Infrared investigations of negatively charged complexes and clusters

The review describes recent progress in the characterization of size-selected negatively charged clusters in the gas phase using vibrational predissociation spectroscopy. Examples from the authors' laboratory are used to demonstrate the way in which spectra are obtained and interpreted to provide information on anion-neutral interactions. Infrared studies of simple dimer complexes consisting of hydrogen molecules attached to halide anions (Cl--H-2, Br--H-2, I--H-2) are described. From rotationally resolved spectra in the 2.5 mum H-H stretch region one can deduce that the complexes have linear equilibrium structures and can ascertain intermolecular separations. Corresponding spectra of the Cl--D-2 and Br--D-2 isotopomers display a series of clearly resolved doublets, highlighting the importance of hindered internal rotation of the D-2 subunit. Studies of Cl--(C2H2)(n), Br--(C2H2)(n) and I--(C2H2)(n) clusters containing up to nine C2H2 molecules illustrate how infrared spectra can be used to explore the progressive 'solvation' of halide anions. The smaller clusters (n less than or equal to 6) have morphologies in which equivalent acetylene molecules are hydrogen bonded to the interior halide anion. For n > 6 there is evidence for structures in which one or more acetylene molecules are situated in the second solvation shell and also for the existence of multiple isomeric forms. The article concludes by discussing prospects for extending spectroscopic studies to hitherto uncharacterized anion complexes.