Continuum-subtracted spectrum of IRAS 20126 in 4.35–4.5 µm range. The shaded gray area shows the 3σ uncertainty on optical depth. The H₂ emission line at ~4.41 µm is shown in gray. Left: two of the most extreme CH₃CN ice mixtures in peak position with large FWHM from Rachid et al. (2022) are shown in red and purple to demonstrate that the shaded green region cannot be explained by CH₃CN mixtures, but the shaded pink region can. Red is the CH₃CN:H₂O:CO₂ (1:5:2, 50 K) mixture and purple is the CH₃CN:CO (1:10, 80 K). Right: mixtures of CH₃CN:CO (1:10) at 15 (blue) and 80 K (orange) demonstrate that the one at 80 K has a larger FWHM and peak position. In addition, the red circle shows the shoulder that does not exist in the pure CH₃CN (dashed green) laboratory spectra of Rachid et al. (2022) and is produced in the CH₃CN ice feature when mixed with CO.