Gas phase infrared spectroscopy of cluster anions as a function of size: The effect of solvation on hydrogen-bonding in Br"„HBr...1,2,3 clusters

The gas phase vibrational spectroscopy of Br•~HBr!1,2,3 clusters has been studied between 6 and 16 mm ~625 and 1700 cm! by multiphoton infrared photodissociation spectroscopy using the output of the free electron laser for infrared experiments. Infrared ~IR! spectra were recorded by monitoring the mass-selected ion yield. In all three systems neutral HBr loss is found to be the dominant photofragmentation channel. BrHBr exhibits a weak absorption band at 1558 cm which is assigned to the overtone of the antisymmetric stretching mode n3 . A series of strong absorption bands was observed for Br•~HBr!2 at energies in the 950–1450 cm 21 range. The Br•~HBr!3 spectra reveal two absorption bands at 884 and 979 cm, which are assigned to two H-atom stretching modes. Evidence for the localization of the H atom and destruction of the symmetric BrHBr hydrogen bond in the larger clusters is presented. Standard electronic structure calculations fail to reproduce the experimental IR spectra, indicating a breakdown of the harmonic approximation. © 2002 American Institute of Physics. @DOI: 10.1063/1.1506308#