LOW ENERGY INTERNAL ROTATION IN THE $S_{1}$ AND GROUND CATION STATES OF 2,6-DEFLOUROTOLUENE

1. R.A. Walker, E.C. Richard, K-T. Lu, J.C. Weisshaar, work in progress.Author Institution: Department of Chemistry, University of Wisconsin; Department of Chemistry, University of California-Berkeley; Department of Chemistry, University of WisconsinEmploying techniques of resonance enhanced two photon ionization and threshhold electron field ionization, we have measured both the $S_{1} \leftarrow S_{0}$ and cation $\leftarrow S_{1}$ spectra of 2,6-difluorotoluene (DFT)$^{1}$ DFT’s $S_{1} \leftarrow S_{0}$ origin lies at $37948 cm^{-1} (\pm 0.3 cm^{-1})$, and DFT shows an adiabatic ionization potential of $73673 cm^{-1} (\pm7.0 cm^{-1})$. Slightly to higher energy of the $S_{1}$ and cation origins. We observe a number of band which we assign to internal rotation of the DFT methyl group. Fitting these features to a sixfold symmetric potential $\left(\frac{\nu_{4}}{2}(1 - \cos 6 \alpha)\right)$ suggests a very low barrier to methyl rotation in both the $S_{1}$ $(|V_{6}(S_{1})| \sim 20 \ cm^{-1})$ state and cation ground state $(|V_{6}(D_{o})| \sim 10 cm^{-1})$. These measured barriers are consistent with ab initio calculations predicting a small $V_{6}$ in the ground state of DFT $(V_{6}(S_{o})= 17.4 cm^{-1})$. This talk will focus on the origin of these small $V_{6}$ terms in the context of $V_{6}$ potentials found for other six-fold symmetric molecules. Additionally, a coupling mechanism invoked to explain several nominally forbidden transitions observed in the spectra will be discussed