Internal rotation of the methyl group in fluorotoluene cations as studied by pulsed field ionization-zero kinetic energy spectroscopy

Pulsed field ionization spectra of o-, m-, and p-fluorotoluenes were observed. The spectrum shows a well-resolved structure due to internal rotation of the methyl group in fluorotoluene cations. Observed level energies and relative intensities were reproduced by a one-dimensional periodic potential with a free-rotor basis set. The analysis shows a great increase in the rotational barrier by ionization for all the fluorotoluenes. It was also found that rotational constant of the internal rotation decreases significantly when the barrier height increases drastically by ionization. From the correlation between the barrier height and the internal rotational constant, the increase of the rotational barrier is explained by the increase of the steric hindrance due to the geometrical change by ionization. The vibronic coupling between very high Rydberg states through the methyl rotational motion was suggested by the strong appearance of the a2–a1 forbidden transition in the PFI spectrum.