Journal of Physical Chemistry A, Vol.111, No.31, 7322-7328, 2007
Interactions between the chloride anion and aromatic molecules: Infrared spectra of the Cl--C6H5CH3, Cl--C6H5NH2 and Cl--C6H5OH complexes
The Cl--C6H5CH3 center dot Ar, Cl--C6H5NH2 center dot Ar, and Cl--C6H5OH center dot 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--C6H5NH2 and Cl--C6H5OH, the Cl- anion is attached to the substituent group by a single near-linear hydrogen bond. For Cl--C6H5CH3, the Cl- is attached to an ortho-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 hydrogen-stretching 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.