Recent models of the structures of naturally-occurring bitumens, obtained through a variety of physical-chemical techniques, indicate the presence of crystallites composed of stacked layers of alkyl-substituted polynuclear aromatics randomly distributed in a continuous mesomorphic medium. The present studies reveal that bitumens exhibit four vibrational bands, located at 865, 815, 760 cm-' for aromatic C-H out-of-plane bending vibrations, and at 730 cm-' corresponding to in-plane methylene rocking vibrations. The first three bands exhibit a blue shift proportional in magnitude to the dielectric con stant of the polar medium. The presence of complexing reagents such as nitro-or cyano-bearing acceptors effects significant blue shifts which are accompanied by a broadening of the bands. The extent of the aromatic C-H bending-vibration shifts was similar to those observed for samples of pure polynuclear aromatic hydrocarbons, approximately 1 O-l 5 cm-l. Polynuclear aromatics within the bituminous matrix behave as donors in chargetransfer processes owing to the presence of the substituted alkyl groups. The blue shift for the donor and the red shift of the acceptor correlate well with molecular orbital m-values, suggesting that the change in vertical energy not only affects the extinction coefficient of a vibrational band, but also influences the force constant of the C-H bending mode. Donor-acceptor 71-n associations result in polynuclear aromatics stack formation in bitumen crystallites, thus optimizing the extent of interaction. Metalloporphyrins occurring in bitumens also exhibit similar types of association (14-18 kcal/mol* dissociation energy).