Unravelling the Deprotonation Isomers of Sulfobenzoic Acid Produced via Electrospray Ionization: A Combined Infrared Spectroscopy and Quantum Chemical Study

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Abstract
Mass spectrometric identification of the regioisomers of Sulfobenzoic acid (SBA), an important chemical intermediate, is complicated by the system containing both sulfonic acid and carboxylic acid deprotonation sites. Here, we apply infrared multiple-photon dissociation (IRMPD) spectroscopy for the first time to ortho-, meta-, and para-SBA deprotonated anions, [SBA-H]-, formed via electrospray ionization, to determine the identity of the deprotonation isomers. IRMPD spectra were acquired over the fingerprint region (800 – 2000 cm-1) at the FELIX free-electron laser, with experimental measurements being interpreted with the aid of quantum chemical calculations at the CAM-B3LYP/6-311++G(2d,2p) level. For all three regioisomers, the IRMPD spectra are consistent with deprotonation occurring only at the sulfonic acid group with characteristic sulfonate stretches observed (~1220 and 1060 cm-1, for the symmetric and asymmetric stretches). Importantly, we find no spectroscopic evidence for the presence of carboxylate deprotomers. Our spectroscopic results are fully consistent with the quantum chemical predictions of the relative deprotomer energies of the [SBA-H]- anions, since the meta- and para-[SBA-H]-, gas-phase sulfonate deprotomers are calculated to be substantially lower in energy than the corresponding carboxylates (by 65.6 and 73.3 kJ/mol, respectively). The ortho-[SBA-H]- regioisomers optimizes to a single structure, where the sulfonate group is deprotonated and forms an intramolecular hydrogen bond with the -OH group of the adjacent carboxylic acid. We discuss the collision-induced dissociation patterns of the [SBA-H]- regioisomers, and present a full interpretation of the observed fragmentation pathways consistent with the electrosprayed anions corresponding solely to sulfonate structures.

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