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 Fab[JaJb + JbJa] to an accuracy of 3% from first principles.
We compare calculated transition frequencies using the best-fit Fab to those with Fab = 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 Kc = 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.