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Interaction between p-toluenesulfonic acid (pTSA) and water is studied at -20 ℃ 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 ℃) 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-p 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. [ABSTRACT FROM AUTHOR]