Infrared multiphoton dissociation spectra of protonated p-aminobenzoic acid generated by electrospray ionization (ESI) from aqueous methanol and acetonitrile solutions were recorded in the gas phase from 2800-4000 cm(-1). The O-protonated ion is more stable than the N-protonated structure in the gas phase, whereas the opposite is true in both solutions. When CH(3)OH/H(2)O was used as the ESI solvent, only the O-protonated ion was observed. In contrast, a 70:30 mixture of the O- and N-protonated species were produced from CH(3)CN/H(2)O. These structural assignments are based on an assortment of experimental data (action spectra, photofragments, photofragmentation kinetics, and H/D exchange) and are fully supported by extensive computations. This work shows that ESI can lead to isomerization and that the ionization site may be varied by changing the solvent from which the substrate is analyzed.

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Gas-phase deprotonation enthalpies were measured for chloric and perchloric acids and found to be 313.2 +/- 3.3 and 299.9 +/- 5.7 kcal mol(-1), respectively. These values were combined with the previously reported electron affinities of ClO(3) and ClO(4) to obtain BDE(H-OClO(2)) = 97.6 +/- 4.0 kcal mol(-1) and BDE(H-OClO(3)) = 107.4 +/- 6.1 kcal mol(-1). These energetic determinations represent the first measurements of these quantities or extensive revisions of the currently available values. B3LYP, M06, M06-2X, G3, and G3B3 computations also were carried out to provide acidities, electron affinities, bond dissociation energies, and heats of formation via atomization energies for ClO(x) and HClO(x), where x = 1-4. All of the methods do a reasonable job for the first three thermodynamic quantities but only M06 does a satisfactory job for the heats of formation, and its performance is similar to the highly accurate but extremely time intensive W4 method.