Relative Energetics of the Gas Phase Protomers of p-Aminobenzoic Acid and the Effect of Protonation Site on Fragmentation.

Guided ion beam tandem mass spectrometry (GIBMS) is used to investigate the energy-dependent threshold collision-induced dissociation (TCID) of the two protonated isomers (protomers) of p-aminobenzoic acid (p-ABA). The O-protomer of p-ABA (protonated at the carbonyl oxygen) was generated via electrospray ionization (ESI) from a methanol/water solution, whereas the N-protomer (protonated at the amine) was produced via ESI from an acetonitrile/water solution. The two protomers are clearly distinguishable from differences in the onsets of the fragmentation channels and in the abundance and identity of the products observed. Additional GIBMS experiments were performed on protonated benzoic acid (BA) and aniline, which serve as simple models for fragmentation driven by protonation at the carbonyl or amine group of p-ABA and provide insights into the dissociation channels of the two p-ABA protomers. Theoretical calculations were carried out to determine the dissociation mechanisms and for comparison with the experimental thermochemistry, which is obtained from modeling the kinetic energy dependent TCID cross sections after accounting for multiple collisions, kinetic shifts, competition between channels, and sequential dissociations. Calculations together with the energy-resolved experiments show that a product ion (C5H5+, m/z 65) common to both p-ABA protomers is produced from sequential dissociations via two different mechanisms. Furthermore, we demonstrate here that the relative stabilities of the two p-ABA protomers can be determined directly from experiment via shared product channels for which there are no barriers exceeding their asymptotic energies. Our results indicate that the two p-ABA protomers are nearly isoenergetic in the gas phase with a relative energy of 0.01 ± 0.26 eV favoring the O-protomer over the N-protomer. This result is in good agreement with calculations at the MP2 and CCSD(T) levels of theory.