Structure and vibrational dynamics of the CO2 dimer from the sub-Doppler infrared spectrum of the 2.7 μm Fermi diad

Abstract

Sub‐Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm⁻¹ using an optothermal molecular beam color‐center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm⁻¹ from the corresponding ν₁+ν₃/2ν⁰₂+ν₃ CO₂ bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C_2h symmetry. The C‐‐C separation and C‐‐C–O angle are determined to be 3.599(7) Å and 58.2(8)°, respectively. The nearest neighbor O‐‐C distance is 3.14 Å which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm⁻¹. No interconversion tunneling is observed.