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 o bserved. 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 flu orotoluenes. It was also found that rotational constant of the interna l rotation decreases significantly when the barrier height increases d rastically by ionization. From the correlation between the barrier hei ght and the internal rotational constant, the increase of the rotation al barrier is explained by the increase of the steric hindrance due to the geometrical change by ionization. The vibronic coupling between v ery high Rydberg states through the methyl rotational motion was sugge sted by the strong appearance of the a2-a1 forbidden transition in the PFI spectrum.