Investigating the C-Cl antibonding character in the pi pi* excited state of vinyl, allyl, and propargyl chloride: Emission spectra and ab initio calculations

We report the emission spectra of dissociating vinyl, allyl, and propargyl chloride upon photoexcitation at 199 nm. To provide a better understanding of the mixed electronic character in the Franck-Condon region of the excited states accessed, we also present ab initio calculations at the configuration interaction level for these three molecules. These experimental and theoretical results indicate large differences in the contribution of n sigma*(C-Cl) and pi sigma*(C-Cl) character to these predominantly pi pi*(C=C/C=C) transitions. We present arguments based on the symmetry of the pertinent molecular orbitals to explain this observed variation. We conclude by considering how the differing electronic character of the excited state in these molecules may influence the branching ratios to C-Cl fission and HCl elimination products observed in the photodissociation of vinyl, allyl, and propargyl chloride.