Coupling of internal rotation of methyl group with proton transfer in the S1 state of 5-methyltropolone

The fluorescence excitation and hole-burning spectra in the S1–S0 region of 5-methyltropolone have been measured in a supersonic free jet. It has been found that the electronic origin of 5-methyltropolone splits into four components corresponding to the transitions between the sublevels of the zero-point levels in the S0 and S1 states originating from coupled motion of the internal rotation of the methyl group and proton transfer. The tunneling rate of the zero-point level in the S1 state is one order smaller than that of tropolone due to this coupling, but the excitation of the internal rotational levels promotes proton tunneling.