N-H and N-C Bond Dissociation Pathways in Ultraviolet Photodissociation of Dimethylamine

We investigated the interlinked N-H and N-C photochemistry of primary and secondary amines via the state-resolved detection of vibrationally excited CH₃ product and H atom product by 200-235 nm dimethylamine photodissociation using resonance-enhanced multiphoton ionization (REMPI) and velocity map imaging (VMI) techniques. The out-of-plane bending (ν₂) vibrationally excited CH₃ showed a bimodal translational energy distribution that became unimodal with a near-zero product yield at longer photolysis wavelengths (λ_photolysis). In contrast, a unimodal distribution was observed for the C-H stretching (ν_CH) vibrationally excited CH₃ products with an almost constant product yield in the examined λ_photolysis region. We ascribed the state-specific energy releases of the CH₃ products to two reaction pathways based on calculations of the potential energy surface (PES): the direct N-CH₃ dissociation pathway and the indirect N-CH₃ dissociation pathway via the N-H bond conical intersection. Meanwhile, the H atom product showed a bimodal energy distribution similar to the ammonia photodissociation model, with an excited-state counterproduct channel that became accessible at a shorter λ_photolysis. These results suggest that the N-H and N-C bond dissociations are connected, and these dissociations cause different photochemistry between primary/secondary amines and tertiary amines.