Interactions between the chloride anion and aromatic molecules: Infrared spectra of the Cl-C₆H₅CH₃, Cl-C ₆H₅NH₂ and Cl-C₆H₅OH complexes

The Cl^--C₆H₅CH₃-Ar, Cl ^--C₆H₅N₃Ar, and Cl ^--C₆H₅OH.Ar anion complexes are investigated using infrared photodissociation spectroscopy and ab initio calculations at the MP2/aug-cc-pVDZ level. The results indicate that for Cl^--C ₆H₅NH₂ and CI₅-C₆H ₅OH, the Cl^- anion is attached to the substituent group by a single near-linear hydrogen bond. For Cl^--C₆H ₅CH₃, the Cl₅ is attached to an orifco-hydrogen atom on the aromatic ring and to a hydrogen atom on the methyl group by a weaker hydrogen bond. The principal spectroscopic consequence of the hydrogen-bonding interaction in the three complexes is a red-shift and intensity increase for the CH, NH, and OH stretching modes. Complexities in the infrared spectra in the region of the hydrogen-bonded XH stretch band are associated with Fermi resonances between the hydrogenstretching vibrational modes and bending overtone and combination levels. There are notable correlations between the vibrational red-shift, the elongation of the H-bonded XH group, and the proton affinity of the aromatic molecule's conjugate base.