Upper panel: section of experimental spectrum between 264 and 282 GHz (in black) compared to calculated spectra obtained from our fit parameters described in Table 2. Lower panel: expanded view of the 271.96−273.96 GHz regionshowing the assignments (A-species of the GS in red, E-species of the GS in blue, A-species of the ν₁₈ = 1 mode in green, E-species of the ν₁₈ = 1 mode in orange, and A-species of the ν₁₈ = 2 mode in magenta).