Molecular Distortions and the Phosphorescence of Benzene

The relative intensities of the members of a progression (Franck–Condon factors) can be used to determine changes in molecular geometry upon electronic excitation. A detailed examination of the normal coordinates shows that for benzene and similar species only one vibrational mode (ν1, ∼990 cm−1) is expected to form progressions, even for deuterated benzenes having many totally symmetric vibrations which, according to group theory, might be expected to form progressions. For a D2h distorted benzene a second coordinate (ν8, ∼1600 cm−1) is expected to form progressions. The phosphorescence of all isotopically substituted benzenes has only one main progression frequency (950–990 cm−1), while the dominant progressions for the methylbenzenes involve the 1600-cm−1 mode. Quantitative calculations show that in the lowest triplet state of benzene the difference between long and short bonds is less than 0.01 Å, while for toluene a value of ∼0.07 Å is found.