S0 and S1 spectroscopy of jet cooled 9-cyano-10-methylanthracene: The methyl group as a molecular rotor

Observed positions and relative intensities of the methyl internal rotational bands were interpreted in terms of transitions calculated based on the quantum mechanical one-dimensional rotor. The low-frequency vibrational bands were interpreted also with the all electron quantum mechanical calculations within the RHF/6-31G(d,p), CIS/3-21G and CIS/6-31G(d,p) approximations. It is predicted that in the case of MA the eclipsed geometry (one C-H in the plane of the ring) is most stable in both S0 and S1 states. Conformation of the methyl group in CMA is suggested to change upon S1 ← S0 excitation (π/12 phase shift of the methyl group). The predicted energy barrier for methyl group rotation in the S0 state of CMA is considerably higher (72 cm−1) than that in the S1 state (22 cm−1). Following the present quantum mechanical calculations, the carbon atom of the methyl group belongs to the aromatic plane in the S0 ground state but it deviates from this plane in the S1 excited state. These in turn suggest that the calculated barrier for methyl group rotation in CMA has a 6-fold symmetry in the S0 ground state and roughly a 4-fold symmetry in the S1 state.