UV photodissociation action spectra of protonated formylpyridines

Abstract

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.