Knowledge Bank

Using resonant two photon ionization we have measured the low frequency torsional vibrational structure of 2-chloro-6-fluorotoluene in a skimmed molecular beam. The $S1\leftarrow So$ origin occurs at 37036 $cm-1$ (adiabatic I.P.= 9.127 $\pm$ 0.006 eV). The spectrum has considerable low frequency vibrational activity with many vibrational bands () showing relatively large intensities. Using a threefold symmetric potential we are able to assign the low lying torsional states of the methyl rotor $(|V3|\sim 140cm-1)$. {Ab-initio} calculations predict a large barrier to internal rotation in $S0$ ($|V3|-230cm-1$) with the most stable confirmation having one C-H bond eclipsed with the C-F bond. In contrast to the monosubstituted ortho toluene remains relatively large. The present study extends our examination of the nature of hyperconjugative interactions involving electron donation from the $nx(X=C1,F)$ p-type orbitals into the $\sigma \ast CH$ orbitals of the methyl group. The analysis of both $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">37</mrow>}Cl$ systems will be presented.