Infrared spectra of I--(C2H2)n (1≤n≤4) anion complexes

Mid-infrared vibrational predissociation spectra of mass-selected I--(C2H2)(n) (1 less than or equal to n less than or equal to 4) complexes have been recorded in the 2700-3400 cm(-1) range. The spectra each feature a single compact band (fwhm less than or equal to 40 cm(-1)), that is shifted from the nu(3) C-H stretch band of free acetylene by -216.9, -176.7, - 59.7 and -150.7 cm(-1) for the n = 1, 2, 3 and 3 clusters, respectively. The main absorption band of the I--C2H2 dimer is consistent with a linear complex, in which excitation of the hydrogen-bonded C-H stretch induces a stiffening and-contraction of the intermolecular I- ... C2H2 bond. Spectra of the larger I--(C2H2)(n) complexes are in accordance with structures where the I- is solvated by roughly equivalent hydrogen-bonded acetylene molecules. (C) 1999 Elsevier Science B.V. All rights reserved.