METHYL INTERNAL ROTATION IN THE $S_{1}$ STATED OF 2-CHLORO-6-FLUOROTOLUENE: EXPERIMENT AND THEORY
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Date
1994
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Ohio State University
Abstract
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 $S_{1} \leftarrow S_{o}$ 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 ($<500 cm^{-1}$) showing relatively large intensities. Using a threefold symmetric potential we are able to assign the low lying torsional states of the methyl rotor $(|V_{3}|\sim 140 cm-1)$. {Ab-initio} calculations predict a large barrier to internal rotation in $S_{0}$ ($|V_{3}|-230 cm^{-1}$) 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 $n_{x} (X=C1, F)$ p-type orbitals into the $\sigma^{*}CH$ orbitals of the methyl group. The analysis of both $^{35}Cl$ and $^{37}Cl$ systems will be presented.
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Author Institution: Department of Chemistry, University of Wisconsin; Department of Chemistry, University of California; Department of Chemistry, University of Wisconsin