Two-dimensional tunneling Hamiltonian treatment 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(CH(3))-CH=O] and exhibits two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is interesting because the transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the ring, i.e., HO-CH=C(CH(3))-CH=O-->O=CH-C(CH(3))=CH-OH, which then triggers a 60 degrees 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 Sanders [J. Mol. Spectrosc. 86, 27 (1981)], 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 megahertz. 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 Sanders' 2-MMA-d0 and 2-MMA-d1 [DO-CH=C(CH(3))-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 CH(3) torsion in 2-MMA is similar, from the viewpoint of molecular symmetry, to the interaction between CNH(2) inversion and CH(3) torsion in methylamine. These similarities are discussed in some detail.