SEMIEMPIRICAL STUDY OF ELECTRON EXCHANGE INTERACTION IN ORGANIC HIGH-SPIN PI-SYSTEMS - CLASSIFYING STRUCTURAL EFFECTS IN ORGANIC MAGNETIC MOLECULES

A semiempirical AM1 molecular orbital plus configuration interaction model is applied to pi-conjugated organic open-shell molecules in order to understand qualitatively the manner and degree to which they are spin-coupled by various electronic mechanisms, either in simpler open-shell systems or, in some examples, in larger polyradicals. A ''library'' of spin-coupling spacer groups (X in radical-X-radical or nitrene-X-nitrene) is delineated with relative exchange interaction strengths and natures described, including alkenes and polyalkenes, alkynes, phenyl rings, heteroatoms, and nonalternant pseudoaromatic systems. Example systems in which ferromagnetic (high-spin) coupling is observed in diradical models are predicted to remain high-spin as higher oligomeric species with like connectivity. The implications of these semiquantitative computational predictions in relation to simple qualitative models of radical-radical spin coupling are discussed in several cases. Overall, the semiempirical MO-CI method is shown to be both conceptually useful and highly cost effective for semiquantitative study and classification of broad classes of electron exchange coupling effects in non-Kekule molecules.