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Two-Dimensional Tunneling Hamiltonian Treatment of the Microwave Spectrum of 2-Methylmalonaldehyde



Yung-Ching Chou, Jon T. Hougen


The molecule 2-methylmalonaldehyde (2-MMA) exists in the gas phase as a six-membered hydrogen-bonded ring [HO-CH=C(CH3)-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. The microwave spectra of 2-MMA-d0, 2-MMA-d1, and 2-MMA-d3 were studied previously by N. D. Sanders [J. Mol. Spectrosc. 86 (1981) 27-42], who used a rotating-axis-system program for two-level inversion problems to fit rotational transitions involving the nondegenerate A(+) and A(-) sublevels to several times their measurement uncertainty. A global fit could not be carried out at that time because no appropriate theory was available. In particular, observed-minus-calculated residuals for the E(+) and E(-) sublevels were sometimes as large as several MHz. In the present work, we use a tunneling-rotational Hamiltonian based on a G12m group-theoretical formalism to carry out global fits of Sanders 2-MMA-d0 and 2-MMA-d1 [DO-CH=C(CH3)-CH=O] spectra nearly to measurement uncertainty, obtaining root-mean-square deviations of 0.12 and 0.10 MHz, respectively The formalism used here was originally derived to treat the methylamine spectrum, but the interaction between hydrogen transfer and CH3 torsion in 2-MMA is similar, from the viewpoint of molecular symmetry, to the interaction between CNH2 inversion and CH3 torsion in methylamine. These similarities are discussed in some detail.
Journal of Chemical Physics


2-methylmalonaldehyde, G18 permutation-inversion group, large-amplitude motions, microwave spectrum, tunneling-rotational Hamiltonian


Chou, Y. and Hougen, J. (2006), Two-Dimensional Tunneling Hamiltonian Treatment of the Microwave Spectrum of 2-Methylmalonaldehyde, Journal of Chemical Physics (Accessed May 28, 2024)


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Created October 25, 2006, Updated October 12, 2021