DESIGN OF HIGH-SPIN MOLECULES INCORPORATING CHARGED PLUS NEUTRAL SPINCENTERS

A series of diradicals comprised of m-phenylene exchange coupling unit s and spin-bearing centers of the same (home-spin) and different (hete ro-spin) types were compared using the semiempirical AM1-CI molecular orbital method. Two meta-coupled neutral (or charged) hetero-spin cent ers result in a high-spin ground stare, while coupling of one neutral and one charged spin center gives rise to a low-spin ground state in t he 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 o f the monocharged ''hetero-spin'' cases. Effects of substitution and H OMO-LUMO splitting may be evaluated by computational methods to identi fy systems where resonance effects may override spin parity effects. ( C) 1998 John Wiley & Sons, Inc.