  Bright HNC 1--0 emission has been found towards several Seyfert galaxies.
This is unexpected since traditionally HNC is a tracer of cold (10 K) gas, and
the molecular gas of luminous galaxies like Seyferts is thought to have bulk
kinetic temperatures surpassing 50 K. In this work we aim to distinguish the
cause of the bright HNC and to model the physical conditions of the HNC and HCN
emitting gas. We have used SEST, JCMT and IRAM 30m telescopes to observe HNC
3-2 and HCN 3-2 line emission in a selection of 5 HNC-luminous Seyfert
galaxies. We estimate and discuss the excitation conditions of HCN and HNC in
NGC 1068, NGC 3079, NGC 2623 and NGC 7469, based on the observed 3-2/1-0 line
intensity ratios. We also observed CN 1-0 and 2-1 emission and discuss its role
in photon and X-ray dominated regions. HNC 3-2 was detected in 3 galaxies (NGC
3079, NGC 1068 and NGC 2623). HCN 3-2 was detected in NGC 3079, NGC 1068 and
NGC 1365. The HCN 3-2/1-0 ratio is lower than 0.3 only in NGC 3079, whereas the
HNC 3-2/1-0 ratio is larger than 0.3 only in NGC 2623. The HCN/HNC 1-0 and 3-2
line ratios are larger than unity in all the galaxies. The HCN/HNC 3-2 line
ratio is lower than unity only in NGC 2623, similar to Arp 220, Mrk 231 and NGC
4418. In three of the galaxies the HNC emissions emerge from gas of densities
n<10^5 cm^3, where the chemistry is dominated by ion-neutral reactions. In NGC
1068 the emission of HNC emerges from lower (<10^5 cm^3) density gas than HCN
(>10^5 cm^3). Instead, the emissions of HNC and HCN emerge from the same gas in
NGC 3079. The observed HCN/HNC and CN/HCN line ratios favor a PDR scenario,
rather than an XDR one. However, the N(HNC)/N(HCN) column density ratios
obtained for NGC 3079 can be found only in XDR environments.
