On the Structure of Lead(II) Complexes in Aqueous Solutions. III. Hexanuclear Clusters

Optimum geometries based on experimental structures and corresponding electronic structures of [Pb6(μ3-OH)8]4+ and [Pb6O(μ3-OH)6]4+ clusters as well as of their hydrated [Pb6(μ3-OH)8(H2O)6]4+ and [Pb6O(μ3-OH)6(H2O)6]4+ analogues are investigated using the semiempirical MNDO method of quantum chemistry. Direct Pb-Pb and O-O bonds are vanishing in the systems under study. Small differences in interatomic Pb-Pb and Pb-O distances between these clusters indicate that they cannot be distinguished by recent experimental techniques in solution. Both the mentioned structures are stable and might coexist in solution.

Molecular electronic structure of subphthalocyanine macrocycles

Quantum chemical calculations at semiempirical (MNDO methods) and ab initio (6-31G and STO-3G basis ses) levels have been performed on boron(III) subphthalocyanines 1-10. Theoretical calculations predict a cone-shaped structure for these compounds independently of the kind of peripheral substitution and even of compositional changes in the central region of the macrocycle (for example, substitution of the boron atom by two hydrogens). The theoretical calculations are in excellent agreement with previous X-ray determinations.

Theoretical studies of heterofullerenes C68X2 (X = N, B)

A systematic investigation on the molecular structures of the possible energetically favored isomers of C68X2 (X = N, B) has been performed using the semiempirical MNDO, AM1, and PM3 methods. The most stable isomer of C68X2 was computed, and the static properties such as heats of atomization, ionization potentials, electron affinities, absolute electronegativities, and global hardness have been predicted theoretically. The proposition that the changes of hybridization from sp2 to sp3 are the predominant factors of the regiochemistry for both C60X2 and C58X2 are supported by the calculations, and the regioselectivity of the doped fullerenes may be deduced in light of the knowledge of the fullerene adducts based on this proposition.Key words: C68N2, C68B2, MNDO, AM1, PM3.

On the Reliability of the Structures of Lead(II) Hydroxo Complexes Obtained by MNDO Method

Optimum geometry of tetrahedro-Pb4[(μ3-OH)4]4+ cluster obtained by the semiempirical MNDO method corresponds to Td symmetry. The calculated Pb-Pb (3.70 · 10-10 m) and Pb-O (2.33 · 10-10 m) distances are in reasonable agreement with experimental X-ray data. Additional H2O inclusion makes the agreement worse.

The Chemisorption of C on Al(111)

The chemisorption of C atoms on Al(111) was studied by means of the semiempirical MNDO method. The incorporation of the C adatom into the clusters was considered and the most favorable chemisorption site was evaluated. The hcp in-surface site was identified as the most stable adsorption site, in agreement with recent scanning tunneling microscopy (STM) experiments. This fact allowed us to use the MNDO method to evaluate the interaction energy of two chemisorbed C atoms in order to understand the observed formation of an ordered carbon underlayer. The low mobility of single isolated C adatoms revealed in STM experiments is also discussed.