15min:

A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, ON K1A 0R6, Canada; J. NOROOZ OLIAEE, M. DEHGHANY AND N. MOAZZEN-AHMADI, Department of Physics and Astronomy, University of Calgary, 2500 University Dr., N.W., Calgary, AB T2N 1N4, Canada.

We recenty reported assignments of specific infrared bands in the CO_{2 3} region ( 2350 cm^-1) to (CO₂)₆, (CO₂)₇, (CO₂)₉, (CO₂)₁₀, (CO₂)₁₁, (CO₂)₁₂, and (CO₂)₁₃. Spectra are obtained by direct absorption using a rapid-scan tuneable diode laser spectrometer to probe a pulsed supersonic slit-jet expansion and assignments are facilitated by recent calculations of Takeuchi based on the Murthy potential.\footnote[2]H. Takeuchi, J. Phys. Chem. A \textbf107, 5703 (2008); C.S. Murthy, S.F. O'Shea, and I.R. McDonald, Mol. Phys. \textbf50, 531 (1983). (CO₂)₆ is a symmetric top with S₆ point group symmetry which can be thought of as a stack of two planar cyclic trimers. (CO₂)₁₃ is also an S₆ symmetric top, and consists of a single CO₂ monomer surrounded by an slightly distorted icosahedral cage. The remaining clusters are asymmetric tops without symmetry.

Here we report additional CO₂ cluster results. Calculations based on the SAPT-s potential indicate that the structure of (CO₂)₁₀ may be slightly different from that given by Takeuchi/Murthy. An additional band is observed for each of (CO₂)₁₃ and (CO₂)₁₀. A feature observed at 2378.2 cm^-1 is assigned as a (CO₂)₆ parallel combination band involving the sum of a fundamental and a low-lying intermolecular vibration. Most significantly, two bands are assigned to a second isomer of (CO₂)₆. This is also a symmetric top, but now with S₄ symmetry. The two symmetric hexamer isomers observed spectroscopically correspond well with the lowest energy structures given by both the SAPT-s and Murthy intermolecular potentials.