Knowledge Bank

Previously, two distinct isomers of the CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ trimer have been identified by means of infrared spectroscopy. The first is a symmetric top with a cyclic planar structure and the second is an asymmetric top barrel-shaped structure with C$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ symmetry. We have observed two new parallel ($\Delta $$_{2}$ trimer at $2364$ and $2370.5$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ which are each assigned as a combination of an intramolecular CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ monomer $\nu 3$ stretch and an intermolecular out-of-plane torsion, giving the torsional frequencies of $12.3$ and $18.8$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, respectively. The band at $2364$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ is surprisingly strong and completely unperturbed, providing a rare and near perfect example for a parallel band of a symmetric top molecule with C symmetry and zero nuclear spins. The second band at 2370.5 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ is somewhat weaker and has not been fully analysed because of spotty coverage of our diode lasers. However, it is still easily assigned as the second anticipated combination band involving an out-of-plane torsional mode of the cyclic CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ trimer. The trimers are generated in a pulsed supersonic expansion from a slit-jet nozzle and probed with a tunable infrared diode laser.