Ultraviolet Photodissociation of the ,-Dimethylformamide Cation.

,-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 ,-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 + NCH 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 + NCH product pair, which can be accessed from the 3A' 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 NCH can be formed efficiently at all three wavelengths. The primary NCH product is relatively long-lived, but the high degree of internal excitation often results in secondary fragmentation via a variety of pathways to form CH, NH, NCH, and NCH, 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 NCH 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 1A' ground state and 2A' first excited state of the DMF cation.