Methyl Rotor State-Dependent Quenching of OH Tunneling in 2,6-Dimethylphenol

2,6-Dimethylphenol
has three internal rotors: two methyl rotors
taking up positions ortho to the phenol–OH
group and the OH rotor itself. We recorded the broadband microwave
spectrum of 2,6-dimethylphenol over the 7.5–17.5 GHz range
under jet-cooled conditions in the gas phase. In contrast to the tunneling
doublets observed in phenol, a-type rotational transitions
in 2,6-dimethylphenol appear as equally spaced triplets, with the
two outer components split by ± 48.54 MHz relative to the central
line. We fit the spectrum using a model in which OH tunneling either
occurs or is quenched, depending on whether the two methyl rotors
are in AA/EE states (tunneling present) or AE/EA states (tunneling
quenched). Tunneling of the OH group is quenched in the AE/EA methyl
rotor states due to OH/CH3 coupling that modulates the
methyl rotor barrier height by almost a factor of 2, depending on
the OH orientation relative to the methyl group. This, in turn, changes
the energy of the AE and EA methyl rotor states, producing an effective
asymmetry in the OH tunneling coordinate that is significantly greater
than the tunneling splitting. This localizes the OH torsional wave
functions in one or the other of the two wells. By contrast, the a-type rotational transitions in the AA and EE methyl rotor
states possess a tunneling splitting similar to that observed in phenol.
We compare and contrast the case of state-dependent quenching of tunneling
encountered here to the reduction in tunneling splitting that can
occur in asymmetric vibrational states.