Investigating the Photolysis Mechanism of Propylene Oxide Using DFT and TD-DFT

Propylene oxide (PO) is a chiral molecule initially identified in the Sagittarius B2 star-forming region through telescopic observations under interstellar conditions. The possibility of homochirality of PO by photolysis is interesting for elucidating the cosmic origin of life on Earth. Existing research shows that photolysis of PO in the gas phase at 185 nm yields propanal and acetone, respectively. This study investigates the detailed photolysis mechanisms of PO using density functional theory (DFT) and time-dependent DFT calculations based on previously proposed mechanisms (J. Org. Chem. 1977, 42, 1252-1254). Our findings suggest that PO photolysis can progress through diradical pathways, as previously proposed. Potential energy diagrams for the excited states indicated that both pathways are more favorable under 185 nm irradiation than in their ground states. In particular, the potential energy diagrams show that the reaction can proceed toward the product through the excited states, even though the activation energies in the ground state are quite high for these reactions to occur. The potential energy surfaces along the intrinsic reaction coordinate partially support the preferential formation of propanal. This study expands our understanding of PO photolysis mechanisms and offers insights into processes that could affect the homochirality of PO.