UV Absorption Spectrum and Photodissociation Channels of the Simplest Criegee Intermediate (CH₂OO)

The lowest-lying singlet states of the simplest Criegee intermediate (CH ₂ OO) have been characterized along the O-O dissociation coordinate using explicitly correlated MRCI-F12 electronic structure theory and large active spaces. It is found that a high-level treatment of dynamic electron-correlation is essential to accurately describe these states. a significant well on the B-state is identified at the MRCI-F12 level with an equilibrium structure that differs substantially from that of the ground X-state. This well is presumably responsible for the apparent vibrational structure in some experimental UV absorption spectra, analogous to the structured Huggins band of the iso-electronic ozone. the B-state potential in the Franck-Condon region is sufficiently accurate that an absorption spectrum calculated with a one-dimensional model agrees remarkably well with experiment.