Spin–orbit ab initio and density functional theory study of vinyl iodide: Molecular properties and photodissociation dynamics

We present a comprehensive theoretical investigation of vinyl iodide (VI), examining its molecular properties and photodissociation dynamics using high-level ab initio and density functional theory methods explicitly incorporating spin–orbit coupling (SOC). To align with experimental results, accurately determining the bond dissociation energy requires an explicit consideration of SOC. For ab initio calculations, correcting for basis set superposition error proves essential for obtaining quantitatively accurate values consistent with the experimental value. We calculate vertical excitation energies and systematically characterize the potential energy curves (PECs) along the C–I dissociation coordinate. This study establishes explicit excited state assignments for VI for the first time, highlighting the significant role of triplet states, particularly the 4A′ and 4A″ states in photodissociation dynamics. These states are found to contribute the broad UV absorption band around 250 nm mainly through σ* ← n″ and σ* ← n′ excitations rather than the previously proposed π* ← n″ excitation. Our calculated PECs provide theoretical validation and detailed explanations for the experimentally observed wavelength-dependent quantum yields and anisotropy parameters of I(2P3/2) and I*(2P1/2) species.