Vibrational spectroscopy of phenanthrene : linear dichroism spectra and new assignments

Linear Dichroism (LD) spectra of phenanthrene aligned in stretched polymer sheets were recorded in the mid-IR region, with the electric vector of the linearly polarised radiation parallel and perpendicular to the stretching direction of the polymer. After baseline correction, the resulting spectra made it possible to determine transition moments directions for symmetry-allowed transitions and thus assign the corresponding excited states to the symmetry-allowed a 1 , b 1 , and b 2 species in the C 2 v point group of the molecule, with reference to theoretical calculations. A careful study of the LD spectra and comparison with earlier assignments of the vibrational transitions for this important molecule lead to some new assignments.