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-CH=C(CH3)-CH=0] 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 degrees 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-CH=C(CH3)-CH=O] and 2-MMA-d1 [DO-CH=C(CH3)-CH=0] 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 G(12)(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. (C) 2008 Elsevier Inc. All rights reserved.