Cl--C6H6, Br--C6H6, and I--C6H6 anion complexes:: Infrared spectra and ab initio calculations

Vibrational predissociation spectroscopy is used to obtain infrared spectra of the Cl--C6H6, Br--C6H6, and I--C6H6 complexes in the region of the benzene CH stretch vibrations (2800-3200 cm(-1)). The infrared spectra of the three dimers are similar, each exhibiting several narrow bands (full width at half maximum <10 cm-1) that are only slightly redshifted from the absorptions of the free benzene molecule. Ab initio calculations predict that the most stable form of the three complexes is a planar C-2v structure in which the halide is hydrogen bonded to two adjacent CH groups. The planar C-2v structure in which the halide is linearly H bonded to a single CH group is predicted to be slightly less stable than the bifurcated form. Comparisons between experimental and theoretically predicted infrared spectra confirm that the bifurcated structure is indeed the most stable conformer for all three complexes. Ab initio calculations show that the electron density transfer from the halide to the benzene is not limited to the sigma*(CH) orbitals adjacent to the halide, but extends to the sigma domain of the benzene ring, consistent with the moderate shift of the CH stretch frequencies. The presence of weak satellite bands is explained in terms of Fermi resonances reminiscent of the benzene Fermi tetrad or hot bands involving the in-plane intermolecular bend vibration. (C) 2003 American Institute of Physics.