Theoretical study of isomerism of acetylene compounds with aluminum clusters doped with 3d transition metals

Abstract

Structural characteristics, vibrational frequencies, and energies of ten isomers of acetylene compounds with the centered aluminum cluster Al13 and its analogues Al12M doped with 3d transition metal atoms (M = Ti-Ni) in the states of different multiplicity have been calculated by the density functional theory method. In addition to “coordinated” intermediates in which the C2H2 molecule is coordinated through its C-C bond to the M vertex, an M-Al edge, or a trigonal face of the [MAl2] cluster, “fragment” isomers have been considered in which the acetylene molecule is broken into fragments (C2H + H, CH + CH, H + CH + C, and 2P + 2H) differently inserted into the aluminum cage and enlarging it to Al12MC2. For most compounds, low-lying isomers have structures 1–4 (the C2H2 molecule is coordinated to an Al2M face), 1–5 (two CH fragments are added to adjacent Al2M faces), and 1–8 (with a five-coordinate C* atom). Structure 1–1, in which the C2H2 molecule is coordinated through the C-C bond to the M dopant is unstable against transformation into 1–4 with a low barrier. An isomer with unusual structure 1–9 has been localized in which two five-coordinate C* atoms built into the aluminum cage are located in adjacent quasi-planar tetragonal [MAl3] faces and are bonded to the central aluminum atom (Alc) through the fifth bonds. The substitution of electronegative substituents X= F and Cl for H atoms in isomers 1–8 and 1–9 makes the latter more basic and clearly more favorable. The five-coordinate C* atoms in them are able to add acceptor ligands of the BH3 and AlH3 type and to increase the coordination number of the carbon atom to six with a considerable decrease in energy. The trends in the change in structural characteristics and relative energies of isomers with a change in M dopants along the 3d series, electronegativity of X substituents, and electronic state multiplicity have been analyzed.