Internal rotation of methyl group in 2- and 1-methylanthracene studied by electronic spectroscopy and DFT calculations

The S₁-S₀ fluorescence excitation and dispersed fluorescence spectra of 2- and 1-methylanthracene are measured in a supersonic free jet expansion. The barrier heights to internal rotation in the S ₀ and S₁ states of 2-methylanthracene are determined to be 69 and 335 cm^-1, respectively, by using a one-dimensional free-rotor model. A prominent 0-0 transition has been observed in the fluorescence excitation spectrum of 1-methylanthracene, but no methyl rotational bands have been detected in both the excitation and dispersed fluorescence spectra. The potential energy curves of the methyl rotation are obtained for 2- and 1-methylanthracene with density functional theory (DFT) calculations and time-dependent (TD)-DFT calculations at the B3LYP/6-31 + G(d,p) level. The barrier heights and the phase of the potential energy curve are very different between 2- and 1-methylanthracene and substantially depend on the electronic state. These differences are consistently explained by a π*-σ * hyperconjugation effect introduced by Nakai and Kawai [Chem. Phys. Lett. 307 (1999) 272].