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 spec troscopy have been applied to cis- and trans-o-cresol. The internal rotatio nal structure in S-1 has been re-assigned for the cis-isomer, and the poten tial curve for the internal rotation has been determined. In the PFI-ZEKE s pectra recorded via different internal rotational levels in the S-1 state, well-resolved low-frequency bands have been observed. The low-frequency ban ds are assigned to the internal rotational motion of the methyl group in th e cation. Level energies and relative transition intensities are reproduced well by a one-dimensional rotor model with a three-fold axis potential. Po tential curves for the internal rotation have been determined for both cis- and trans-o-cresol cations. The barrier height for the internal rotation i s different for the two isomers in the cation, while it becomes similar in S-1. Contributions of steric and electronic factors to the rotational barri er are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.