On the role of symmetry in quenching OH tunneling in 2,6-dimethylphenol

We model the spectroscopy of the two methyl torsional degrees of freedom coupled to the OH torsional motion in 2,6-dimethylphenol. Recent gas-phase rotational transitions [Welsh et al., J. Phys. Chem. Lett. 17, 3749-3758 (2026)] have been interpreted to be the result of certain symmetry states of the methyl groups leading to the quenching of the OH torsional motion. Both a three-dimensional model and an adiabatic model, in which the OH torsion is treated as the slow mode, are developed to test these conjectures. Good agreement is found between the exact and approximate adiabatic models. The adiabatic model is developed to interpret and elucidate the key couplings leading to the quenching of the tunneling splitting.