Hindered Rotation in the Ground and Excited Electronic States of o-Xylene

A study has been made of the hot bands in the ultraviolet absorption spectrum of ortho-xylene and some deuterated derivatives. It is shown that these hot bands arise from excited levels of torsional vibrations of the methyl groups. The spectrum can be explained fairly well assuming independent oscillators with a barrier to internal rotation of 700 ± 50 cm−1 in the ground state and 240 ± 35 cm−1 in the excited state. The origin of the barrier and the applicability of the independent oscillator model are also discussed.