Two ketones of atmospheric
interest, methyl glyoxal and
methyl
vinyl ketone, are studied using explicitly correlated coupled cluster
theory and core–valence correlation-consistent basis sets.
The work focuses on the far-infrared region. At the employed level
of theory, the rotational constants can be determined to within a
few megahertz of the experimental data. Both molecules present two
conformers, trans/cis and antiperiplanar (Ap)/synperiplanar
(Sp), respectively. trans-Methyl glyoxal
and Ap-methyl vinyl ketone are the preferred structures. cis-Methyl glyoxal is a secondary minimum of very low stability,
which justifies the unavailability of experimental data in this form.
In methyl vinyl ketone, the two conformers are almost isoenergetic,
but the interconversion implies a relatively high torsional barrier
of 1798 cm–1. A very low methyl torsional barrier
was estimated for trans-methyl glyoxal (V
3 = 273.6 cm–1). Barriers of 429.6 and
380.7 cm–1 were computed for Ap- and
Sp-methyl vinyl ketone. Vibrational second-order perturbation
theory was applied to determine the rovibrational parameters. The
far-infrared region was explored using a variational procedure of
reduced dimensionality. For trans-methyl glyoxal,
the ground vibrational state was estimated to split by 0.067 cm–1, and the two low excited energy levels (1 0) and
(0 1) were found to lie at 89.588 cm–1/88.683 cm–1 (A2/E) and 124.636 cm–1/123.785 cm–1 (A2/E). For Ap- and Sp-methyl vinyl ketone, the ground vibrational state
splittings were estimated to be 0.008 and 0.017 cm–1, respectively.