Structure and bonding of tetramer clusters: Theoretical understanding of the aromaticity

The ground state geometries and energetics of charged (Si4+2, Ge4+2, Si2Ge2+2, Si3Al+, Si3Ga+ Ge3Al+, and Ge3Ga+) and neutral (Al2Si2, Al2Ge2, Ga2Si2, Ga2Ge2, Si3Be, Si3Mg, Ge3Be, Ge3Mg, Al3P, Al3As, Ga3P, and Ga3As) tetramer clusters have been performed using ab initio molecular-orbital calculations. Harmonic vibrational analysis has been performed to ensure the stability of the optimized geometries. From the comparison of the total energies between possible isomeric structures it is found that both homo- and hetero-atomic tetramer clusters favor planar configuration as the lowest energy structure. The aromatic character of these clusters has been analyzed based on planarity, electronic stability, Nuclear Independent Chemical Shift (NICS) Values and their molecular orbitals pictures. The extra electronic stability of these clusters towards donating or accepting of electrons is reflected in the calculated hardness, large ionization potential and low electron affinity. To check the chemical stability of these tetramer clusters further calculations have been carried out for the interactions of hydrogen atoms with few representative cluster like Si4+2, Al4−2 and Al2Si2. Indeed, it is found that even after formation of four bonds with hydrogen atoms the planarity of these clusters retained and more interestingly, the delocalized π-electron cloud of the HOMO energy levels remained undisturbed.