Ultraviolet Photodissociation\nof the <i>N</i>,<i>N</i>‑Dimethylformamide\nCation

<i>N</i>,<i>N</i>-Dimethylformamide\n(DMF) provides\na useful small-molecule model for studying features of the peptide\nbond that forms the backbone of proteins. We report results from a\ncomprehensive multimass velocity-map imaging study into the ultraviolet\n(UV) photolysis of the <i>N</i>,<i>N</i>-dimethylformamide\ncation (DMF<sup>+</sup>) at wavelengths of 225, 245, and 280 nm. Electronic\nstructure calculations on DMF and DMF<sup>+</sup> were employed to\nhelp interpret the experimental results. DMF<sup>+</sup> ions are\ngenerated by 118 nm single-photon ionization of neutral DMF. Subsequent\nUV photolysis is found to lead to selective cleavage of the N–CO\namide bond. This yields HCO + NC<sub>2</sub>H<sub>6</sub><sup>+</sup> as major products, with virtually\nall of the excess energy released into internal modes of the fragments.\nThe data also indicate a small branching ratio into the HCO<sup>+</sup> + NC<sub>2</sub>H<sub>6</sub> product pair, which can be accessed\nfrom the 3<sup>2</sup>A′ electronic state of DMF<sup>+</sup>. N–CO bond dissociation can also be accompanied by simultaneous\nintramolecular hydrogen transfer from the oxygen to the nitrogen end\nof the amide bond, in which case NCH<sub>4</sub><sup>+</sup> can be formed efficiently at all three wavelengths.\nThe primary NC<sub>2</sub>H<sub>6</sub><sup>+</sup> product is relatively long-lived, but the\nhigh degree of internal excitation often results in secondary fragmentation\nvia a variety of pathways to form CH<sub>3</sub><sup>+</sup>, NH<sub>4</sub><sup>+</sup>, NCH<sub>2</sub><sup>+</sup>, and NC<sub>2</sub>H<sub>4</sub><sup>+</sup>, with secondary dissociation more likely\nat higher photon energies. The isotropic velocity-map images recorded\nfor the various fragments attest to the long lifetime of NC<sub>2</sub>H<sub>6</sub><sup>+</sup> and also\nimply that dissociation most probably occurs from the same set of\nelectronic states at all wavelengths studied; these are thought to\nbe the 1<sup>2</sup>A′ ground state and 2<sup>2</sup>A′\nfirst excited state of the DMF<sup>+</sup> cation.