Reflection-absorption infrared spectra of thick CH₃CN ice (850 ML; upper panel) and of thick CH₃NC ice (890 ML; lower panel), condensed at 90 K on polycrystalline gold. Indicated assignments are detailed in Table 1. Peaks denoted with (*) correspond to identified combination modes. Structures observed at ≈2350 cm^-1 are associated with a modification of the reference spectra due to a change of gaseous CO₂ concentration in the spectrometer during the experiment time.