusters

Hartree-Fock (HF) and density functional (B3LYP) calculations of geometrica l, vibrational, magnetic, and energetic parameters of the endohedral I-h st ructure of X@Al12H122- closo-alane and X@G(a1)2H(12)(2-) closo-gallane anio ns with interstitial noble gas atoms and hydride and halide ions (X = Ng, H -, Hal(-)) in the center of icosahedral [Al-12] and [Ga-12] clusters were p erformed with the 6-31G* and 6-311+G** basis sets. For the majority of syst ems studied, the endohedral structure corresponds to a minimum on the poten tial energy surface, which is 28 (He), 52 (Ne), 158 (Ar), 180 (Kr), 78 (H-) , 87 (F-), 199(Cl-), and 210 (Br-) kcal/mol higher than the dissociation li mit for alanes and 39 (He), 73 (Ne), 213 (Ar), 194 (Li+), and 120 (Na+) kca l/mol higher than the dissociation limit for gallanes. The potential energy barrier for the interstitial heteroatom X to come out through an edge (via the structure C-2v(b)) or a face (structure C-3v(t)) Of the [Al-12] cluste r is rather high for light atoms and ions X = He, Ne, H-, and F- (20-50 kca l/mol). Thus, the endohedral I-h structure for these compounds (like for si milar salts with the cation L = Li+, Na+, and Mg2+) is kinetically stabiliz ed and can exist as a high-energy intermediate. Characteristic features of vibrational spectra and Al-27 NMR chemical shifts for molecules with the en dohedral I-h structure are considered from the standpoint of the possibilit y of their identification by IR and NMR spectroscopy. The trends in the str uctural, energetic, and spectral properties of the molecules with an increa se in the radius and electronegativity of the central X atom upon its chang e along the subgroup and in isoelectronic series are considered, and the di fference between the properties of alanes and gallanes is discussed.