UV photodissociation action spectra of protonated formylpyridines

The first pi pi* 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). Published under an exclusive license by AIP Publishing.

Automated summary

This study investigates the first pi pi* transition for protonated 2-, 3-, and 4-formylpyridine (FPH+) using mass spectrometry coupled with photodissociation action spectroscopy. The findings reveal the photodissociation pathways and product distributions for different ions. While the elimination of CO is thermodynamically preferred, the loss of CH2O also shows kinetic competitiveness. This study is important for understanding the photochemical properties of these ionized compounds.