Raman spectrum of supercritical C¹⁸O₂ and re-evaluation of the Fermi resonance

We report the first Raman spectra of fully ¹⁸O-labeled supercritical CO₂ (scCO₂) and various isotopic mixtures. The experimental results, coupled with ab initio molecular dynamics calculations, demonstrate that the frequencies assigned to the Fermi dyad of the CO₂ molecule transpose upon isotopic labeling of both oxygen atoms. Although the transposition of the Fermi dyad of CO₂ gas due to isotopic substitution has been discussed before, this is the first confirmation of the effect in the Raman spectrum of the supercritical fluid and provides necessary groundwork for future Raman spectroscopy studies of reactions in this important medium. More importantly, the work yields a quantitative assessment of the mixing of states upon labeling that provides the needed clarification concerning the pedigree of the assignments for the dyad of CO₂ under supercritical conditions.