Internal methyl group rotation in o-cresol studied by pulsed field ionization-ZEKE photoelectron spectroscopy

Pulsed field ionization-ZEKE photoelectron spectroscopy and (1+1) R2PI spectroscopy have been applied to cis- and trans-o-cresol. The internal rotational structure in S1 has been reassigned for the cis-isomer, and the potential curve for the internal rotation has been determined. In the PFI-ZEKE spectra recorded via different internal rotational levels in the S1 state, well-resolved low-frequency bands have been observed. The low-frequency bands are assigned to the internal rotational motion of the methyl group in the cation. Level energies and relative transition intensities are reproduced well by a one-dimensional rotor model with a three-fold axis potential. Potential curves for the internal rotation have been determined for both cis- and trans-o-cresol cations. The barrier height for the internal rotation is different for the two isomers in the cation, while it becomes similar in S1. Contributions of steric and electronic factors to the rotational barrier are discussed.