(Sub-)millimeter-wave spectroscopy of gauche-propanal

A detailed analysis of (sub-)millimeter-wave spectra of the vibrational ground state (υ=0) combined with the energetically lowest excited vibrational state (υ24=1; aldehyde torsion) of gauche-propanal (g-C2H5CHO) up to 500GHz is presented. Both vibrational states, υ=0 and υ24=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 ν24 of gauche-propanal is redetermined to 68.75037(30)cm−1.