Sweetness and light: computation of the rotational spectra of proto-saccharides

To date, four C2H4O2 isomers have been identified in the interstellar medium: acetic acid, glycolaldehyde, methyl formate, and (Z)-1,2-ethenediol. The detection of these species raises the question of whether other related carbohydrates could be present in similar astrophysical environments. In this study, we report a systematic investigation of six C2H4O2 isomers not previously observed spectroscopically in space: anti-(1)-, (2)- and (3)-CH2=CHOOH, methyldioxirane, and anti- and syn-(2S)-oxiran-2-ol. For each species, we employ coupled-cluster theory at the AE-CCSD(T)/cc-pCVQZ level to provide accurate equilibrium parameters, rotational constants, quartic centrifugal distortion parameters, and permanent electric dipole moment components. Anharmonic force fields were determined within vibrational perturbation theory to second order and evaluated using the MP2 method, thus allowing the evaluation of vibrational corrections. These computations provided, as a by-product, sextic centrifugal distortion constants. Rotational spectra have subsequently been simulated in the millimeter/submillimeter-wave frequency for the first time. It is hoped and anticipated that the accurate theoretical rovibrational spectral data presented herein will aid in experimental investigations of these title molecules in the interstellar medium.