Reinvestigation of the microwave spectrum of 2-methylmalonaldehyde

The molecule 2-methylmalonaldehyde (2-MMA) exists in the gas phase as a six-membered hydrogen-bonded ring [HO{single bond}CH{double bond, long}C(CH₃){single bond}CH{double bond, long}O] and exhibits two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is interesting because transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the ring, which then triggers a 60° internal rotation of the methyl group attached to the ring. We present a new experimental study of the microwave spectra of the 2-MMA-d0 [HO{single bond}CH{double bond, long}C(CH₃){single bond}CH{double bond, long}O] and 2-MMA-d1 [DO{single bond}CH{double bond, long}C(CH₃){single bond}CH{double bond, long}O] isotopologs of the molecule. The new measurements were carried out by Fourier-transform microwave (FTMW) spectroscopy in the 8-24 GHz frequency range and by conventional absorption spectroscopy in the 49-149 GHz range. In the present work, we use a tunneling-rotational Hamiltonian based on a G₁₂^m group-theoretical formalism to carry out global fits of 2578 2-MMA-d0 transitions and 2552 2-MMA-d1 transitions to measurement uncertainty, obtaining a root-mean-square deviation of 0.015 MHz for both isotopologs. This represents a significant improvement in fitting accuracy over past attempts. Some problems associated with calculating barrier heights from the observed tunneling splittings and assumed tunneling paths are also considered.