Electronic Spectroscopy of the Halogenated Criegee Intermediate, ClCHOO: Experiment and Theory.

A chlorine-substituted Criegee intermediate, ClCHOO, is photolytically generated using a diiodo precursor, detected by VUV photoionization at 118 nm, and spectroscopically characterized via ultraviolet-visible (UV-vis)-induced depletion of m/z = 80 under jet cooled conditions. UV-vis excitation resonant with a π* ← π transition yields a significant ground state depletion, indicating a strong electronic transition and rapid photodissociation. The broad absorption spectrum peaks at 350 nm and is attributed to contributions from both syn (∼70%) and anti (∼30%) conformers of ClCHOO based on spectral simulations using a nuclear ensemble method. Electronic structure theory shows significant differences in the vertical excitation energies of the two conformers (330 and 371 nm, respectively) as well as their relative stabilities in the ground and excited electronic states associated with the π* ← π transition. Natural bond orbital analysis reveals significant and nonintuitive nonbonding contributions to the relative stabilities of the syn and anti conformers.