Reinvestigation of the microwave spectrum of 2-methylmalonaldehyde

• Published in: Journal of molecular spectroscopy 2008-09, Vol.251 (1), p.56-63
• Main Authors: Ilyushin, Vadim V., Alekseev, Eugene A., Chou, Yung-Ching, Hsu, Yen-Chu, Hougen, Jon T., Lovas, Frank J., Picraux, Laura B.
• Format: Article
• Language: English
• Subjects: Hydrogen transfer; Internal rotation; Large-amplitude motion; Microwave spectrum; Tunneling Hamiltonian; Tunneling splittings
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/j.jms.2008.01.005

The molecule 2-methylmalonaldehyde (2-MMA) exists in the gas phase as a six-membered hydrogen-bonded ring [HO CH C(CH 3) CH 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 CH C(CH 3) CH O] and 2-MMA-d1 [DO CH C(CH 3) CH 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 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.