Pulsed field ionisation-ZEKE photoelectron spectrum of o-, m- and p-tolunitrile

Pulsed field ionisation-ZEKE photoelectron spectroscopy has been applied to o-, m- and p-tolunitrile in a supersonic jet. The PFI-ZEKE photoelectron spectra of m- and p-tolunitrile show well-resolved anharmonic structures in the low-frequency region, which are assigned to bands due to 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 o-tolunitrile, no band due to internal rotation was found in PFI-ZEKE spectrum. It is suggested that the o-tolunitrile cation has the high barrier for internal rotation and the stable conformation that is the same as that in S1 and S0. The barrier height and the conformation are compared with other toluene derivatives, and the relation between the electronic character of -CN and the internal rotational motion has been discussed.