Interactions between the chloride anion and aromatic molecules: infrared spectra of the Cl-_C6H5CH3, Cl-_C6H5NH2 and Cl-_C6H5OH complexes

The Cl --C 6H 5CH 3-Ar, Cl --C 6H 5N 3Ar, and Cl --C 6H 5OH.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 6H 5NH 2 and CI 5-C 6H 5OH, the Cl - anion is attached to the substituent group by a single near-linear hydrogen bond. For Cl --C 6H 5CH 3, the Cl 5 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.