Ultraviolet Photodissociation of the N‑Methylpyridinium Ion: Action Spectroscopy and Product Characterization

The ultraviolet photodissociation of gas-phase N-methylpyridinium ions is studied at room temperature using laser photodissociation mass spectrometry and structurally diagnostic ion–molecule reaction kinetics. The C₅H₅N–CH₃⁺ (m/z 94), C₅H₅N-CD₃⁺ (m/z 97), and C₅D₅N–CH₃⁺(m/z 99) isotopologues are investigated, and it is shown that the N-methylpyridinium ion photodissociates by the loss of methane in the 36 000 – 43 000 cm^–1 (280 – 230 nm) region. The dissociation likely occurs on the ground state surface following internal conversion from the S₁ state. For each isotopologue, by monitoring the photofragmentation yield as a function of photon wavenumber, a broad vibronically featured band is recorded with origin (0–0) transitions assigned at 38 130, 38 140 and 38 320 cm^–1 for C₅H₅N–CH₃⁺ C₅H₅N-CD₃+ and C₅D₅N–CH₃⁺, respectively. With the aid of quantum chemical calculations (CASSCF­(6,6)/aug-cc-pVDZ), most of the observed vibronic detail is assigned to two in-plane ring deformation modes. Finally, using ion–molecule reactions, the methane coproduct at m/z 78 is confirmed as a 2-pyridinylium ion.