Mechanism and kinetic properties for the gas-phase ozonolysis of β-ionone

Abstract

The gas-phase degradation mechanisms of β-ionone with O₃ are investigated using density functional theory (DFT). Possible reaction pathways are presented and discussed. The calculated results show that O₃ addition to the double bond in the side chain is favored over that in the ring in the initial steps. The experimentally observed products of 2-oxopropanal and 2,6,6-trimethylcyclohex-1-ene-1-carbaldehyde are mainly from O₃ addition to the double bond in the β-ionone side chain. Three energetically favorable product pathways are revealed for the first time, in which a H₂O molecule acts as an activator of OH transfer. The main products of O₃-initiated β-ionone reaction are 2-oxopropanal, 2,6,6-trimethylcyclohex-1-ene-1-carbaldehyde, 3-(2,6,6-trimethylcyclohex-1-en-1-yl)dioxira-ne and (E)-6,6-dimethylundec-3-ene-2,5,10-trione, which have great potential to form secondary organic aerosols because of their high polarity and water solubility. The total rate constant of the O₃ addition reaction is predicted to be 1.45 × 10⁻¹⁷ cm³ per molecule per s at 298 K and 760 Torr total pressure. The gas-phase residential time of β-ionone determined by O₃ is around 28.5 hours. This work provides a comprehensive investigation of the ozonolysis of β-ionone and will be helpful in understanding its environmental fate.