Infrared spectra of X-•CO2•Ar cluster anions (X = Cl,Br,I)

Ion-molecule clusters of the heavier halide anions X-.CO2 (X=Cl-,Br-,I-) with CO2 have been studied by gas phase infrared photodissociation spectroscopy, using Ar evaporation from the complexes X-.CO2.Ar upon infrared excitation. We observe that the asymmetric stretch vibrational mode of the CO2 molecule is red-shifted from the frequency of free CO2, with the red-shift increasing toward the lighter halide ions. A similar trend is repeated in the region of the Fermi resonance of the combination bands of the asymmetric stretch vibration with two quanta of the bending vibration and the symmetric stretch vibration. We discuss our findings in the framework of ab initio and density functional theory calculations. (C) 2004 American Institute of Physics.