Tunneling of methyl groups in toluquinone: Dependence on the electronic state of the molecule

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Abstract
The torsional frequency and the tunneling splitting of the methyl group of crystalline toluquinone are reported for different electronic states of the molecule. Whereas in the ground state both values can be derived from a conventional cos 3φ potential with a barrier height of approximately 8 kJ/mol, the values in the lowest excited triplet state cannot be reconciled with such a simple potential. It is suggested that a chemical interaction between the methyl protons and the adjacent oxygen of the excited carbonyl group is responsible for the modification of the potential. Numerical calculations based on a simple modeling of this chemical interaction support this idea.