Structure and Spectroscopic Insights for CH₃PCO Isomers: A High-Level Quantum Chemical Study

The exploration of phosphorus-bearing species stands as a prolific field in current astrochemical research, particularly within the context of prebiotic chemistry. Herein, we have employed high-level quantum chemistry methodologies to predict the structure and spectroscopic properties of isomers composed of a methyl group and three P, C, and O atoms. We have computed relative and dissociation energies, as well as rotational, rovibrational, and torsional parameters using the B2PLYPD3 functional and the explicitly correlated coupled cluster CCSD(T)-F12b method. Based upon our study, all the isomers exhibit a bent heavy atom skeleton with CH₃PCO being the most stable structure, regardless of the level theory employed. Following in energy, we found four high-energy isomers, namely, CH₃OCP, CH₃CPO, CH₃COP, and CH₃OPC. The computed adiabatic dissociation energies support the stability of all [CH₃, P, C, O] isomers against fragmentation into CH₃ and [P, C, O]. Torsional barrier heights associated with the methyl internal rotation for each structure have been computed to evaluate the occurrence of possible A-E splittings in the rotational spectra. For the most stable isomer, CH₃PCO, we found a V₃ barrier of 82 cm^-1, which is slightly larger than that obtained experimentally for the N-counterpart, CH₃NCO, yet still very low. Therefore, the analysis of its rotational spectrum can be anticipated as a challenging task owing to the effect of the CH₃ internal rotation. The complete set of spectroscopic constants and transition frequencies reported here for the most stable isomer, CH₃PCO, is intended to facilitate eventual laboratory searches.