Tentative detection of HC5NH+ in TMC-1
  Using the Yebes 40m radio telescope, we report the detection of a series of
seven lines harmonically related with a rotational constant B0=1295.81581 +/-
0.00026 MHz and a distortion constant D0=27.3 +/- 0.5 Hz towards the cold dense
cloud TMC-1. Ab initio calculations indicate that the best possible candidates
are the cations HC5NH+ and NC4NH+. From a comparison between calculated and
observed rotational constants and other arguments based on proton affinities
and dipole moments, we conclude that the best candidate for a carrier of the
observed lines is the protonated cyanodiacetylene cation, HC5NH+. The
HC5N/HC5NH+ ratio derived in TMC-1 is 240, which is very similar to the
HC3N/HC3NH+ ratio. Results are discussed in the framework of a chemical model
for protonated molecules in cold dense clouds.
