How a (nearly) free methyl rotor accelerates intramolecular vibration relaxation. Theory and Experiment

Abstract

The presence of a methyl group as a nearly free rotor on an aromatic ring can accelerate the rate of intramolecular vibrational redistribution (IVR) by two orders of magnitude. We show how the molecule p-difluorobenzene and its methyl-substituted variant p-fluorotoluene (pFT) provide a good experimental opportunity to learn whether the acceleration is associated with the internal rotational motion of the methyl or due to other effects of methyl substitution. A theory of the interactions between internal rotation and ring vibrations has been proposed earlier to account for the increased IVR. Based on van der Waals interactions between methyl hydrogens and adjacent ring atoms, the theory yields selection rules for level coupling that are now being tested spectroscopically with S₁→S₀ jet-cooled fluorescence from pFT. The tests now provide secure evidence that the methyl internal rotation states are central to the IVR acceleration. Cold-jet fluoresence spectra have been obtained from S₁ molecules with selective excitation of specific internal rotation states. These spectra show specific state-to-state couplings that are consistent with the selection rules of the theory.