Infrared spectra reveal box-like structures for a pentamer and hexamer of mixed carbon dioxide–acetylene clusters

Abstract

Except for a few cases like water and carbon dioxide, identification and structural characterization of clusters with more than four monomers is rare. Here, we provide experimental and theoretical evidence for existence of box-like structures for a pentamer and a hexamer of mixed carbon dioxide–acetylene clusters. Two mid-infrared cluster absorption bands are observed in the CO₂ν₃ band region using a tunable diode laser to probe a pulsed supersonic jet. Each requires the presence of both carbon dioxide and acetylene in the jet, and (from observed rotational spacings) involves clusters containing about 4 to 7 molecules. Structures are predicted for mixed CO₂ + C₂H₂ clusters using a distributed multipole model, and the bands are assigned to a specific pentamer, (CO₂)₃–(C₂H₂)₂, and hexamer, (CO₂)₄–(C₂H₂)₂. The hexamer has a box-like structure whose D_2d symmetry is supported by observed intensity alternation in the spectrum. The pentamer has a closely related structure which is obtained by removing one CO₂ molecule from the hexamer. These are among the largest mixed molecular clusters to be assigned by high-resolution spectroscopy.