Molecular complexes. Part 13.—Influence of charge transfer interactions on the structures of π-π* electron donor-acceptor molecular complexes

A theoretical study has been made of the influence of charge-transfer resonance stabilization of the ground state on the relative orientation of the superposed planar donor and acceptor molecules in π-π* electron donor-acceptor complexes.Molecular orbital coefficients and energies of the isolated donor and acceptor molecules were calculated by PPP π-electron theory. Charge-transfer interactions were then estimated for a large number of possible donor-acceptor orientations in each complex.It is concluded that (a) consideration of only the lowest-energy charge-transfer state gives a poor representation of the magnitude and orientation-sensitivity of charge-transfer stabilization, and (b) in the absence of large intermolecular forces, such as those of hydrogen bonding or ion-dipole interactions, the charge-transfer stabilization energy of the complex is maximized in the observed donor-acceptor orientation. The few exceptions to the latter are provided by certain complexes in which such stabilization would be maximized by a centre-on centre donor-acceptor orientation. In these complexes crystal packing forces may exclude the possibility of such a structure unless donor and acceptor molecules are of similar size and shape.