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-ccpVDZ) most of the observed vibronic detail is assigned to two in-plane ring deformation modes. Finally, using ion-molecule reactions the methane co-product at m/z 78 is confirmed as 2-pyridinylium ion.