Spectra showing the HFS transitions corresponding to the J = 4 → 3 (top), J = 5 → 4 (middle) and J = 3 → 2 (bottom), fine-structure levels observed using the Onsala 20 m telescope toward W3 IRS5. The individual fits to the HFS components are displayed by red curves. The NS contamination near 69 019.180 MHz that blends with the J, F = 3, 4 → 2, 3 o-CH₂ HFS line at 69 017.995 MHz is marked in light blue. The relative contribution of the NS feature is modelled using the nearby NS F = 5∕2 → 3∕2 transition at 69 002.890 MHz and is displayed in light blue. The velocity scale is set by the F′ − F′′ = J–J − 1 HFS line (see Table 1).