Infrared Spectroscopy of Ni⁺(C₂H₂)<i>_n</i> Complexes:  Evidence for Intracluster Cyclization Reactions

Abstract

Ni + (C 2 H 2 ) n ion−molecule complexes are produced by laser vaporization in a pulsed nozzle source. These ions are mass analyzed and size-selected with a reflectron time-of-flight spectrometer. Infrared excitation with a pulsed OPO/OPA laser induces photodissociation of these complexes that proceeds by sequential loss of C 2 H 2 molecules. IR resonance-enhanced photodissociation (IR-REPD) spectra are measured in the C−H stretch region for Ni + (C 2 H 2 ) n complexes ( n = 3−6). The C−H stretch vibrations are shifted to lower frequencies than those in free acetylene, consistent with the formation of metal cation π-complexes. At the cluster sizes of n = 5 and larger, new bands are measured attributed to surface acetylene molecules. Additional red-shifted bands at n = 5 and beyond are assigned to an intracluster cyclization reaction, forming π-bonded cyclobutadiene.