A study of some electron donor-acceptor complexes

Abstract

In 1961 Dewar published a paper applying Molecular Orbital Theory to the spectra of polycyclic aromatic hydrocarbon/1,3,5-trinitrobenzene complexes. The present investigation was undertaken to ascertain whether the spectra of complexes derived from 1,2-diarylethylenes, 1,4-diaryl 1,3-butadienes and various π acceptors could be treated by the theory proposed by Dewar. The Huckel Molecular Orbital energy levels for the donors have been calculated and satisfactory correlations with the energies of the observed charge transfer bands have been found. -- The possibility of Diels-Alder adduct formation in some systems has been examined, subsequent to observing the decay in the intensity of the charge transfer bands when certain donors and acceptors were mixed. The system 1,4-diphenyl-1,3-butadiene/2,3-dichloro-5,6-dicyano-1,4-benzoquinone has been investigated in some detail and the adduct isolated and characterised. The rates of the disappearance of both the 1,4-diphenyl-1,3-butadiene and of the charge transfer band have been measured in an attempt to determine whether the electron donor-acceptor complex is an intermediate in this particular example of the Diels-Alder reaction. These results are unfortunately inconclusive, and although the idea that the electron donor-acceptor complex is an intermediate in this reaction is intuitively attractive, conclusive proof of this proposal must await a more detailed kinetic investigation of this particular system.