S₁ Torsional Potentials of Substituted Anthracenes

The vibrational structures of jet-cooled 2-phenylanthracene, 9-vinylanthracene, and 9-(2-naphthyl)anthracene have been investigated by laser-induced fluorescence in the frequency range 29900–27600 cm^-1. For each molecule the excitation spectrum contains a progression that can be assigned to an excited-state mode corresponding primarily to torsional motion about the bond joining the anthracene ring to the substituent. The experimentally observed vibrational spacings for this mode are modeled well by a simple one-dimensional effective potential in the torsional angle ϕ, of the form V(ϕ) =¹/₂Σ_nV_n(1 - cos nϕ). The best-fit potential parameters for the molecule are the following: 2-phenylanthracene, V₂= 864 cm^-1and V₄= -77 cm^-1; 9-vinylanthracene, V₂= 1909 cm^-1and V₄= -106 cm^-1; and 9-(2-naphthyl)anthracene, V₂= -1087 cm^-1and V₄= -1000 cm^-1. However, a comparison of observed vibronic intensities and isotope shifts in the vibrational spacings with those predicted for pure torsional motion shows that the simple one-dimensional-model potential is not completely adequate.