Theoretical Spectroscopic Study of Two Ketones of Atmospheric Interest: Methyl Glyoxal (CH 3 COCHO) and Methyl Vinyl Ketone (CH 3 COCH═CH 2 ).

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 (A p )/synperiplanar (S p ), respectively. trans -Methyl glyoxal and A p -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 A p - and S p -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 (A 2 /E) and 124.636 cm -1 /123.785 cm -1 (A 2 /E). For A p - and S p -methyl vinyl ketone, the ground vibrational state splittings were estimated to be 0.008 and 0.017 cm -1 , respectively.