Probing a strong hydrogen bond with infrared spectroscopy: Vibrational predissociation of BrHBr−⋅Ar

The gas phase vibrational spectroscopy of BrHBr−, a prototypical strongly hydrogen-bonded species, has been studied between 6 and 17 μm (590 and 1670 cm−1) by infrared vibrational predissociation of the BrHBr−⋅Ar ion. Infrared excitation was accomplished using the output of the free electron laser for infrared experiments (FELIX). Predissociation spectra were recorded by monitoring depletion of mass-selected BrHBr−⋅Ar ions as a function of excitation wavelength. Four prominent absorption bands are observed at 733, 890, 1048, and 1201 cm−1. They are assigned to the fundamental of the hydrogenic stretching mode ν3 and a sequence of ν3+nν1 combinations (n=1–3). Additional features to the blue of these bands spaced by ∼21 cm−1 are attributed to combination bands involving motion of the Ar messenger atom. Differences in the relative intensities of the ν3+nν1 combinations bands in comparison to previous matrix experiments are rationalized on the basis of the underlying dissociation dynamics.