Abstract
A series of diradicals comprised of m-phenylene exchange coupling units and spin-bearing centers of the same (homo-spin) and different (hetero-spin) types were compared, using the semiempirical AM1-CI molecular orbital method. Two meta-coupled neutral (or charged) hetero-spin centers result in a high-spin ground state, while coupling of one neutral and one charged spin center gives rise to a low-spin ground state in the cases studied. The latter result is ascribed to the large splitting of partially occupied molecular orbitals by substitution, leading to dominance of purely ionic resonance structures in the singlet states of the monocharged “hetero-spin” cases. Effects of substitution and HOMO–LUMO splitting may be evaluated by computational methods to identify systems where resonance effects may override spin parity effects. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:161–167, 1998
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