1_{0,1}$ rotational lines fall within the coverage of the CHESS spectral survey. A clear detection has been obtained in the 2b band for both $J = \tfrac{5}{2}-\tfrac{3}{2}$ and $J = \tfrac{3}{2}-\tfrac{1}{2}$ fine-structure components of the $N_{Ka,\,Kc} = 2_{1,2} \leftarrow 1_{0,1}$ transitions of the *para* species. They are shown in Fig. \[fig:nd2\_21\]. The $N_{Ka,\,Kc} = 1_{1,1} \leftarrow 0_{0,0}$, $J = \tfrac{3}{2}-\tfrac{1}{2}$ of the *ortho* species, illustrated in Fig. \[fig:nd2\_10\], is located at 527GHz.

![Spectrum of the $J = \tfrac{3}{2} - \tfrac{1}{2}$ component of the $N_{Ka,Kc} = 1_{1,1} - 0_{0,0}$ *o*- transition around 527GHz observed towards IRAS 16293-2422. The filled gray histograms show the observed spectra, while the blue trace is our HFS fit. The red sticks represent the position of the HFS components assuming LTE intensities.[]{data-label="fig:nd2_10"}](nd2_1-0_15-05.pdf){width="\hsize"}

In this spectral region, covered by the HIFI 1a band, the continuum emission of the source is only 200mK; hence, the resulting absorption feature is rather faint and the detection is only marginal, reaching at most $5\sigma$. The other fine-structure component (intrinsically less intense) is just below the noise level and not detected. Given the weakness of all the three components observed for , their analysis had to be carried out adopting suitable assumptions for 