Insights into the Ultrafast Dynamics of CH2OO and CH3CHOO Following Excitation to the Bright 1ππ State: The Role of Singlet and Triplet States

ABSTRACT Criegee intermediates make up a class of molecules that are of significant atmospheric importance. Understanding their electronically excited states guides experimental detection and provides insight into whether solar photolysis plays a role in their removal from the troposphere. The latter is particularly important for large and functionalized Criegee intermediates. In this study, the excited state chemistry of two small Criegee intermediates, formaldehyde oxide (CH2OO) and acetaldehyde oxide (CH3CHOO), was modeled to compare their specific dynamics and mechanisms following excitation to the bright ππ* state and to assess the involvement of triplet states to the excited state decay process. Following excitation to the bright ππ* state, the photoexcited population exclusively evolves to form oxygen plus aldehyde products without the involvement of triplet states. This occurs despite the presence of a more thermodynamically stable triplet path and several singlet/triplet energy crossings at the Franck‐Condon geometry and contrasts with the photodynamics of related systems such as acetaldehyde and acetone. This work sets the foundations to study Criegee intermediates with greater molecular complexity, wherein a bathochromic shift in the electron absorption profiles may ensure greater removal via solar photolysis. Criegee intermediates of various sizes are produced through ozonolysis of biogenic and anthropogenic alkenes in the Earth’s atmosphere. The S2 excited electronic state is populated via a spectroscopically bright π*←π transition that occurs in the UV‐Vis region of the electromagnetic spectrum. Following near‐UV excitation of CH2OO and CH3CHOO, prompt O‐O bond fission occurs to form O + aldehyde products. Two regions of conical intersection control photodissociation through non‐adiabatic processes. The role of intersystem crossing in this photodissociation is explored for the first‐time utilizing trajectory surface hopping simulations.