Infrared multiple photon dynamics and spectroscopy of cationic PABA and its dehydroxylated fragment ion

The gas-phase infrared spectra of cationic para-amino benzoic acid (PABA, H2N - C6H4 - COOH) and its dehydroxylated fragment ion, the para-amino benzoyl (PABz, H2N - C6H4 - CO) cation, are presented in the 500 - 2500 cm(-1) range. The spectra are obtained via mass-selective infrared multiple photon dissociation (IRMPD) using a continuously tunable free electron laser. Although both spectra are in good overall agreement with results from density functional theory, some interesting discrepancies are also observed. Particularly in the spectrum of the PABz cation, the strongly anharmonic NH2 out-of-plane bending mode causes substantial deviations from theory. The relative intensity of this out-of-plane bending mode in both the PABA and PABz cation spectra is much lower than calculated, similar to what has been reported for cationic aniline. For the PABz cation, two distinct fragmentation products are observed and recording the infrared spectra on these individual decay channels yields information on the dissociation mechanism. Moreover, it is experimentally shown that the decay rate into the two competing channels can qualitatively be described using statistical models.