Weakly-bound clusters of atmospheric molecules: infrared spectra and structural calculations of (CO2)n-(CO)m-(N2)p, (n, m, p) = (2, 1, 0), (2, 0, 1), (1, 2, 0), (1, 0, 2), (1, 1, 1), (1, 3, 0), (1, 0, 3), (1, 2, 1), (1, 1, 2)
Abstract
Structural calculations and high-resolution infrared spectra are reported for trimers and tetramers containing CO2 together with CO and/or N2. Among the 9 clusters studied here, only (CO2)2-CO was previously observed by high-resolution spectroscopy. The spectra, which occur in the region of the 3 fundamental of CO2 (~2350 cm-1), were recorded using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The trimers (CO2)2-CO and (CO2)2-N2 have structures in which the CO or N2 is aligned along the symmetry axis of a staggered side-by-side CO2 dimer unit. The observation of two fundamental bands for (CO2)2-CO and (CO2)2-N2 shows that this CO2 dimer unit is non-planar, unlike (CO2)2 itself. For the trimers CO2-(CO)2 and CO2-(N2)2, the CO or N2 monomers occupy equivalent positions in the 'equatorial plane' of the CO2, pointing toward its C atom. To form the tetramers CO2-(CO)3 and CO2-(N2)3, a third CO or N2 monomer is then added off to the 'side' of the first two. In the mixed tetramers CO2-(CO)2-N2 and CO2-CO-(N2)2, this 'side' position is taken by N2 and not CO. In addition to the fundamental bands, combination bands are also observed for (CO2)2-CO, CO2-(CO)2, and CO2-(N2)2, yielding some information about their low-frequency intermolecular vibrations.
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