Ultraviolet Photodissociation of the N,N-Dimethylformamide Cation

N,N-Dimethylformamide (DMF) provides a useful small-molecule model for studying features of the peptide bond that forms the backbone of proteins. We report results from a comprehensive multimass velocity-map imaging study into the ultraviolet (UV) photolysis of the N,N-dimethylformamide cation (DMF⁺) at wavelengths of 225, 245, and 280 nm. Electronic structure calculations on DMF and DMF⁺ were employed to help interpret the experimental results. DMF⁺ ions are generated by 118 nm single-photon ionization of neutral DMF. Subsequent UV photolysis is found to lead to selective cleavage of the N–CO amide bond. This yields HCO + NC₂H₆⁺ as major products, with virtually all of the excess energy released into internal modes of the fragments. The data also indicate a small branching ratio into the HCO⁺ + NC₂H₆ product pair, which can be accessed from the 3²A′ electronic state of DMF⁺. N–CO bond dissociation can also be accompanied by simultaneous intramolecular hydrogen transfer from the oxygen to the nitrogen end of the amide bond, in which case NCH₄⁺ can be formed efficiently at all three wavelengths. The primary NC₂H₆⁺ product is relatively long-lived, but the high degree of internal excitation often results in secondary fragmentation via a variety of pathways to form CH₃⁺, NH₄⁺, NCH₂⁺, and NC₂H₄⁺, with secondary dissociation more likely at higher photon energies. The isotropic velocity-map images recorded for the various fragments attest to the long lifetime of NC₂H₆⁺ and also imply that dissociation most probably occurs from the same set of electronic states at all wavelengths studied; these are thought to be the 1²A′ ground state and 2²A′ first excited state of the DMF⁺ cation.