New line intensities for the far infrared bands of the Trans- and Cis-conformer of nitrous acid (HONO), new determination of the Trans-Cis conformer barrier and its impact on the astrophysical detection of nitrous acid in protostellar clouds

•Determination of the energy difference (ΔECis-Trans) between the Cis- and Trans-HONO conformers of nitrous acid.•Relative line intensities for the B-type transitions of pure rotational bands of Trans-HONO and Cis-HONO measured.•Far infrared FTS spectra on SOLEIL.•Line list for HONO in the far infrared region.