Phenol tunnels between two equivalent minima. The tunneling splitting is small because the tunneling barrier is high. The transition frequencies were summarized by
(1) L. Koslesniková, A. M. Daly, J. L. Alonso, B. Tercero, and J. Cernicharo, 2013, J. Mol. Spectosc. 289, 13.
The data set contained some tunneling-rotation transitions from
(2) E. Mathier, D. Welti, A. Bauder, and H. H. Günthard, 1971, J. Mol. Spectosc. 37, 63;
and some weak pure rotational transitions from
(3) C. Tanjaroon and S. G. Kukolich, 2009, J. Phys. Chem. A 113, 9185.
The pure rotational transitions obey a-type selection rules, the tunneling-rotation transition obey b-type selection rules. Four equivalent H nuclei cause a 5 : 3 intensity alteration for transitions with K_a + v_t being being even and odd, respectively. The "errors" in (3) were assumed to be 3σ uncertainties. About half of the lines weighted out in (1) were weighted in because of small residuals of the blends; several of these were modified. Some lines with large residuals were omitted from the fit. The uncertainty used for most of the lines from (1) (30 kHz) were close to the implicitely assumed value of 35 kHz. Some sextic distortion parameters were used in the present fit. Predictions with uncertainties larger than 0.5 MHz should be viewed with caution. This is not of importance for astronomical observations.
The dipole moment was derived from experiments by
(4) N. W. Larsen, 1979, J. Mol. Struct. 51, 175.