A study has been made of the infrared spectra of 33 poly-nuclear aromatic compounds including some known carcinogens. In most cases, the number of fused rings may be determined by measuring the frequency of the characteristic aromatic (C-H) stretching vibration with a LiF prism. For unsubstituted fused rings, the (C-H) stretching vibration shifts to lower frequencies as the number of fused rings increases. In addition, the position of the fused rings can be established from data obtained from the out-of-plane (C-H) bending vibrations in the range of 700 to 900 cm−1.
Stephen E. Wiberley and Richard D. Gonzalez, "Infrared Spectra of Polynuclear Aromatic Compounds in the C-H Stretching and Out-of-Plane Bending Regions," Appl. Spectrosc. 15, 174-177 (1961)