Infrared spectra of Cl--(C2H2)n (1 ≤ n ≤ 9) anion clusters:: Spectroscopic evidence for solvent shell closure

Mid-infrared vibrational predissociation spectra of mass selected Cl--(C2H2)(n) (1 less than or equal to n less than or equal to 9) complexes have been recorded in the vicinity of the acetylene nu(3) vibrational band (2700-3400 cm(-1)). For clusters containing up to 6 acetylene ligands, the spectra each feature a single dominant band, shifted to lower frequency from the nu(3) C-H stretch band of free acetylene, and are consistent with interior solvation structures, whereby roughly equivalent acetylene molecules are bound end-on to a central chloride anion. Spectra of the n=7, 8, and 9 complexes, display multiple peaks and provide evidence for acetylene molecules situated in a second solvation shell and also for the existence of multiple isomeric forms. Depending on the cluster size, the inner solvation shell contains 6-8 acetylene molecules. (C) 1999 American Institute of Physics. [S0021-9606(99)01818-8].