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 CH 2 O 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 CH 2 O 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 .