Matrix Isolation Spectroscopy: Aqueous p-Toluenesulfonic Acid Solvation

Interaction between p-toluenesulfonic acid (pTSA) and water is studied at -20 °C in a CCl₄ matrix. In CCl₄ water exists as monomers with restricted rotational motion about its symmetry axis. Additionally, CCl₄ is transparent in the hydrogen-bonded region; CCl₄ thus constitutes an excellent ambient thermal energy matrix isolation medium for diagnosing interactions with water. Introducing pTSA-nH₂O gives rise to two narrow resonances at 3642 cm^-1 and at 2835 cm^-1 plus a broad 3000-3550 cm^-1 absorption. In addition, negative monomer symmetric and asymmetric stretch features relative to nominally dry CCl₄ indicate that fewer water monomers exist in the cooled (-20 °C) acid solution than in room-temperature anhydrous CCl₄. The negative peaks along with the broad absorption band indicate that water monomers are incorporated into clusters. The 3642 cm^-1 resonance is assigned to the OH-π interaction with a cluster containing many water molecules per acid molecule. The 2835 cm^-1 resonance is assigned to the (S-)O-H stretch of pTSA-dihydrate. The coexistence of these two species provides insights into interactions in this acid-water CCl₄ system.