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 (λphotolysis). In contrast, a unimodal distribution was observed for the C–H stretching (νCH) vibrationally excited CH3 products with an almost constant product yield in the examined λphotolysis 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 λphotolysis. 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.