(Sub-)millimeter-wave spectroscopy of gauche-propanal
  A detailed analysis of (sub-)millimeter-wave spectra of the vibrational
ground state ($\upsilon=0$) combined with the energetically lowest excited
vibrational state ($\upsilon_{24}=1$; aldehyde torsion) of gauche-propanal
(g-C$_2$H$_5$CHO) up to 500 GHz is presented. Both vibrational states,
$\upsilon=0$ and $\upsilon_{24}=1$, are treated with tunneling rotation
interactions between their two respective tunneling states, which originate
from two stable degenerate gauche-conformers; left- and right-handed
configurations separated by a small potential barrier. Thanks to
double-modulation double-resonance (DM-DR) measurements, important but weak
$c$-type transitions connecting the tunneling states could be unambiguously
assigned. In addition, Coriolis interaction as well as Fermi resonance between
the two vibrational states needed to be taken into account to derive fits with
experimental accuracy using Pickett's SPFIT program in a reduced axis system
(RAS). Based on the rotational analysis, the fundamental vibrational frequency
$\nu_{24}$ of gauche-propanal is redetermined to 68.75037(30) cm$^{-1}$.
