2D-Graph of intermolecular interactions predicts radical character of anion-π type charge-transfer complexes

The molecular orbital (MO) theory is one of the most useful methods to describe the formation of a new chemical bond between two molecules. However, it is less often employed for modelling non-bonded intermolecular interactions because of the small charge-transfer contribution. Here we introduce two simple descriptors, the energy difference ( E DA ) of the HOMO of an electron donor and the LUMO of an acceptor against such HOMO-LUMO overlap integral ( S DA ), to show that the MO theory could give a unified charge-transfer picture of both bonding and non-bonding interactions for two molecules. It is found that similar types of interactions tend to be closer to each other in this 2D graph. Notably, in a transition region from strong bonding to single-electron transfer, the interacting molecular pairs appear to present a "hybrid" between chemical bonding and a radical pair, such as anion-π* interactions. It is concluded that the number of nodes in the HOMO and LUMO play a crucial role in determining the bonding character of the molecular pair. An alternative description of bimolecular interactions in organic chemistry is proposed, where the overlap integral ( S DA ) and energy gap ( E DA ) between donor HOMO and acceptor LUMO are used as two axes in the 2D map.