The vibrational spectrum of acenaphthylene: linear dichroism and ab initio model calculations

Abstract

Complete infrared linear dichroism spectra were recorded between 50 and 3200 cm −1 of acenaphthylene aligned in stretched polyethylene and polyethylene-d 4 as well as Raman solution spectra. The information obtained from the infrared spectra made it possible to separate the electric dipole allowed transitions into three groups, corresponding to three different excited state symmetries. The separation is possible for the C 2 v symmetric molecule, since the three groups of transitions correspond to three mutually perpendicular transition moment directions, which in the stretched polymer obtain different average alignments. This information makes a safe assignment of all allowed fundamental vibrational transitions, belonging to three of the four C 2 v symmetry classes, possible. Similarly, the Raman spectra permitted an assignment of vibrations belonging to the fourth symmetry class. The assignments were supported by ab initio quantum mechanical modelling calculations at various levels. A comparison of the present assignments with those from recent linear dichroism studies of acenaphthylene aligned in a nematic liquid crystal matrix confirmed most assignments, but also led to new assignments and reassignment of several transitions. This was possible because of the additional information that becomes available in the stretched polyethylene method, which provide an excellent separation of the excited state symmetries corresponding to the two in-plane transition moment directions.