MOST STABLE CONFIGURATIONS OF POLYNUCLEAR AROMATIC HYDROCARBON MOLECULES IN PITCHES VIA MOLECULAR MODELING

Molecular mechanics calculations were performed on a series of polynuclear aromatic hydrocarbon molecules in various geometric arrangements and combinations. Homologous aromatic hydrocarbons associate strongly face-to-face, in a parallel shifted stack arrangement with a displacement of about 4.7 angstrom. Heteromerous aromatic hydrocarbons also prefer a shifted stack configuration of two or three molecules high. A third or fourth molecule added to a stack will prefer to orient perpendicular to the stack such that its face is against the edge of the stack. Methyl groups act to encourage stacking interactions between dissimilar aromatic hydrocarbons with the methyl groups preferring to be inside the stack and thus, having a minimal effect on edge-to-edge interactions between stacks. These results support the colloidal model for the behavior of pitches.