Theoretical Insights into Potential Interstellar Molecules: Protonated Acrolein and Methylketene

In this work, highly accurate ab initio calculations have been employed to predict the spectroscopic properties of protonated acrolein (both cis- and trans-isomers) and methylketene. These C₃H₅O⁺ systems are of astrochemical interest owing to their neutral analogues having already been detected in the interstellar medium. In total, we have characterized six species: syn and anti conformers of both O-protonated cis- and trans-acrolein, O-protonated gauche-methylketene and C-protonated methylketene. Specifically, the CCSD(T) method (coupled-cluster with all single and double substitutions and a quasiperturbative treatment of triple excitations) has been used in conjunction with extrapolation to the complete basis-set limit, and corrections for core-valence effects. To support experimental spectroscopy measurements aiming at guiding astronomical observations, we have focused on determining best theoretical estimates of rotational and vibrational spectroscopic parameters. The V₃ barriers to internal methyl rotation for both protonated methylketene species (O- and C-) have additionally been computed, and an estimate of the reduced rotation constant for internal rotation, F₀, has been deduced.