Polarization of the Second Ultra-violet System in Hexamethylbenzene

Abstract

ALLOWED electronic transitions in aromatic molecules between states approximately described by combinations of pπ atomic orbitals must all be polarized in the molecular plane, and the same is true of the much weaker transitions which take on allowed character only because of the disturbing effect of nuclear vibrations. This is the polarization property actually found in band systems so far studied experimentally, almost all of which have been lowest-energy transitions. Specifically, Scheibe¹ showed that the 2800-A. system of hexamethylbenzene is polarized with the active electric vector in the molecular plane. This system certainly corresponds to the well-known 2600 A. system of benzene itself, which is assigned A_1g — B_2u from vibrational analysis. Hexamethylbenzene crystallizes with all the molecular planes parallel to one another, and perpendicular to the main faces of a sublimed crystal, and so is much better suited to polarization measurements than benzene.