Knowledge Bank

We recenty reported \textbf{13}, 1297 (2011).} assignments of specific infrared bands in the CO$2$ \nub{3} region ($\sim$2350 $\backslash wn$) to (CO$2$)$6$, (CO$2$)$7$, (CO$2$)$9$, (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}$, (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}$, (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}$, and (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$. 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. \textbf{107}, 5703 (2008); C.S. Murthy, S.F. O'Shea, and I.R. McDonald, \textit{Mol. Phys.} \textbf{50}, 531 (1983).} (CO$2$)$6$ is a symmetric top with $S$$6$ point group symmetry which can be thought of as a stack of two planar cyclic trimers. (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$ is also an $S$$6$ symmetric top, and consists of a single CO$2$ monomer surrounded by an slightly distorted icosahedral cage. The remaining clusters are asymmetric tops without symmetry. Here we report additional CO$2$ cluster results. Calculations based on the SAPT-s potential \textbf{110}, 3785 (1999).} indicate that the structure of (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}$ may be slightly different from that given by Takeuchi/Murthy. An additional band is observed for each of (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$ and (CO$2$)$\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}$. A feature observed at 2378.2 $\backslash wn$ is assigned as a (CO$2$)$6$ 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 (CO$2$)$6$. This is also a symmetric top, but now with $S$$4$ 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.