On the contributions of van der Waals interactions to vibrational level mixing. Torsion–vibration coupling in p-fluorotoluene

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Abstract
As an example demonstrating the importance of van der Waals interactions to vibrational level mixing, a model is developed for the intramolecular coupling of internal methyl torsion to aromatic ring vibrations in p‐fluorotoluene (pFT). The coupling is a consequence of van der Waals interactions between the methyl rotor and the ring. Fermi resonance matrix elements are generated in a manner analogous to previous calculations for collision‐induced vibrational energy transfer and van der Waals molecule predissociation. Using these matrix elements, the extent of vibrational mixing is calculated for the level 6a1 in S1 pFT. The calculated result is in agreement with spectroscopic observations. Arguments for the generality of van der Waals interactions as mediators of intramolecular vibrational level mixing are presented.