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 ν₃ 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.