UV photodissociation action spectra of protonated formylpyridines

The first ππ* transition for protonated 2-, 3-, and 4-formylpyridine (FPH⁺) (m/z 108) is investigated by mass spectrometry coupled with photodissociation action spectroscopy at room temperature and 10 K. The photoproduct ions are detected over 35 000-43 000 cm^-1, and the major product channel for 3-FPH⁺ and 4-FPH⁺ is the loss of CO forming protonated pyridine at m/z 80. For 2-FPH⁺, the CO loss product is present but a more abundant photoproduct arises from the loss of CH₂O to form m/z 78. Plausible potential energy pathways that lead to dissociation are mapped out and comparisons are made to products arising from collision-induced dissociation. Although, in all cases, the elimination of CO is the overwhelming thermodynamically preferred pathway, the protonated 2-FPH⁺ results suggest that the CH₂O product is kinetically driven and competitive with CO loss. In addition, for each isomer, radical photoproduct ions are detected at lower abundances. SCS-CC2/aug-cc-pVTZ Franck-Condon simulations assist with the assignment of vibrionic structure and adiabatic energies (0-0) for 2-FPH⁺ at 36 560 cm^-1, 37 430 cm^-1 for 3-FPH⁺, and 36 140 cm^-1 for 4-FPH⁺, yielding an accurate prediction, on average, within 620 cm^-1.