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

Mid-infrared vibrational predissociation spectra of mass-selected I−–(C2H2)n (1≤n≤4) complexes have been recorded in the 2700–3400 cm−1 range. The spectra each feature a single compact band (fwhm≤40 cm−1), that is shifted from the ν3 C–H stretch band of free acetylene by −216.9, −176.7, −159.7 and −150.7 cm−1 for the n=1, 2, 3 and 4 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.