The low-frequency vibrations of triptycene

Abstract

The ground-state vibrational spectra of triptycene (9,10-dihydro-9,10[ 1',2']benzenoanthracene) were studied by fluorescence emission and IR and Raman spectroscopies, as well as by semiempirical (AMI) and ab initio (HartreeFock) quantum chemical calculations. Comparison of experimental vibrational frequencies and intensities in the range 60-1oOO cm-l with the semiempirical and the ab initio values is made. Excellent agreement is found between experiment and ab initio calculation with respect to vibrational frequencies. Agreement with the frequencies predicted by the semiempirical AM1 calculation is less satisfactory. The interpretation and assignment of the lowest frequency vibrational modes is also discussed in terms of a vibrational excimer model in a basis of symmetry-adapted combinations of local benzene ring coordinates. The two lowest frequency modes at 64 and 21 1 cm-' are identified by shape and symmetry (e' and ai, respectively), which is very important for the understanding of the Jahn-Teller effect in the first excited state 'E' of triptycene.