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.
ARC Centre of Excellence for Free Radical Chemistry and Biotechnology and School of Chemistry, University of Wollongong , Wollongong, New South Wales 2522, Australia.
Christopher S
CS
Hansen
Benjamin B
BB
Kirk
Stephen J
SJ
Blanksby
Adam J
AJ
Trevitt