  Widespread interest in weakly bound molecular clusters of medium size (5-50
molecules) is motivated by their complicated energy landscapes, which lead to
hundreds or thousands of distinct isomers. But most studies are theoretical in
nature, and there are no experimental results which provide definitive
structural information on completion of the first solvation shell. Here we
assign rotationally resolved mid-infrared spectra to argon clusters containing
a single carbon dioxide molecule, CO2-Ar15 and CO2-Ar17. These mark completion
of the first solvation shell for CO2 in argon. The assignments are confirmed by
nuclear spin intensity alternation in the spectra, a marker of highly symmetric
structures for these clusters. Precise values are determined for rotational
parameters, and for shifts of the CO2 vibrational frequency induced by the
argon atoms. The spectra indicate possible low frequency (~2 cm-1) vibrational
modes in these clusters, posing a challenge for future cluster theory.
