Computational rotational-vibrational spectroscopic analysis of isomeric species in the interstellar gas-phase stereoinversion of amino acid threonine

The gas-phase stereoinversion of amino acid threonine under the condition of interstellar medium (ISM) has been predicted to proceed through isomeric species with diverse chemistry. These species including ammonium ylides, epoxides, contain a variety of functional groups such as geminal-diol, triol besides alkenyl, carboxy, keto, hydroxy, and amino groups. The detection of these species in ISM can help in unravelling the enantiomeric excess observed in meteoritic samples. Towards this, the present work reports rotational and vibrational spectroscopic data computed for the conformers and isomeric intermediates predicted along the stereoinversion pathways of proteinogenic threonine under conditions akin to ISM. The rotational parameters are computed using quantum mechanical methods employing Moller-Plesset perturbation theory whereas for the vibrational analysis, density functional computations are performed using dispersion corrected exchange-correlation functionals. The anharmonic corrections are also computed using vibrational second-order perturbation theory, which, however, fails to account for the hydrogen bonded interactions in the species investigated. The rotational and vibrational transitions predicted for the conformers of threonine are observed to be in good agreement with the available experimental data. The gas-phase spectroscopic data computed for other isomeric species of threonine is quite reliable and can be used to search threonine or other amino acids in ISM by resolving the astrophysical data observed in the microwave and mid-infrared regions.