A FREE ELECTRON MODEL FOR $\pi$ MOLECULAR COMPLEXES$^{\ast}$

Author Institution: Applied Physics Laboratory, The Johns Hopkins UniversityThe free electron model for conjugate molecules is extended to a discussion of molecular complexes formed from conjugate molecules. A well-known example of these are the complexes formed from aromatic amines and nitro compounds. These complexes, following the nomenclature of M. J. S. Dewar, may be termed $\pi$ molecular complexes. The discussion of the spectra of these $\pi$ molecular complexes follows along the lines given by Kuhn and Bayliss for conjugate organic dyes. The main modification is the necessity of taking account of the perturbation (splitting) of the energy levels in the molecular complex as it is formed from the molecular components. This complex formation is equivalent, quantum mechanically, to a change from two one-minimum potentials to one two-minima potential. The stability of the complex may be attributed to the delocalization of the $\pi$ electrons as they penetrate the potential barrier between the molecular components to move more or less freely throughout the whole complex. A simplified quantum mechanical treatment for a symmetrical linear $\pi$ complex indicates that the intensity of the complex absorption is comparable to that for the molecular components and may thus be quite strong. A calculation as to the effect of hydrostatic pressure on the position of the absorption maxima indicates, in agreement with experimental observations, that the main absorption peak will be shifted toward the red; some of the weaker, and as yet not observed, subsidiary absorption maxima should shift toward the ultraviolet. A comparison of this free electron model with the model suggested by other investigators shows good agreement in regard to the salient factors governing the formation, stability, and the spectra of molecular complexes