Implications of molecular orbital symmetries and energies for the electron delocalization of inorganic clusters

Isostructural clusters exhibit contrasting magnetic properties when the no. of electrons differs. Surprisingly, the same is true even for isoelectronic cages (e.g. Oh B6H62- is diatropic, whereas Oh Si62- is paratropic) or for those with different substituents (e.g. Td B4H4 is paratropic, whereas Td B4F4 is diatropic). Indeed, the total nucleus-independent chem. shift (NICS) values, based on shieldings computed at cluster centers, may range considerably in magnitude and even change from diatropic (upfield shifted) to paratropic (downfield shifted). Similarly, individual dissected canonical MO contributions to the total NICS values computed at the "gauge-including AOs" level vary greatly. This contrasting behavior arises from MO energy differences, from the extent of orbital overlap and from symmetry-based selection rules derived from group theory. Differences in magnetic properties may originate from the symmetry of the orbitals; specifically from the forbidden nature of the HOMO->LUMO electronic excitation weighted by the occupied-unoccupied orbital energy difference. Thus, HOMO-NICS values are generally highly paratropic if the HOMO->LUMO rotational transition is allowed by symmetry selection rules. [on SciFinder (R)]