Theoretical study of isomerism of compounds of CO and CO2 molecules with aluminum clusters Al13, Al12Ti, and Al12Ni

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Structural characteristics, vibrational frequencies, and energies of isomers of compounds of CO and CO2 molecules with the centered aluminum cluster Al13 and its doped analogues Al12M (M = Ti and Ni) have been calculated by the density functional theory method. For the Al12MCO compounds, the most favor-able are two “fragment” isomers in which the C and O atoms are separated and built into the cluster cage, completing it to a 14-vertex polyhedron. In one of them, the C and O atoms are in the capping positions over adjacent trigonal MAl2 faces; in the second isomer, there is the five-coordinate C* atom located in the center of a tetragonal MAl3 face and bound to the central Al atom through the long fifth bond. The “coordinated” isomers, in which the CO molecule is coordinated as a ligand to a cluster vertex, edge, or face, are unstable to removal of CO for Al13CO, close in energy to the fragment isomers for Al12NiCO, and considerably higher on the energy scale than the fragment isomers but remain stable to CO removal for Al12TiCO. For the Al12MCO2 compounds, the most favorable is the fragment isomer in which both oxygen atoms are in the capping positions over adjacent faces and the C* atom is five-coordinate. The alternative oxo carbonyl isomer Al12MO(CO) is close to the lowest-lying one in the case of M = Ni and is ∼56 kcal/mol higher on the energy scale in the case of M= Ti. The less stable Al12M(CO2) isomer is the complex in which the CO2 ligand is coordinated to an M-Al edge. According to calculations, addition of CO to Al12MO and addition of CO2 to Al12M to form, respectively, Al12MO(CO) and Al12M(CO2) can occur without noticeable barrier. The Al12M(CO2) and Al12MO(CO) isomers are separated by a barrier, moderate for M = Ti (∼16 kcal/mol) and small for M = Ni (∼6 kcal/mol).