Charge transfer complexes studied by semiempirical and ab initio methods: The effect of geometrical constraints on complex stability

The AM1 parameter set was tested for its ability to describe weak charge-transfer (CT) complexes between aromatic donors and either aromatic or quinone acceptor molecules. After concluding that the method is generally suitable, the effects of various geometrical constraints upon the complex stabilization energies were studied. Substantial flexibility is found for the complexes, suggesting that minor geometrical constraints, which might be imposed in the case of intramolecular complexes, may not be destructive for complex formation. A qualitative trend was found, relating the complex strengths to the calculated CT-band λ_max values and the calculated oscillator strengths. An analysis of the frontier MOs of the complex systems suggests that various high-lying occupied MOs interact pairwise with various low-lying unoccupied MOs.