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.