Broadband Microwave Spectroscopy of Perfluorophenol: OH Tunneling in a Polar, Near-Oblate Symmetric Top

The broadband microwave spectrum of perfluorophenol (C₆F₅OH) was recorded over the 7.5-17.5 GHz and 26-40 GHz frequency ranges under jet-cooled conditions in the gas phase. 895 lines were assigned to the ¹²C₆F₆OH parent molecule. We also detect torsion-rotation transitions of the singly ¹³C and ¹⁸O-substituted isotopologues in natural abundance and fit a total of 405 transitions of the isotopologues. Tunneling due to OH internal rotation in a V₂ potential leads to a 0₊/0_- tunneling splitting of ΔE = 24.826 MHz that appears on the a-type transitions. Using a properly symmetrized relaxed potential energy curve, we carry out discrete-variable reference (DVR) calculations to predict both the tunneling splitting and the Coriolis-like coupling term F_ab[J_aJ_b + J_bJ_a] to an accuracy of 3% from first principles. We compare calculated transition frequencies using the best-fit F_ab to those with F_ab = 0 to identify sets of rotational levels in the 0₊ and 0_- tunneling levels that are shifted significantly by Coriolis-like mixing between them, which is maximized for K_c = 1,2 in this oblate symmetric top limit. Finally, in the singly substituted isotopologues of PFP (¹³C or ¹⁸OH), the potential energy surface for tunneling is still symmetric, but asymmetry in the kinetic terms of the Hamiltonian that are introduced by asymmetric ¹³C-substitution at the positions ortho and meta to the OH group quench the OH tunneling. While the v = 0₊ and 0_- torsional wave functions for these levels are completely localized in ortho-¹³C PFP, we predict partial delocalization in v = 0₊ and 0_- torsional substates of meta-¹³C PFP. However, the predicted transitions between tunneling levels in meta-¹³CC₅F₅OH could not be assigned experimentally.